LM3 – Programming the A/D converter of the MC9S12DP256B/C
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1. icr i 0 Led Qx00 i Data amp Ox3FF i x3FF 8 axxwnbie LED bit pattern led 1ed 1 0x01 a cet LED orray 10 new value e OGTE char led iso SESE RBALSE MSO ue coss Ejere eoo liie 0 BASCMRMNA OO ss Figure LM3 3 Adding files adc c and adc h to project group Sources Compile your code and download it to the Dragon12 board target Monitor Step through your code to verify its proper operation Click on the Start Continue button green arrow to run the program at full speed Turn the potentiometer and observe that the converted value is instantly displayed on the LEDs Click on Stop to halt execution The debugger might complain about the loss of communication Should this be the case simply reset the board close down the debugger and start it again Exit from the debugger Set the mode switch SW7 to RUN the mode indicator LED should change from EVB to EEPROM Operate the RESET button of the Dragon12 board The program should now run uninterrupted ADC interrupt In this section we will modify our ADC program to trigger an interrupt every time the ADC has finished converting a value This way we can keep the ADC running in the background and use an interrupt service routine ISR to store the result in a global variable The main program will read this variable and display its value Close down the CodeWarrior IDE Open the Windows File Explorer and go to the folder in2. ANO7 and displays the result of the conversion on the LEDs connected to port B Channel ANO7 has been connected to the on board potentiometer 0 V 5 V The display should be implemented as inear level gauge At the minimum conversion value 0 all LEDs should be off at the maximum conversion value Ox3FF 1023 all LEDs should be on in between number of LEDs lit is proportional to converted value Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C Hints 1 Begin with creating a new project based on stationery Dragon12_flat Save this empty project under the name myADC mcp 2 This exercise will make use of 3 special function registers A control register select channel configure ADC a status register test for end of conversion don t use interrupts yet and a data register conversion result 3 It is good practice to write a separate source code file for each hardware unit you use e g one for the ADC one for serial communication one for the timer etc Functions which are defined in any of these source files can be made available to the main module via an associated header file see Figure LM3 2 It is recommended that you create the following pair of files adc c contains the definition of all ADC related functions e g ADC_Init ADC_Read the corresponding header file adc h declares these functions by listing their function prototypes You will have to modify your main program main c to include t
3. Where would you place it Once you have modified your code re compile and download to the Dragon12 board Run your code at full speed Start Continue green arrow and verify that the LEDs properly display the acquired value Concluding remark The CodeWarrior simulator seems to struggle with the simulation of interrupts You may have noticed that once an interrupt has been triggered once we immediately re enter the ISR as soon as we get to its end This strange behaviour might have to do with the way we have configured the simulator Possibly we have stumbled across a bug in the program Fortunately the debugger seems to work when used in conjunction with the monitor program on the Dragon12 You should therefore always prefer the actual Dragon12 board over its simulation at least when working with interrupts 10 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C Summary Programming interrupts 1 Create an interrupt service routine ISR This is a sub routine of type _ interrupt void ISR name void 2 Install this ISR in the interrupt vector table This can be achieved by including an explicit interrupt vector table in your project as done here Alternatively you can include an interrupt vector number in the definition of your ISR This causes the compiler to take care of the installation of the interrupt vector Example _ interrupt 0x16 void ISR name void 3 Allow all interrupts to happe
4. soo zns cmt sim_pesticed cms Bcstlcad crzmatd file cocrectiy executed Breakpoint pA wet real Loans quete ADO TER cem ateos conversion twault in ACC Data ani restart comrersicnh sverrupo voMADC IS81v0 9 B return velue im qgintal variable A2C Data e MAII Data ATZCCRO AIDXCILS e Qk374 mert sequence shansel RO result signt sligned e22248282 3335035355 jaj BB RS lt I gt wlsttle CURASSIR Deve Q maipej tor B PORTAS o gt volatile PORIABSIR SEBRSSESSSSZA8S e 8 2289ESESSSEXSQSAS 538333338538353 SSBSSSLS288882 eos 528 BOERS T mone omi sim_posticed cms wmaelizes unsigned iat tec zr aliscacet utzignad char Pcwllcad command file correctiy execcted Ereeypotnt in Figure LM3 7 Placing a breakpoint inside ADC_ISR 9 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C Run the program by clicking onto toolbar button Start Continue green arrow Observe that the CPU is running CPU cycles are counting up in Register window However the program doesn t seem to do much the breakpoint inside ADC SR is never reached What could be the reason for this Halt the CPU red Halt button of the toolbar Source file main c reappears with the program stuck somewhere within the display Re open source file adc c and scroll down to our interrupt service routine ADC SR Notice the second line of this ISR This re starts the ADC after the previous result ha
5. the microcontroller This ensures that conversions always begin in synchrony with a CPU cycle 1 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C Study the ADC user manual S12ATD10B8CV2 pdf to find the nominal conversion time of a single 10 bit A D conversion hint you can Edit Search for keywords such as frequency conversion time sample time etc What is the maximum ATD clock frequency What is the default ATD clock frequency reset value How long would you expect a 10 bit conversion to last for when the ADC unit is driven at the default ATD clock frequency In which register can you expect to find the conversion result What register s is are used to configure the ADC for a single conversion How can you check if a conversion is still on going or has finished Note The bus clock of the MC9S12DP256B C is controlled by a built in Phase Locked Loop circuit PLL to a constant 24 MHz ATD_10B8C Bus Clock Clock ATD clock Conversion Complete Interr Mode and Timing Control VRHE E Successive m K Approximation Register SAR VDDA I and DAC AN7 PAD7 X AN6 PAD6 R AN5 PAD5 K AN4 PADA R AN3 PAD3 AN2 PAD2B Comparator AN1 PAD1E ANO PADO R Port AD Data Register Figure LM3 1 ADC unit of the MC9S12DP256B C Now that you have familiarised yourself a little with the function of the ADC unit you should be able to write a small program that reads an analogue voltage from channel
6. E 0 External Trigger Level Edge Control bit 3 ETRIGP 0 External Trigger Polarity bit 2 ETRIGE 0 External Trigger Mode Enable bit 1 ASCIE 0 ATD Sequence Complete Interrupt Enable bit 0 ASCIF 0 ATD Sequence Complete Interrupt Flag ATDOCTL4 0x05 configure conversion bit 7 SRES8 0 A D Resolution Select 1 2 8 bit resolution 0 10 bit resolution bit 6 SMP1 0 Sample Time Select bit 5 SMP0 0 2 clock period bit 4 PRS4 0 ATD Clock Prescaler divide by 12 bit 3 PRS3 0 ATD Clock Prescaler bit 2 PRS2 1 ATD Clock Prescaler bit 1 PRS1 0 ATD Clock Prescaler bit 0 PRSO 1 ATD Clock Prescaler 5 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C To instruct the ADC to trigger an end of conversion interrupt we will have to set the ASCIE flag in ATDOCTL2 ATD Sequence Complete Interrupt Enable Make the required code changes to enable interrupts Find the end of conversion function ADC_Read In its current form ADC_Read first starts a new conversion sequence by writing the channel number to ATDOCTLS It then waits for the conversion to be complete Conversion Complete Flag CCFO in ATDOSTAT 1 goes high before returning the 10 bit result from data register ATDODRO VXXXX ADC Read Ck ckckckckck ckck ckck ckck kk perform 10 bit analog to digital conversion input chan is 0 to 7 specifying analog channel to sample output 10 bit ADC sample left justified analog input
7. Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C Objectives To write a small application for the Motorola MC9S12DP256B C microcontroller that reads the A D converter ADC and displays the converted value on the on board LEDs of the Dragon12 development board To learn about interrupts and the use of interrupt service routines ISR Introduction Digital signal processing DSP and digital control both rely on the timely evaluation of the system equations e g the difference equation of a filter or any other transfer function A typical cycle consists of 1 Reading the ADC 2 Evaluation of the system equations 3 Writing the result s to the DAC The implement of such a cycle on a microcontroller or a digital signal processor requires some form of timing engine This can be achieved in a variety of ways Advanced systems often employ the services of a real time operating system RTOS which usually allows for multi tasking and the scheduling of tasks at predefined sample rates Real time operating systems are particularly useful when working with multi rate control systems However many digital control systems are designed for a single sample rate In this case a simple timer interrupt service routine ISR can be used to provide the required time base With this laboratory session we begin to investigate how this can be done We will write a number of small test applications to learn about A D converters interrupt
8. he new header file File ade c read ADC channel chan unsigned int ADC Read unsigned int chan G main void main void initialize ADC void ADC Init void s extern void ADC Init void Figure LM3 2 Separating main program and user functions Having created ADC c and ADC h you will have to add both files to the Sources group of your CodeWarrior project This can be done by right hand mouse clicking onto Sources and selecting Add Files Upon adding the source files your project browser should look as shown in Figure LM3 3 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C Metrowerks ade Warm f e Yew Zee rows Zeb Plcanorfacet Yircew en nzn smgHovcc 5tanzajgmoot maun cs Example program for thm Vytec Dragon 12 MCS512D6258C e CEZ jBvot n Fies ju oe Tagai include mr3msi2dp256 h gt derivative information Jinclude pll h e defines BYSCUME seto buo frequency to 3include h ADC Init ADC Reed void wmeinivoid unsigned int Deta i mignaed cher led eet ayeter clock irwquancy to _BUSCIOCK MHz 24 oz 4 PIL_Init gt set port B as output LETS DORS xii Pert B iz output ADC a ai ad a a a oa 5 a ba a a activat ADC Init foresvey tort t reed ADC Data ADC Beadi x97 right justified unsigned channel ANo detersine IED bit pattern
9. left justified right justified 0 000 0 0 0 005 0040 Ji 0 010 0080 2 1 250 4000 100 2 500 8000 200 5 000 FFCO 3FF uses busy wait synchronization bit 7 DJM Result Register Data Justification 1l right justified O left justified bit 6 DSGN Result Register Data Signed or Unsigned Representation 1 signed 0 unsigned bit 5 SCAN Continuous Conversion Sequence Mode 1 continuous 0 single bit 4 MULT Multi Channel Sample Mode 12multiple channel O single channel bit 3 0 bit 2 0 CC CB CA channel number 0 to 7 unsigned short ADC Read unsigned short chan ATDOCTL5 unsigned char chan start sequence while ATDOSTAT1 amp 0x01 0 wait for CCFO return ATDODRO Waiting for the ADC to finish a conversion seems a waste of precious CPU time Bear in mind that the microcontroller is supposed to evaluate a filter equation or a transfer function and feed the result to an external DAC in the shortest possible time So the smaller the number of avoidable delays we built into this cycle ADC evaluation gt DAC the better The ADC delay can be avoided using the end of conversion interrupt Using this technique the ADC unit can work in parallel with the CPU only interrupting the main program for a very short time in order to transfer the conversion result from the ADC data register ATDODRO to memory This takes significantly less time than waiting for an entire conver
10. m the body of the ISR Furthermore we no longer return the conversion result to a calling function remember an ISR does not get called explicitly Instead the contents of the data register ATDODRO will have to be moved to a global variable The reason why this variable has to be global is simple We are accessing this variable from within 2 7 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C different functions Values are written to it in ADC_ SR while they are read from it in main Defining the result variable in either one of these two functions would hide it from the other one This data exchange via global variables is a general technique which is used with all interrupt service routines you may ever come across Complete the transition from ADC_Read to ADC_ISR by modifying your code in accordance to the explanations given above Your ISR might end up looking somewhat like this RRKKKKKK ADC ISR Ckckckckck ck ck ckck ck k kk store conversion result in ADC Data and restart conversion _ interrupt void ADC ISR void return value in global variable ADC Data ADC Data ATDODRO ATDOCTL5 0x87 start sequence channel ANO7 result right aligned Note As mentioned before it is good programming style to define the global variable say unsigned int ADC_Data in source file adc c This maintains the strict separation of ADC related functions from general purpose functions However s
11. n by clearing the CCR Interrupt disable flag A simple way to achieve this is by using the inline assembler instruction cli clear interrupt disable flag In the next section we will set up a short timer program Everything we ve learned about interrupts will be used in this program 11
12. s and how these interrupts can be serviced All programs will be simulated followed by a run on the Dragon12 development board Note During this exercise you will frequently have to find information from sources such as the user manual of the MC9S12DP256B C or the data sheet of the D A converter It may be a good idea to open this user guide now Accessing the A D converter unit of the MC9S12DP256B C Figure LM3 1 shows a block diagram of the 8 10 bit A D converter unit ATD_10B8C of the MC9S12DP256B C Central to this unit are the sample and hold amplifier SHA and the successive approximation register SAR The unipolar analogue voltage to be converted Vin is selected by the multiplexer from pins ANO AN7 Vin must fall within the range from 0 V Vssa S Vin VppA 5 V The digital value of the successive approximation register is converted to an analogue voltage Vpac This D A conversion is based on a reference voltage range from 0 V Vg to Vay 5 V The voltage Vpac is then compared to the output voltage of the sample and hold amplifier Vsar Depending on the result of this comparison the value within the successive approximation register is doubled Vpac lt Vsar or halved Vpac gt Vsar Each comparison decides the state of one bit of the SAR A total number of 10 comparisons are required to convert an analogue voltage with to an overall accuracy of 10 bit Note that the ADC is synchronized with the bus clock of
13. s been stored in ADC Data return value in global variable ADC Data ADC Data ATDODRO ATDOCTL5 Ox87 start sequence channel ANO7 result right aligned Does this explain to you why this ISR is never triggered Exit from the debugger and return to the CodeWarrior IDE Modify function ADC nit to ensure the ADC is actually running Re compile and re start the debugger simulator Place a breakpoint inside ADC_ISR as before and run the code What do you observe Does the code behave as expected now It looks like we still haven t resolved the problem The interrupt service routine remains inactive Click on the Halt button to stop code execution Click on Reset then Start Continue The simulator should run through the code until it reaches the breakpoint at the beginning of main Locate the Condition Code Register CCR of the CPU What seems to be the state of the nterrupt disable flag Double click onto this flag I to toggle its current state then continue running the code green arrow Does this resolve the problem Exit from the debugger and modify your source code to allow interrupts to happen To alter the state of the Interrupt disable flag within the Condition Code Register of the CPU you can use the following inline assembler directives asm cli clears the interrupt disable flag asm sei sets the interrupt disable flag Which one of these instructions would you need to add to your code
14. s that isr vectors c has to be extended to include an external declaration of ADC_ISR extern void interrupt ADC ISR void It is good programming practice to place this declaration in header file adc h and include this file in isr_vectors c That way all ADC related functions can be found in adc c and or adc h an advantage you will learn to acknowledge as soon as a project comprises of several 10s of source files In summary your project should be structured somewhat like this now Figure LM3 5 File adc c File main c read ADC channel chan interrupt void ADC Read void initialize ADC void ADC Init void o ee File sdeh o declare ADC Read t extern __interrupt void ADC Read void CER e ADC Init d o o Yy Figure LM3 5 Logical project structure To complete the conversion from a regular sub routine to an interrupt service routine we will have to modify the way data is transferred from the ADC data register to memory Previously ADC_Read took care of this by polling the conversion complete flag CCFO see code fragment above In our new approach we have automated this process We no longer have to check for an end of conversion flag our interrupt service routine ADC_ISR is invoked automatically whenever a value has been converted We therefore can and have to remove the test for CCFO fro
15. sion to be finished We now have to turn function ADC_Read into an interrupt service routine ISR and install its starting address in the interrupt vector table An ISR differs from a regular sub routine only by the additional declarative keyword __interrupt and the fact that there are no call up parameters and no return values Change the definition of sub routine ADC_Read to the following ISR RRKKKKKK ADC ISR Ckckckckck ck ck ckck ck kc ko kk store conversion result in ADC Data and restart conversion interrupt void ADC ISR void 521 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C From the user manual of the MC9S12DP256B C find the vector number of the end of conversion interrupt of the ADC unit ATDO see chapter 5 of the user manual We will now have to install our ISR in the vector table of our project source file isr_vectors c Open this file and replace the placeholder vector Unimplemented iSR of the ATDO interrupt by the name of our interrupt service routine ADC_ SR These two modifications turn a regular sub routine into an interrupt service routine However if we were to compile this program right now we would get a compiler error message claiming an unresolved external reference to ADC_ISR This is because our ISR has been defined in source file adc c whereas we use it in isr_vectors c To avoid this error we will have to reassure the compiler that ADC ISR is indeed defined elsewhere This mean
16. ticking ADC_Data in adc c implies that we also have to extend the associated header file adc h by an external declaration of this variable That way the compiler won t freak out when it tries to compile main c in which we use ADC_Data without explicitly defining it Including header file adc h now reassures the compiler that ADC_Data is defined elsewhere It would also be good to initialise this variable to zero in function ADC_Init Confused Well have a look on myUni at Mechatronics IIIM Course Documents Tutorials gt 9812 adc interrupt Compile your program and load it into the debugger simulator target Simulator Step over the first few instruction until you come to the display of the converted value What do you observe Does the program behave as expected Open file adc c by right hand mouse clicking inside the Source window of the debugger and selecting Open Source File Figure LM3 6 Place a breakpoint onto the first line of the interrupt service routine ADC SR This can be done from the Debug menu or by right hand mouse clicking onto the first line of ADC ISR and selecting Set Breakpoint Figure LM3 7 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C include cospslsaesit h gt dutivetive izfCrwation e neluse pll h defines SOKLI secs bus frequency to X ADC Init AUC Read seesseeeese2 SSeS eS0505H4505 cuoc 555 ec oco 535555 22998ES8 5282528252 gt 5
17. which you stored your ADC project Duplicate copy the entire ADC project folder myADC and rename the copied folder from Copy of myADC to ADC interrupt Figure LM3 4 For consistency you may also want to change the project file name mcp of the newly created project i e the one in folder myADC interrupt from myADC mcp to myADC interrupt mcp This is not really required but it may help keeping things organised Finally delete the sub folder myADC Data of the new 4 Mechatronics IIIM LM3 The A D converter of the MC9S12DP256B C project i e myADC_interrupt myADC_Data The new project is now ready to be worked on BP o aneas aed Settings Awornde My Documents iak Microcontwaller 9S 12002560 ia KAC E ales xi fe p dew fecrm Dem bb r Q gt 4 z p E Foes gt x o IH irem 5 Ceconem 1 terres rie Mv Docurents aK Jgs t fed s s zPEM JEmio 2 comm BING pet Fresno SB Prog aneers img EE rE TES 11309 Figure LM3 3 Duplicating a project Launch CodeWarrior by double clicking onto the project file myADC interrupt mcp Open the source file of the A D converter adc c and find the initialisation function ADC Init In its present form ADC_ Init consists of 2 instructions ATDOCTL2 0x80 enable ADC bit 7 ADPU 1 enable bit 6 AFFC 0 ATD Fast Flag Clear All bit 5 AWAI 0 ATD Power Down in Wait Mode bit 4 ETRIGL
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