DM6854 Isolated Digital I/O dataModule User`s Manual
Contents
1. Pin Description Description Pin 1 Port C 0 Ext Trigger Output 2 3 Port C 1 GND 4 5 Port C 2 GND 6 7 Port C 3 GND 8 9 Port C 4 GND 10 11 Port C 5 GND 12 13 Port C 6 GND 14 15 Port C 7 GND 16 17 Port B 0 GND 18 19 Port B 1 GND 20 21 Port B 2 GND 22 23 Port B 3 GND 24 25 Port B 4 GND 26 27 Port B 5 GND 28 29 Port B 6 GND 30 31 Port B 7 GND 32 33 Port A 0 GND 34 35 Port A 1 GND 36 37 Port A 2 GND 38 39 Port A 3 GND 40 41 Port A 4 GND 42 43 Port A 5 GND 44 45 Port A 6 GND 46 47 Port A 7 GND 48 49 5 V fused GND 50 PC 104 Bus Connectors CN1 and CN2 Connectors CN1 and CN2 provide PC 104 bus connections CN1 carries XT bus signals and CN2 carries additional signals for the AT bus The signals on CN1 and CN2 conform to the IEEE P966 standard for the PC 104 bus The PC 104 connector pinout may be found at www pc104 org Note Two locations on the bus have mechanical keying pins to help prevent misconnection of the PC 104 bus These keying pins are a part of the PC 104 standard and we strongly recommend you leave them in place If you have other modules without keying pins we suggest you modify them to include keying Reserved Connector CN9 CNO is reserved for factory use Nothing should be attached to this connector Page 14 of 33 DM6854 User s Manual Chapter 5 HARDWARE DESCRI2. 17 OPtOCOUplerS rest eter pietre RI eye E UE EN ipte res 17 TRIGGER OUTPUT 3 5 d Tthtusoro tto Im vetita teas riis cepi a iie 17 INTER uUa NH LEE 17 CHAPTER6 BOARD OPERATION AND PROGRAMMING ee ee ecce eene eese enne tese sees eee ee toes een 18 DEFINING THE I O MAP cccsccccccecessessccecececeessscaecececeeseseaecececsesensnaececececsesaaeceeececeesaaececececsessaaeaecececeeneasaesececeenes 18 BA 0 Digital input register Write eene eene trennen trennen 18 BA 0 Digital inputs Read i eie d tue re e e i ERE D E TREE a Tie esed 18 1 Input mask register Read Write 0 18 BA 2 Input inversion register Read Write 0 after 1 19 BA 3 Configuration digital output register Read Write 0 after reset essere 19 Page 4 of 33 DM6854 User s Manual 4 7 Read Write sxc sacado epe tton ede teen tai een uneven qs 19 BA 5 PPI Port B Read Write 19 BA 6 PPI Port 20 7 PPI Control Byte Write only ir tate Rm net e RH 20 BA 400h BA 401h RTD ID Data read only 8 bit or I6 bit esses ener 21 402h RTD ID Reset Pointer read only 8 bit 21 RID AD Data Read Indexes nee ee te PREISE NEUE E Hee Ue ar ER eet eR E e EE 21 PROGRAMMIN
3. Transfers the 8 bit Port A digital input and output data between the module and an external device A read transfers data from the external device through connector J5 and into port A a write transfers the written data from port A through J5 to the external devices BA 5 PPI Port B Read Write Transfers the 8 bit Port B digital input and output data between the module and an external device A read transfers data from the external device through connector J5 and into port B a write transfers the written data from port B through J5 to the external devices Page 19 of 33 DM6854 User s Manual BA 6 Read Write Transfers the 4 bit low nibble of the Port C digital input and output data between the module and an external device A read transfers data from the external device through connector J5 and into bits 0 7 of port C a write transfers the written data bits 0 7 from port C through J5 to the external devices 7 PPI Control Byte Write only Table 6 2 Control Byte Control Word Mode Bit Name Description D7 Mode Set Flag 1 Active When bit 7 is set to 1 a write programs the PPI configuration Group 1 D6 D5 Mode select for group 1 06 D5 Mode 0 0 0 0 1 1 1 1 2 D4 Direction of Port A 0 Output 1 Input D3 Direction of Port C Upper bits 4 7 0 2 Output 1 Input Group 2 D2 Mode select for group 2 0 Mode
4. eese 12 RECOMMENDED PROCEDURE pec evi ee per petet vede ivre spes reine Pete c Evae baee e EN oe eraat 12 CHAPTER4 CONNECTING THE DATAMODULE eee ee ee eoe eee tone ee etta es seen eese ee eese e seen 13 BINDING PIN 1 OP CONNEGTORS ocurre etna exe eec Ove piene ee ea bet 13 ISOLATED I O CONNECTIONS JP15 amp 16 13 EXPANSION INTERFACE CONNECTIONS JP 14 14 PC 104 BUS CONNECTORS 2 14 CONNECTORS CNO 5 S 14 CHAPTER5 HARDWARE DESCRIPTION sccccssssscscssssccsssssccssssscccsssscccsssscccessscccecsssessesseccsessnsecscsscsecees 15 DIGITAL I O PROGRAMMABLE PERIPHERAL INTERFACE 15 ISOLATED OPTOCOUPLER INPUTS 2tesesesceteidcsteesetcetendeavessecedeateadesssstecee cauveduves seecedevueegensa Tee vex Pere EN p PER PE ETE Ree eI 16 Input a Irae 16 OptoCOUplers ss Svo PEN PEU ses FS ee 16 Input bit MASK stage i epe hem eate dee e AeA ive hie d ops AA oe eder Rei 16 Input bit inverter stage ettet deber Re bee Pe e Feeder Ste edet s etd t ES 16 ISOLATED OPTOCOUPLER OUTPUTS sssssscsccecesssssscecececsesesseaecececscsensnaesecececsesaaeceeececeeeaaeeecececsessaaeaecececsenesaaeeeeeceenes 17 Output data TE BISTCT xe A eet e RI ert err e eR nee 17
5. 0 1 Mode 1 DI Direction of Port B 0 Output 1 Input DO Direction of Port C Lower bits 0 3 0 Output 1 Input Table 6 3 Control Byte Bit Set Mode Bit Name Description D7 Mode Set Flag 0 Active When bit 7 is set to 0 a write can program individual lines of port C D6 D4 Reserved Can be any value D3 D1 Bit Select D3 D2 D1 Bit Select 0 0 0 PCO 0 0 1 PCI 0 1 0 2 0 1 1 1 0 0 PC4 1 0 1 PC5 1 1 0 PC6 1 1 1 7 Page 20 of 33 DM6854 User s Manual DO Value 0 Set bit to 0 1 Set bit to 1 BA 400h BA 401h RTD ID Data read only 8 bit or 16 bit RTD ID is a method to identify a board on the ISA bus There are two 8 bit registers mapped at BA 400h and 401th The registers can be read as two 8 bit or one 16 bit An internal pointer is auto incremented with every read to either address so the data read will step through each index as indicated below The pointer is set to zero at reset and can be reset to zero by a read to BA 402h BA 402h RTD ID Reset Pointer read only 8 bit only A read to BA 402h will set the internal pointer to zero The pointer is set to zero at reset RTD ID Data Read Indexes Index Data 8 Bit Read 16 Bit Read 0 Device ID 545h 6854h 1 Device ID 68h 2 RTD Vendor ID 35h 1435h 3 RTD Vendor ID 14h 4 EPLD Rev
6. Page 29 of 33 DM6854 User s Manual DC Characteristics Table 7 1 Absolute Maximum Ratings Parameter Symbol Min Max Units Storage Temperature Ts 55 125 Operating Temperature 40 85 Supply Voltage Vcc 0 5 7 V Digital I O non isolated Input Voltage Vi 0 5 0 5 V Output Voltage Vo 0 5 0 5 V Isolated Inputs Input Voltage IN with respect to IN Vi V 5V Range 50 50 12V Range 70 70 24V Range 100 100 Isolation Voltage Viso 1500 Vems Isolated Outputs Collector Current Ic 150 mA Collector Emitter Voltage V cgo 7 30 V Isolation Voltage Viso 1500 Vems Table 7 2 Recommended Operating Conditions Parameter Symbol Min Max Units Supply Voltage 4 75 5 25 Digital I O non isolated Low Input Voltage Vit 0 3 0 8 V High Input Voltage Vin 2 2 0 3 V Low Output Voltage Io 2 5 mA Vor 0 4 V High Output Voltage V 40 HA 4 2 2 5 mA 3 7 Pullup Pulldown Resistor R 9 11 Isolated Inputs Low Input Voltage IN with respect to IN Vir V 5V Range 25 2 0 12V Range 35 2 5 24V Range 50 4 5 High Input Voltage IN with respect to IN Vin V 5V Range 4 5 25 12V Range 6 35 24V Range 14 50 Isolated Outputs Collector Current Ic 0 8 mA Collector Emitter Voltage 0 30 V Page 30 of 33 DM6854 User s Manual Dimension Drawing Figur
7. Port A gt Output Port B Output Port C Upper Input Port C Lower Output Control Byte 88h Isolated digital input programming The optoisolated digital inputs may be addressed directly by reading the digital input register in address 0 The DM6854HR features more advanced modes of input pattern or bit change recognition to reduce software overhead This is possible with the hardware interrupt generated by the onboard comparison logic The following two subsections will illustrate how to program the different modes of the isolated digital input port Software controlled direct read Page 22 of 33 DM6854 User s Manual in data inp BA Loads data from inputs into variable in Interrupt driven read upon change in bits mask 0x55 Odd bits are of interest state 0x01 Any of bits 3 5 7 goes high or bit 1 goes low outp BA 1 mask Program mask register outp BA 2 state Program bit inversion register outp BA 3 0x01 Enable interrupts outp BA 0x00 Clear hardware interrupt if state had already occurred Now if any of bits 3 5 7 goes high or bit 1 goes low it will cause a hardware interrupt In your ISR you will read the input data with in data inp BA Isolated digital output programming The four bit digital output port can directly be programmed from software by writing output data to the high bits 4 8 of I O port address 3 Care must be taken to
8. Range Jumpers set for 24V input range Page 11 of 33 DM6854 User s Manual Chapter 3 INSTALLING THE DATAMODULE Recommended Procedure We recommend you follow the procedure below to ensure that stacking of the modules does not damage connectors or electronics e Turn off power to the PC 104 system or stack e Select and install standoffs to properly position the dataModule on the PC 104 stack e Touch a grounded metal part of the stack to discharge any buildup of static electricity e Remove the dataModule from its anti static bag e Verify the jumper settings of dataModule Check that keying pins in the PC 104 bus connector are properly positioned e Hold the dataModule by its edges and orient it so the bus connector pins line up with the matching connector on the stack e Gently and evenly press the dataModule onto PC 104 stack CAUTION Do not force the module onto the stack Wiggling the module or applying too much force may damage it If the module does not readily press into place remove it check for bent pins or out of place keying pins and try again Page 12 of 33 DM6854 User s Manual Chapter 4 CONNECTING THE DATAMODULE The following sections describe connectors of the dataModule Finding Pin 1 of Connectors A white area silk screened on the PC board indicates the pin 1 end of connectors A square solder pad visible on the bottom of the PC board also indicates it Please make certain you have
9. Writing an Interrupt Service Routine ISR epiese o een rennen tnter trennen trennen 25 Saving the Startup Interrupt Mask Register IMR and interrupt vector 26 Common Interrupt mistakes eese enne enne N nest nr en A tere tenete ee trennen nennen 27 Example of Interrupt vector table setup in 27 CHAPTER 7 SPECIFICATIONS 29 GENERAL SPECIFICATIONS GEAR EEG REED TER s 29 PEE 29 Digital O non isolated iiu ER Re er rte CO RR RENI M e RE 29 Digital isolated iae ha dg eve e ed e ive lest ode ente ege ved pe 29 Digital outputs tsolated 3 2 4 Re EAE ee E ite Ee eti 29 COMME CLOTS scat 29 CHARACTERISTICS 4 Serene Beet eet een avr neu tiere mite tees 30 DIMENSION DRAWING ee Eg eh vele eh imt ee E Nee exte EE e Ent ee MR ebbe E 31 CHAPTER8 RETURN POLICY AND WARRANTY eeeeeeeeeeeee teens enses tastes stessa sees 32 RETURN POLICY Soinion asite I rt e PR ERREUR E Ds e e ERR IER PRINTER e ERREUR RENS URS 32 LIMITED WARRANTY 45 costes ien etenim epe mne 32 Page 5 of 33 DM6854 User s Manual Chapter 1 INTRODUCTION Featur
10. an Email request please include your company s name and address your name your telephone number and a brief description of the problem Page 7 of 33 DM 66854 User s Manual Chapter 2 CONFIGURING THE DATAMODULE The following sections contain information on configuring the dataModule Please read this entire section before attempting to use the dataModule The DM6854HR Isolated digital I O board has jumper settings that you can change to suit your application and I O configuration setup Factory Configured Jumper Settings Table 2 1 illustrates the factory jumper setting for the DM6854HR Figure 2 1 shows the board layout and the locations of the jumpers The following paragraphs explain how to change the factory jumper settings to suit your specific application Table 2 1 Factory Default Jumper Settings Jumper Name Description Number of Jumpers Factory Default BASE JP9 Base Address 5 0x300 IRQ JP10 Interrupt Selection 12 11 1 IRQ5 PD PU Pullup or Pulldown for PPI Ports 3 terminal 4x Pulldown INI 8 Input Range for Isolated input 1 8 3 8x 24 T 1 is FE IN ar 3 E il in WI ELLE a 1 be 0 3 0 2 3277p 4 s a DM6854 Embedded Tec Figure 2 1 Board layout showing jumper locations Page 8 of 33 Base Address JP9 Factory setting 300h DM68
11. complete detailed description of the I O map and a detailed discussion of programming operations to aid you in application programming Defining the I O Map The I O map of the DM6854HR is shown in Table 4 1 below As shown the module occupies 8 addresses The Base Address designated as BA can be set using the jumpers as described in Chapter 1 Module Settings The following sections describe the register contents of each address used in the I O map Table 6 1 DM6854HR I O Address Read Function Write Function 0 Digital Inputs 1 8 Clear IRQ Trigger 1 Read Input Mask Write Input Mask 2 Read Input Inversion Register Write Input Inversion Register 3 Read Configuration Output Register Write Configuration Output Register 4 82C55 Read Port A Data 82C55 Write Port A Data 5 82 55 Read Port B Data 82C55 Write Port B Data 6 82 55 Read Port C Data 82C55 Write Port C Data BA 7 Reserved 82C55 Write to Control Byte 400 RTD ID Data Read next Character n a BA 401h RTD ID Data Read next Character n a BA 402h Reset RTD ID Reset counter n a BA 0 Digital input register Write The trigger condition is latched on the DM5854 6854 The interrupt request will remain high until the host acknowledges the interrupt This is done with a write to BA 0 Example data Clears trigger and interrupt data is irrelevant 0 Digi
12. ensure the lower four bits remain unchanged since they hold the configuration data of your DM6854HR The example below illustrates a safe way of controlling the digital outputs Example in C syntax Step 1 store the current contents of the Control Output register reg data 1 3 reg data holds all information conf data OxOf amp amp reg data mask lower bits to store configuration Step 2 prepare output data stored in variable out data out data out lt lt 4 shift left 4 bits Step 3 combine configuration and output data and write to port outp BA 3 out_data conf data write to correct address This code can be compressed into more elegant and optimum code The steps above illustrate the logic of operation Trigger output programming Interrupts As described in the previous sections an external trigger may be used to indicate to the host or other devices that a predetermined input condition has occurred Before you can use an external trigger you must first program your DM6854HR to enable correct operation The following steps must be taken Program mask and inversion register first Program the polarity of the trigger output to other devices Clear interrupt and trigger register Enable trigger output tristate buffer Enable interrupts this also enables the operation of the external trigger output What is an interrupt Page 23 of 33 DM6854 User s Man
13. of all the bits you do not wish to change by proper use of the bitwise AND and OR operators Using and or operators single or multiple bits can easily be set or cleared in one line operations e To clear a single bit in a port AND the current value of the port with the value where B 255 2 exp bit To set a single bit in a port OR the current value of the port with the value B where 2 exp bit e Bits are numbered from 0 7 for the low byte of a word and from 8 15 for the high byte of word Setting and clearing of multiple bits in a bye or word is more complex e To clear multiple bits in a port AND the current value of the port with the value B where B 255 the sum of the values of the bits to be cleared Note that the bits do not have to be consecutive e To set multiple bits in a port OR the current value of the port with the value B where B sum of the individual bits to be set Initializing the 8255 PPI Before you can operate the non isolated general purpose digital I O through the 8255 PPI it must be initialized This step must be performed every time you start up reset or reboot your computer The 8255 is initialized by writing the appropriate control byte to the I O Port 7 The contents of your control word will vary depending on how you want to configure your I O lines Use the control word description in the previous I O map section to help you program the right value Example
14. performed by writing to the mask register 1 A bit is selected for comparison if itis set to one 1 The currently active mask can be read back from the same address BA 1 After reset condition all bits are 0 and disabled Input bit inverter stage The trigger logic of the DM6854HR uses OR logic from the inputs This means that a transition from low to high on any input will cause a trigger condition To enable bitwise selection of the polarity of the signals an inverter stage may be used Bit inversion is performed by writing to the Page 16 of 33 DM6854 User s Manual input inversion register 2 An input channel is inverted by writing 1 to the corresponding inversion register bit Example outp BA 2 0x0f will invert input channels 1 4 Isolated optocoupler outputs The Isolated optocoupler output stage of the DM6854HR consists of 3 major parts e Output data register e Schmidt triggers Optocouplers Output data register The four bit output register holds the output pattern for the output stage Data can be updated and read from address 3 bits 4 8 The lower bits 1 4 hold the configuration bits of your DM6854HR Make sure these bits are unchanged when writing to this register To avoid accidental changes in configuration read the data in the register BA 3 before writing to it Schmidt triggers The output of the optocoupler has a Schmidt trigger to condition the output of the optocouplers t
15. 54 User s Manual The most common cause of failure when you are first setting up your module is address contention Some of your computers I O space is already occupied by other internal I O devices and expansion boards When the DM6854HR attempts to use its reserved I O addresses already used by another peripheral device erratic performance may occur and data read from the board may be corrupted To avoid this problem make sure you set up the base address first using the jumpers marked BASE which let you choose from 32 different I O addresses in your computers I O map Should the factory installed setting of 300H be unusable for your system configuration you may change this setting to another using the options illustrated in Table 2 2 The table shows the jumper settings and their corresponding I O address in values hexadecimal and decimal Make sure you verify the correct location of the base address jumpers Table 2 2 Base Address Jumper Settings Base Address Jumper Settings Base Address Jumper Settings Hex Decimal A8 A4 Hex Decimal A8 A4 200 512 00000 300 768 10000 210 528 00001 310 784 10001 220 544 00010 320 800 10010 230 560 00011 330 816 10011 240 576 00100 340 832 10100 250 592 00101 350 848 10101 260 608 00110 360 864 10110 270 624 00111 370 880 10111 280 640 01000 380 896 11000 290 656 01001 390 912 11001 2A0 6
16. 72 01010 3A0 928 11010 2B0 688 01011 3B0 944 11011 2C0 704 01100 3C0 960 11100 2D0 720 01101 3D0 976 11101 2E0 736 01110 3E0 992 11110 2F0 752 01111 3F0 1008 11111 Figure 2 2 Base Address jumper block A4 to top A8 to bottom open 0 closed 1 Page 9 of 33 DM6854 User s Manual Interrupt channel JP10 Factory setting IRQ5 G The header connector shown on Figure 2 3 lets you connect the board interrupt to one of the 6 interrupt channels available on the XT AT bus The setting installs a pull down resistor on the selected interrupt The G jumper is used to share an IRQ line between multiple devices Figure 2 3 Interrupt Selection Jumpers Note There must be exactly one G jumper installed on any interrupt line The interrupt is caused by an external trigger event that has transferred the storage latch data to the output latch of the isolated digital output stage PD PU Pull up or Pull down resistor selection Factory setting PA PB and PC pulled Low The 8255 programmable digital I O interface provides 24 TTL CMOS compatible lines which can be interfaced with external devices These lines are divided into three groups eight Port A lines eight Port B lines and eight Port C lines You can connect pull up or pull down resistor networks for ports A B and C There is a separate jumper for the upper four lines of Port C and one for the lower four lines You may want to pull li
17. ARRANTIES FOR MECHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE LIMITED TO THE DURATION OF THIS WARRANTY IN THE EVENT THE PRODUCT IS NOT FREE FROM DEFECTS AS WARRANTED ABOVE THE PURCHASER S SOLE REMEDY SHALL BE REPAIR OR REPLACEMENT AS PROVIDED ABOVE UNDER NO CIRCUMSTANCES WILL RTD Embedded Technologies BE TO THE PURCHASER OR ANY USER FOR ANY DAMAGES INCLUDING ANY INCIDENTAL OR CONSEQUENTIAL DAMAGES EXPENSES LOST PROFITS LOST SAVINGS OR OTHER DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE PRODUCT SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF INCIDENTAL OR CONSEQUENTIAL DAMAGES FOR CONSUMER PRODUCTS AND SOME STATES DO NOT ALLOW LIMITATIONS ON HOW LONG AN IMPLIED WARRANTY LASTS SO THE ABOVE LIMITATIONS OR EXCLUSIONS MAY NOT APPLY TO YOU THIS WARRANTY GIVES YOU SPECIFIC LEGAL RIGHTS AND YOU MAY ALSO HAVE OTHER RIGHTS WHICH VARY FROM STATE TO STATE Page 33 of 33
18. DM6854 Isolated Digital I O dataModule User s Manual RTD Embedded Technologies Inc Real Time Devices Accessing the Analog World 1509001 and AS9100 Certified BDM 610010032 DM 66854 User s Manual RTD Embedded Technologies Inc 103 Innovation Blvd State College PA 16803 0906 Phone 1 814 234 8087 FAX 1 814 234 5218 E mail sales rtd com techsupport rtd com web site http www rtd com Copyright 1999 2010 RTD Embedded Technologies Inc All rights reserved The RTD Logo is a registered trademark of RTD Embedded Technologies cpuModule and dataModule are trademarks of RTD Embedded Technologies PC 104 is a registered trademark of PC 104 Consortium All other trademarks appearing in this document are the property of their respective owners Page 2 of 33 Rev A Rev B Manual Revision History New RTD USA Manual RTD ISA ID added Added AS9100 Information Updated board photo to current PCB revision Updated jumper diagrams to match Added information on CN9 Replaced PC 104 connector pinout with a link to PC 104 Consortium Corrected description of isolated input inversion Added Dimension Drawing DM6854 User s Manual Page 3 of 33 DM6854 User s Manual TABLE OF CONTENTS CHAPTER T JINTRODUGCTI ON ieieen reeeerec cv eset bo eaae o o doge to rro rose ceno se Ehe Verde atop io pd one len ee oae eap de de eR oe 6 FEATURES 5 or ERRARE ETRS 6 GENERAL PURPOS
19. E DIGITAL ini erre eed 6 ISOLATBED DIGITAT INPUTS idi te 6 ISOLATED DIGITAL ba sedie este ied bd edo 6 MECHANICAL DESCRIPTION rete pe e etre reete tov eee de tete veta bine es Eo bes Ul e e ERE US 6 CONNEGTOR DESCRIPTION Accra B eR B Isid 7 WHAT COMES WITH YOUR BOARD eec ede a voe rt eere e Re bod eere rir red ree ied ese erected ore e p e ern 7 BOARD AGCESSORIBS 7 USING THIS MANUAT z 3 5 errare We o E QUE eee EE 7 WHEN YOUNBED HEEP em o eei enin thon dnm nA nimii 7 CHAPTER2 CONFIGURING THE DATAMODULE eee ee ee ee eee toon eee ee tone eese 8 FACTORY CONFIGURED JUMPER SETTINGS ccsecsssccccecsesssececececeensaececececeesensecesececeesesssaeeeeececseaaeceeeeeeseseaaeaeeeeeeeenes 8 BASE ADDRESS eedem eto impetu tite evi etti tee 9 INTERRUPT CHANNEL JP IO ete prece eei cepe pip ede pce pee e e eee ede eed ta 10 PD PU PULL UP OR PULL DOWN RESISTOR SELECTION esee 10 IN1 IN8 OPTOCOUPLER INPUT RANGE ccccesssssccececeesesssaecececeesenseaececececsensaaececececsesaaececececseneaaeaecececsesenseaececeeeeees 11 CHAPTER3 INSTALLING THE DATAMODULE ee ee ee ee ee enne eee tone seta to sese seen
20. G THE editore ie ER S oet XR QR ERE 21 Clearing and setting bits in an I O port ess e de Re te Po a eo Eb en 22 Initializing the 8255 PPL x an tee eee d eet die d ge eed 22 Isolated digital input programming eese eene enne tene teee trente entren eret enne enne enne nnne 22 Software controlled direct fead eee de eee e eei ei pte ene 22 Interrupt driven read upon change in bits sess en rennen tette trennen nennen 23 Isolated digital output programming eese eene enne enne tne tnee tene teen trennen eret ene enne tenen 23 TiS SEF OULPUL PTOSTAMIMING EE 23 cet e a eate ue Tren RE aha d SE ee 23 Whbatais andnterrupt eerte eant eee ettet dee ete inti teed eb e DAs reer 23 Interr pt request Ines ete e re e ie pese ei ibis pee Wee eene epe ep e nb ee pere es 24 8259 Programmable Interrupt Controller sees nennen nennen rennen ener 24 Interrupt Mask Registe IMR a he lere yere n erede Poe oe prec Ee o Pee eua det 24 End of Interrupt EOI Command e ne nennen netten en netten entente enne 24 What exactly happens when an interrupt occurs ce eeseceeececseceeeeeeseceeeecsaeceeeeeaseceneeeaaeceeeeeeaeeceeeeeenaecnees 24 Using Interrupt in your Prograim itte ere e HIR Rire ba Eee re 25
21. PTION PC 104 BUS This chapter describes the major features of the DM6854HR the 8255 based digital I O isolated optocoupler inputs isolated optocoupler outputs interrupts and output triggering Figure 5 1 shows the general block diagram of the DM6854HR Figure 5 1 DM6854HR Block diagram Digital Programmable Peripheral Interface PPI The programmable peripheral interface PPI is used for general purpose digital I O functions This high performance TTL CMOS compatible chip has 24 digital I O lines divided into two groups of 12 lines each e Group A Port A 8 lines and Port C upper 4 lines e Group B Port B 8 lines and Port C lower 4 lines Port A Port B and Port C are available at the 50 pin expansion connector You can use Ports A B and C in one of these three operating modes Mode 0 Basic I O Lets you use simple input output functions for a port Data is written to or read from the specified port Mode 1 Strobed Input Output Lets you transfer data I O from port A in conjunction with strobed or handshake signals Page 15 of 33 DM6854 User s Manual Mode 2 Strobed bi directional input output Lets you communicate with an external device through port A Handshaking is similar to mode 1 Available Port direction definitions A may be Inputs or Outputs e C lower bits may be Inputs or Outputs e C higher bits may be Inputs or Outputs these modes are discussed in detail in
22. able to point to our funtions ie setup for ISR s esds susnsuseBuensusesuensueuht on eesuesuse s eestenientdensian dens void init 1 handlers void 1 disableO old IRQ1 handler dos getvect IRQ1 VECTOR 8 dos setvect IRQ1 VECTOR 8 new IRQ1 handler Gi old mask inp 0x21 outp Ox21 Gi old mask amp 1 lt lt 1 enableO nde cen coder Function restore do this before exiting program Inputs Nothing Returns Nothing Purpose Restore interrupt vector table Page 27 of 33 DM6854 User s Manual void restore void 1 Restore the old vectors disableO dos setvect IRQ1 VECTOR 8 old IRQ1 handler outp 0Ox21 Gi old mask enableO Page 28 of 33 Chapter 7 SPECIFICATIONS General Specifications Interface Jumper selectable base address I O mapped Jumper selectable interrupts Digital I O non isolated Number of lines Logic compatibility Digital inputs isolated Number of lines Input ranges individually jumper selectable Digital outputs isolated Number of lines Output stage Connectors Isolated I O Non Isolated I O DM6854 User s Manual 24 TTL CMOS 8 5V 12V 24V 4 Open collector Header connector 48 Pin H 24 position screw terminal connector S Header connector 50 Pin
23. appens when an interrupt occurs Understanding the sequence of events when an interrupt is triggered is necessary to correctly write interrupt handlers When an interrupt request line is driven high by a peripheral device such as the DM5854 the interrupt controller checks to see if interrupts are enabled for that IRQ and then Page 24 of 33 DM6854 User s Manual checks to see if other interrupts are active or requested and determines which interrupt has priority The interrupt controller then interrupts the processor The current code segment CS instruction pointer IP and flags are pushed onto the system stack and a new set if CS and IP are loaded from the lowest 1024 bytes of memory This table is referred to as the interrupt vector table and each entry to this table is called an interrupt vector Once the new CS and IP are loaded from the interrupt vector table the processor starts to execute code from the new Code Segment CS and from the new Instruction Pointer IP When the interrupt routine is completed the old CS and IP are popped from the system stack and the program execution continues from the point it was interrupted Using Interrupt in your Program Adding interrupt support to your program is not as difficult as it may seem especially when programming under DOS The following discussion will cover programming under DOS Note that even the smallest mistake in your interrupt program may cause the computer to hang up and
24. correctly identified pin 1 of a connector before you connect to it and attempt to use the dataModule Isolated I O connections JP15 amp JP16 The pin assignments for the isolated I O are shown in Table 4 1 The DM6854 is available with either a terminal block connection or a 48 pin header connection In the 48 pin header configuration each column of the header is connected to the same signal i e pin 1 and 2 are both IN1I Pin 1 is to the left side of the connector when board is oriented as in Figure 2 1 ALL SIGNALS ARE ISOLATED Table 4 1 Isolated I O Pin Assignments Terminal Block Pin Header Pin Description 1 1 2 IN1 2 3 4 INI 3 5 6 IN2 4 7 8 IN2 5 9 10 IN3 6 11 12 IN3 7 13 14 IN4 8 15 16 IN4 9 17 18 IN54 10 19 20 IN5 11 21 22 IN6 12 23 24 IN6 13 25 26 IN7 14 27 28 IN7 15 29 30 IN8 16 31 32 IN8 17 33 34 OUT1_COLLECTOR 18 35 36 OUT1_EMITTER 19 37 38 OUT2_COLLECTOR 20 39 40 OUT2_EMITTER 21 41 42 OUT3_COLLECTOR 22 43 44 OUT3_EMITTER 23 45 46 OUT4_COLLECTOR 24 47 48 OUT4_EMITTER Page 13 of 33 Expansion Interface Connections JP14 DM6854 User s Manual Figure 2 4 shows the 50 pin expansion interface connector layout of the DM6854HR This connector is used to interface to non isolated 8255 based digital I O signals Table 4 2 50 pin expansion interface connector pinout
25. d if another interrupt is being processed it decides if the new request should supersede the one in progress or if it has to wait until the one in progress is done The priority level of the interrupt is determined by the number of the IRQ IRQO has the highest priority IRQ7 the lowest Many of the IRQ s are used by the standard system resources IRQO is dedicated for the internal timer 1 is dedicated to the keyboard input IRQ3 for serial port 2 and IRQA for serial port COMI Often interrupts 5 and 7 are free for the user 8259 Programmable Interrupt Controller The chip responsible for handling interrupt requests in a PC is the 8259 Interrupt Controller To use interrupts you will need to know how to read and set the 8259 s internal interrupt mask register IMR and how to send the end of interrupt EOI command to acknowledge the 8259 interrupt controller Interrupt Mask Register IMR Each bit in the interrupt mask register IMR contains the mask status of the interrupt line If a bit is set equal to 1 then the corresponding IRQ is masked and it will not generate an interrupt If a bit is cleared equal to 0 then the corresponding IRQ is not masked and it can generate an interrupt The interrupt mask register is programmed through port 21h End of Interrupt EOI Command After an interrupt service routine is complete the 8259 Interrupt Controller must be acknowledged by writing the value 20h to port 20h What exactly h
26. e 7 1 shows the connector locations The dimensions are specified in thousandths of an inch 3300 00 3250 00 3775 00 3576 DO 3475 DO 3325 00 3575 00 200 00 1050 00 3550 00 Figure 7 1 DM6854HR Dimension Drawing Page 31 of 33 DM6854 User s Manual Chapter 8 RETURN POLICY AND WARRANTY Return Policy If you wish to return a product to the factory for service please follow this procedure Read the Limited Warranty to familiarize yourself with our warranty policy Contact the factory for a Return Merchandise Authorization RMA number Please have the following available Complete board name Board serial number A detailed description of the board s behavior List the name of a contact person familiar with technical details of the problem or situation along with their phone and fax numbers address and e mail address if available List your shipping address Indicate the shipping method you would like used to return the product to you We will not ship by next day service without your pre approval Carefully package the product using proper anti static packaging Write the RMA number in large 1 letters on the outside of the package Return the package to RTD Embedded Technologies Inc 103 Innovation Blvd State College PA 16803 0906 USA Limited Warranty RTD Embedded Technologies Inc warrants the hardware and software products it manufactures and produces to be free
27. e interrupts temporarily disabled you can assign the interrupt vector to point to your ISR Again you can overwrite the appropriate entry in the vector table with a direct memory write but this is not recommended Instead use the DOS function 25h Set Interrupt Vector or if your compiler provides it the library routine for setting up interrupt vectors Remember that interrupt vector 8 corresponds to IRQO vector 9 for IRQI etc If you need to program the source of your interrupts do that next For example if you are using the input compare on the DM6854HR as an interrupt source program it to do that Finally clear the mask bit for your IRQ in the IMR This will enable your IRQ Common Interrupt mistakes e Remember hardware interrupts are from 8 15 are numbered 0 7 e Forgetting to clear the IRQ mask bit in IMR e Forgetting to send the EOI command after ISR code Disables further interrupts Example of Interrupt vector table setup in C code void far _interrupt new 1 handler void ISR function prototype define 1 VECTOR 3 Name for IRQ void Cinterrupt far old 1 dispatcher Ces ds di si bp sp bx dx cx ax ip cs flags Variable to store old Vector void far interrupt new 1 handler void ANC ne ied ed nd desta dnd dod ose Function init irq handlers Inputs Nothing Returns Nothing Purpose Set the pointers in the interrupt t
28. errupt controller Saving the Startup Interrupt Mask Register IMR and interrupt vector The next step after writing the ISR is to save the startup state of the interrupt mask register IMR and the original interrupt vector you are using The IMR is located in address 21h The interrupt vector you will be using is located in the interrupt vector table which is an array of 4 byte pointers addresses and it is locate din the first 1024 bytes of the memory Segment 0 offset 0 You can read this value directly but it is a better practice to use DOS function 35h get interrupt vector to do this Most C compilers have a special function available for doing this The vectors for the hardware interrupts on the XT bus are vectors 8 15 where IRQO uses vector 8 and IRQ7 uses vector 15 Thus if your DM6854HR is using IRQ5 it corresponds to vector number 13 Before you install your ISR temporarily mask out the IRQ you will be using This prevents the IRQ from requesting an interrupt while you are installing and initializing your ISR To mask the IRQ read the current IMR at I O port 21h and set the bit that corresponds to tout IRQ The IMR is arranged so that bit 0 is for IRQO and bit 7 is for IRQ7 See the paragraph entitled Interrupt Mask Register IMR earlier in this discussion for help in determining your IRQ s bit After setting Page 26 of 33 DM6854 User s Manual the bit write the new value to I O port 21h With the startup IMR saved and th
29. es This user s manual describes the operation of the DM5854HR DM6854HR Isolated Digital Interface board 8 channel by channel isolated digital inputs with masking modes 4 Optocoupled digital outputs with open collector configuration 30V range 24 non isolated TTL CMOS compatible 8255 based digital I O lines Optional Pull up or Pull down resistors Software enabled interrupts XT and AT interrupts available Single 45V operation Support for direct PC 104 interface with RTD dataModules XT DM5854HR and AT DM6854HR boards available PC 104 compliant RTD IDAN compatible The following paragraphs briefly describe the major features of the DM6854HR A more detailed discussion is included in Chapter 5 Hardware description and in Chapter 6 Board operation and programming The board setup is described in Chapter 2 Configuring the dataModule General purpose digital I O The DM6854HR board has 24 8255 based TTL CMOS compatible general purpose digital I O lines which can be directly interfaced with external devices or signals to interface to alphanumerical displays sense switch closures trigger digital events or activate solid state relays These lines may be pulled down or pulled up or pulled down with 10K Ohm resistor networks Installation procedures will be described later in this manual Isolated digital inputs Eight optocoupler inputs may be used to connect high voltage signals to a computer Eight channel by channel jumper conf
30. from defects in materials and workmanship for one year following the date of shipment from RTD Embedded Technologies INC This warranty is limited to the original pur chaser of product and is not transferable During the one year warranty period RTD Embedded Technologies will repair or replace at its option any defective products or parts at no additional charge provided that the product is returned shipping prepaid to RTD Embedded Technologies All replaced parts and products become the property of RTD Embedded Technologies Before returning any product for repair customers are required to contact the factory for an RMA number THIS LIMITED WARRANTY DOES NOT EXTEND TO ANY PRODUCTS WHICH HAVE BEEN DAMAGED AS A RESULT OF ACCIDENT MISUSE ABUSE such as use of incorrect input voltages improper or insufficient ventilation failure to follow the operating instructions that are provided by RTD Embedded Technologies acts of God or other contingencies beyond the control of RTD Embedded Technologies OR AS A RESULT OF SERVICE OR Page 32 of 33 DM6854 User s Manual MODIFICATION BY ANYONE OTHER THAN RTD Embedded Technologies EXCEPT AS EXPRESSLY SET FORTH ABOVE NO OTHER WARRANTIES ARE EXPRESSED OR IMPLIED INCLUDING BUT NOT LIMITED TO ANY IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE AND RTD Embedded Technologies EXPRESSLY DISCLAIMS ALL WARRANTIES NOT STATED HEREIN ALL IMPLIED WARRANTIES INCLUDING IMPLIED W
31. his will reduce false triggering Optocouplers Small SMD optocouplers are used to isolate each channel of the isolated outputs Individual optocouplers are used for each channel The optocouplers are directly connected to the Output data register The optocouplers are connected in a non inverting configuration in open collector configuration The maximum switching current is 8mA at 30V Trigger output Your DM6854HR is capable of generating a trigger signal caused by an input condition This feature is especially useful when monitoring for a predefined input condition When this predefined condition occurs an interrupt will indicate this condition to the host This trigger is also available as an external output signal on the 50 pin connector Pin 2 You may use this to trigger other devices or other boards in your system such as the isolated digital output board and DM6854HR The polarity of this trigger signal is programmable and is driven by a tristate buffer This enables several boards to drive the same trigger output signal Interrupts The DM6854HR has an interrupt available to indicate an external trigger This interrupt can be used to tell the host computer that the pre programmed input pattern condition has occurred Chapter 6 will provide programming information on interrupts Page 17 of 33 DM6854 User s Manual Chapter 6 BOARD OPERATION AND PROGRAMMING This chapter shows you how to program and use your DM6854HR It provides a
32. igurable input ranges are available 5V 12V and 24 The optocoupler inputs have a reverse voltage protection diode across the input This enables AC signals to be connected to the isolated optocoupler inputs This input diode also acts as a rectifier The programmable functions of the DM6854HR enable input masking and input pattern recognition with interrupts This relieves the host computer from processor capacity consuming input polling Isolated digital outputs Four open collector optocoupler outputs may be used to directly drive loads such as switches or relays with voltages up to 30V DC Mechanical description The DM6854HR is designed on a PC 104 form factor An easy mechanical interface to both PC 104 and PC 104 Plus systems can be achieved Stack your PC 104 compatible computer Page 6 of 33 DM6854 User s Manual directly on the DM6854HR using the onboard mounting holes A dimension drawing with connector locations is provided in Chapter 7 Connector description There is a 50 pin digital interface connectors on the DM6854HR to directly interface to the non isolated 8255 based digital I O signals The signal definition of this connector is compatible with the digital PC 104 dataModules manufactured by RTD Isolated outputs and inputs are connected to the DM6854HR by either a screw terminal discrete wire connector or with a 50 pin flat ribbon cable header connector Use this type of interface connector with a TB50 screw te
33. ision Revision LSD Revision 5 EPLD Revision Revision MSD 6 9 Reserved Ignore Ignore 10 Board Name String D DM 11 Board Name String M 12 Board Name String 6 68 13 Board Name String 8 14 Board Name String 5 54 15 Board Name String 4 16 Board Name String nul lt nul gt lt nul gt 17 Board Name String lt nul gt m 18 255 Unused FFh FFFFh Programming the DM6854HR This section gives you some general information about programming the DM6854HR board and then walks you through the major programming functions of the board These descriptions will help you as you use the example programs and function libraries included with this board of the program descriptions use decimal values unless otherwise specified The DM6854HR is programmed by writing to and reading from the correct I O port addresses of the board These I O ports were described in the previous section of this chapter The following example shows how to perform a 8 bit read and write I O port addresses using C syntax and assembly code Table 6 4 Code Examples Read Write Page 21 of 33 DM6854 User s Manual C Syntax var inp address outp address data Assembly mov dx address mov dx address in ax dx mov ax data out dx ax Clearing and setting bits in an I O port When you clear or set bits in an I O port you must be careful not to alter the status of other bits You can preserve the status
34. les you to verify you have entered the ISR and executed it successfully For example set a flag in your ISR and in your main program check for the flag Note If you choose to write your ISR in in line Assembly you must push and pop registers correctly and exit the routine with the IRET instruction instead of the RET instruction There are a few precautions you must consider when writing ISR s The most important is do not use any DOS functions or functions that call DOS functions from an interrupt routine DOS is not reentrant that is DOS function cannot call itself In typical programming this will not happen because of the way DOS is written But what about using interrupts Then you could have the situation such as this in your program If DOS function X is being executed when an interrupt occurs and the interrupt routine makes a call to the DOS function X then function X is essentially being called while active Such cases will cause the computer to crash DOS does not support such operation general rule is that do not call any functions that use the screen read keyboard input and any file I O routines should not be used in ISR s The same problem of reentrancy exists for many floating point emulators as well meaning you should avoid floating point mathematical operations in your ISR Page 25 of 33 DM6854 User s Manual Note that the problem of reentrancy exists no matter what programming language you use Even if yo
35. nes up for connection with switches This will pull the lines high if the switch is disconnected Or you may want to pull lines down for connection to relays which control turning motors on or off The port A B and C lines of the 8255 programmable digital I O interface are set as inputs after reset This can cause the external devices connected to these lines to operate erratically Pulling these lines down when the board is powered up the lines will not be active before the 8255 is initialized Figure 2 4 shows both Ports A B and C set in the Pull down configuration Figure 2 4 Pull up and Pull down jumpers for Ports A B and C Page 10 of 33 DM 6854 User s Manual IN1 IN8 Optocoupler Input Range Factory setting 24V range The Optocoupler inputs can be jumper configured for different input ranges Each channel can be set for a different range The different ranges use different current limiting resistors at the optocoupler diode input Make sure you set the range correctly since overloading the optocoupler with over current may cause permanent damage to the device the channels are set in a similar manner Figure 2 5 illustrates one channel set for a 24V level input signal The jumper topmost toward the spring loaded connectors selects 24V input range the middle position 12V and the bottom most toward the center of the board 5 The optocoupler forward current is 0 13 mA for switching Figure 2 5 Optocoupler Input
36. rminal block Please consult the factory for more details on different connector options What comes with your board You receive the following items in your DM6854HR package DM6854HR Isolated digital interface module Companion CD Notes Latest software and drivers can be downloaded from our website If any item is missing or damaged please contact RTD Embedded Technologies Board accessories Several software and hardware accessories are available for the DM6854 Call your distributor for more information on these accessories and for help in choosing the best items to support your control system e Application software and drivers e Hardware accessories such as IDAN aluminum enclosure systems For more information in IDAN please visit our websites at www rtd com e Power Supplies e Terminal Adapter Board TB50 Using this manual This manual is intended to help you install your new DM6854HR card and get it running quickly while also providing enough detail about the board and its functions so that you can enjoy maximum use of its features even in the most demanding applications When you need help This manual and all the example programs will provide you with enough information to fully utilize all the features on this board If you have any problems installing or using this board contact our Technical Support Department 814 234 8087 during EST business hours or send an Email to techsupport rtd com When sending
37. tal inputs Read The eight isolated digital input bits available in connector J4 can be read from address BA 0 BA 1 Input mask register Read Write 0 after reset The input mask register selects which bits are monitored for a trigger condition A 1 corresponds to a selected bit Example outp BA 1 Ox7F Bits 1 7 are selected Page 18 of 33 DM6854 User s Manual BA 2 Input inversion register Read Write 0 after reset The input inversion register may be used to invert input channels selected by the input mask register If a logical 0 on the input bit is should cause a trigger the respective inversion bit should be set to 1 Example Bits 1 7 are monitored for a condition where any bit changes state from condition Ox3E Step 1 Mask required bits outp BA 1 0x7F 0111 1111 Step 2 Set inversion register outp BA 2 0x3E 0010 1110 BA 3 Configuration digital output register Read Write 0 after reset BIT 0 1 gt External host IRQ enabled 0 gt External host IRQ disabled BIT 1 1 gt External trigger output 50 pin header pin 2 tristate buffer enabled 0 gt External trigger output tristate buffer in tristate externally pulled low BIT 2 1 gt External trigger output active low 0 gt External trigger output active high BIT 3 Interrupt trigger active Read only 0 gt Interrupt trigger not active Read only BITS 4 7 4 bit isolated digital output port BA 4 PPI Port A Read Write
38. the 8255 datasheet from Intel Isolated optocoupler inputs The Isolated input stage of the DM6854HR consists of 4 major parts Input protection Input optocouplers Input bit mask stage Input bit inverter stage Input Protection The input circuitry of the DM6854HR consists of a range selection jumper field and an input protection Schottky diode The optocoupler is current driven so the input range is selected by connecting different current limiting resistors in series with the optocoupler input diode To protect the optocoupler input diode from over current and reverse voltage a diode is connected in reverse with the optocoupler input diode Make sure you do not exceed the absolute maximum voltage of each nominal input range Exceeding this limit may cause permanent damage to the optocouplers Optocouplers The SMD optocouplers provide the galvanic isolation between the computer and the process signals as well as isolation between input channels The optocouplers are logically inverting i e a voltage on the isolated digital input will be interpreted as a logical 0 To avoid false triggering on the inputs special Schmidt trigger buffers are used to provide reliable and precise switching of the inputs Input bit mask stage The programmable input processing section of the DM6854HR includes an input bit mask filter stage This mask may be is used to select bits or groups of interesting bits for pattern recognition Bit selection is
39. u are writing your ISR in Assembly language DOS and many floating point emulators are not reentrant Of course there are ways to avoid this problem such as those which involve checking if any DOS functions are currently active when your ISR is called but such solutions are beyond the scope of this manual The second major concern when writing ISR s is to make them as short as possible in term of execution time Spending long times in interrupt service routines may mean that other important interrupts are not serviced Also if you spend too long in your ISR it may be called again before you have exited This will lead to your computer hanging up and will require a reboot Your ISR should have the following structure Push any processor registers used in your ISR Most C compiler do this automatically Put the body of your routine here Clear the interrupt bit by writing to address 3 Issue the EOI command to the 8259 by writing 20h to address 20h Pop all registers Most C compilers do this automatically The following C example shows what the shell of your ISR should be like po Function new IRQ handler Inputs Nothing Returns Nothing Sets the interrupt flag for the EVENT void interrupt far new_IRQ_handler void IRQ_flag 1 Indicate to main process interrupt has occurred Your program code should be here outp BA 0 0x00 Clears interrupt on DM6854HR H 0x20 Acknowledge the int
40. ual An interrupt is an event that causes the processor in your computer to temporarily halt its current process and execute another routine Upon completion of the new routine control is returned to the original routine at the point where its execution was interrupted Interrupts are a very flexible way of dealing with asynchronous events Keyboard activity is a good example your computer cannot predict when you might press a key and it would be a waste of processor time to do nothing while waiting for a keystroke to occur Thus the interrupt scheme is used and the processor proceeds with other tasks Then when a keystroke occurs the keyboard interrupts the processor and the processor gets the keyboard data placed it into memory and then returns to what it was doing before the interrupt occurred Other common devices that use interrupts are network boards A D boards serial ports etc Your DM6854HR can interrupt the main processor when an external trigger event occurs if interrupts are enabled on the board By using interrupts you can write powerful code to deal with real world events Interrupt request lines To allow different peripheral devices to generate interrupts on the same computer the PC XT bus has eight interrupt request channels IRQ s A rising edge transition on one of these lines will be latched into the interrupt controller The interrupt controller checks to see if the interrupts are to be acknowledged from that IRQ an
41. will only restart after a reboot This can be frustrating and time consuming Writing an Interrupt Service Routine ISR The first step in adding interrupts to your software is to write an interrupt service routine ISR This is the routine that will be executed automatically each time an interrupt request occurs for the specified IRQ An ISR is different from other subroutines or procedures First on entrance the processor registers must be pushed onto the stack before anything else Second just before exiting the routine you must clear the interrupt on the DM6854HR by writing to address BA 0 and write the EOI command to the interrupt controller Finally when exiting the interrupt routine the processor registers must be popped from the system stack and you must execute the IRET assembly instruction This instruction pops the CS IP and processor flags from the system stack These were pushed onto the stack when entering the ISR Most compilers allow you to identify a function as an interrupt type and will automatically add these instructions to your ISR with one exception most compilers do not automatically add the EOI command to the function you must do it yourself Other than this and a few exceptions discussed below you can write your ISR as any code routine It can call other functions and procedures in your program and it can access global data If you are writing your first ISR we recommend you stick to the basics just something that enab
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