AN3468, The MMA745xL Digital Accelerometer
Contents
1. INT2 register INT1 and INT2 pins E dby Mod INT2 reg Pulse INTipin DRDY C Measurement Mode INT2 reg Level INT1pin INT ireg INT2pin INT2reg C Level Detection Mode C INT1reg Single Pulse INT2 reg Pulse C INT1pin INT2reg INT2pin INT ireg Pulse Detection Mode LEVEL DETECTION MODE PULSE DETECTION MODE MEASUREMENT MODE absolute OR OR gt Pulse Duration 19ms Latency Time Oms Cg Xa of pos GND 2 C AND lt 4 Ycal g Wc Level Threshold 101 Pulse Threshold 40 gg Zcal 0 2nd Pulse Oms TKT write R EXYZOutpuis wee errem mere 1 X8 14 X10 64 Flags Y8 13 Y10 55 Disable x Level Detect Condition Xlevel z8 6 Z10 24 DisableY Level Detect not selected Ypulse __ T Diseble Z Zpulse Figure 10 Pulse Mode Settings The following are all the available settings that can be used in the pulse detection mode For the OR logic used for single or double pulse detection the following register should be set with PDPL 0 SS 9 regs 0x19 amp 0xFD For the AND logic used for pulse freefall detection the following register should be set with PDPL 1 RegisterWrite 0x19 regs 0x19 0x02 19 CTL2 DRVO PDPL LDPL 19 mask 0 0 0 0 0 0 1 0 19 result DRVO 1 LDPL This is an example of how to toggle th2. 18 mask 1 0 1 1 1 1 1 1 18 result DFBW 0 ZDA YDA XDA INTRG 1 INTRGI0 INTPIN For motion detection there is a positive negative detection condition that can be used RegisterWrite 0x18 regs 0x18 0x40 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 0 1 0 0 0 0 0 0 18 result DFBW 1 ZDA YDA XDA INTRG 1 INTRG 0 INTPIN AN3468 Sensors Freescale Semiconductor 18 For motion detection only there is an or logic condition that can be used LDPL 0 RegisterWrite 0x19 regs 0x19 amp 0xFE 19 CTL2 DRVO PDPL LDPL 19 mask 1 1 1 1 1 1 1 0 19 Result DRVO PDPL 0 For freefall detection only there is an and logic condition that can be used LDPL 1 RegisterWrite1 0x19 regs 0x19 0x01 19 CTL2 ka as E DRVO PDPL LDPL 19 mask 0 0 0 0 0 0 0 1 19 result DRVO PDPL 1 This is an example of how to toggle the XDA bit to enable or disable the X axis RegisterWrite1 0x18 regs 0x18 0x08 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 0 0 0 0 1 0 0 0 18 result DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN This is an example of how to toggle the YDA bit to enable or disable the Y axis RegisterWrite1 0x18 regs 0x18 0x10 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 0 0 0 1 0 0
3. The 10 bit data is available by reading X 00 01 Y 02 03 and Z 04 05 In 2 g mode and 4 g mode the ten bit data will match the 8 bit data In 8 g mode the 10 bit data is going to be 4 times that of the 8 bit data Measurement Mode RegisterWrite 0x16 0x01 Dynamic Range Settings In Measurement Mode 2g RegisterWrite 0x16 0x05 4g RegisterWrite 0x16 0x09 8g RegisterWrite 0x16 0x01 Level Detection Mode RegisterWrite 0x16 0x02 Note Dynamic range setting is automatically 8 g Pulse Detection Mode RegisterWrite 0x16 0x03 Note Dynamic range setting is automatically 8 g CALIBRATION OF THE MMA745XL The offset can be calibrated by storing the offset values in the designated offset drift registers 10 to 15 in the accelerometer These values will be stored until the part loses power and therefore it is a good idea to store these values in the memory of a microcontroller used in conjunction with the sensor This will provide automatic calibration of the sensor each time the sensor is turned back on In order to calibrate the MMA745xL Og offset the predetermined digital offset values should be subtracted from the reading of the actual digital sensing values The following procedure is a recommendation for how this can be accomplished Step 1 After power up set up the Mode Control Register Register 16 to be in measurement mode by writing 05 into Register 16 Then read the X Y and Z offset values from the Registers
4. 0 0 18 result DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN This is an example of how to toggle the ZDA bit to enable or disable the Z axis RegisterWrite1 0x18 regs 0x18 0x20 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 0 0 1 0 0 0 0 0 18 result DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN This is the register for setting the threshold value In Threshold Detection Mode the sensitivity is 16 counts g The threshold is setin the LDTH 1A 8 bit Register 1 shows the hex values with the associated counts and g values to understand how to program in the level value that corresponds to the acceleration threshold desired 1A LDTH LDTH 7 LDTH 6 LDTH 5 LDTH 4 LDTH S LDTH 2 LDTH 1 LDTH O In Threshold Detection Mode the sensitivity is 16 counts g The threshold is set in the LDTH 1A 8 bit Register Sensors Freescale Semiconductor AN3468 19 Table 1 Setting Threshold Values Binary Hex Decimal Counts g value 0000 0000 0x00 0 0 0g 0001 0000 0x10 16 16 1g 0010 0000 0x20 32 32 2g 0011 0000 0x30 48 48 3g 0100 0000 0x40 64 64 4g 0101 0000 0x50 80 80 5g 0110 0000 0x60 96 96 6g 0111 0000 0x70 112 112 7g 0111 1111 Ox7F 127 127 8g 1000 0000 0x80
5. 0 0 0 0 1 0 AN3468 Sensors Freescale Semiconductor 24 Table 2 User registers Summary Address Name Bit 7 Bit 6 Bit 5 Bit 4 Bit 3 Bit 2 Bit 1 Bit 0 00 XOUTL XOUT 7 XOUT 6 xour 5 xour 4 xour 3 xour 2 xour 1 xourT 0 01 XOUTH xouT 9 xouT 8 02 YOUTL YouT 7 vour 6 vour 5 vour 4 vour 3 vour 2 vour 1 vourT 0 03 YOUTH YOUT 9 YOUT 8 04 ZOUTL zouT 7 zouT 6 zour 5 zouT 4 zour 3 zour 2 zour 1 zour 0 05 ZOUTH zouT 9 ZOUT 8 06 XOUT8 xouT XOUT 6 xour 5 xour 4 xour 3 xour 2 xour 1 xourT 0 07 YOUT8 vouT YOUT 6 vour 5 vour 4 YouT 3 vour 2 vour 1 vour 0 08 ZOUT8 zouT 7 zouT 6 zour 5 zouT 4 zour 3 zour 2 zour 1 zour 0 09 STATUS PERR DOVR DRDY 0A DETSRC LDX LDY LDZ PDX PDY PDZ INT2 INT1 0B TOUT TMP 7 TMP 6 TMP 5 TMP 4 TMP 3 TMP 2 TMP 1 TMP 0 0C 0D I2CAD I2CDIS DADF 6 DAD 5 DAD 4 DAD 3 DAD 2 DAD 1 DAD 0 0E USRINF UI 7 UI 6 UI 5 Ul 4 UI 3 Ul 2 uij UI 0 0F WHOAM ID 7 ID 6 ID 5 ID 4 ID 3 ID 2 ID 1 ID 0 10 XOFFL XOFF 7 XOFF 6 XOFF 5 XOFF 4 XOFF 3 XOFF 2 XOFFT1 XOFF 0 11 XOFFH XOFF 10 XOFF 9 XOFF 8 12 YOFFL YOFF 7 YOFF 6 YOFF 5 YOFF 4 YOFF 3 YOFFI2 YOFF 1 YOFF 0 13 YOFFH YOFF 10 YOFF 9 YOFF 8 14 ZOFFL ZOFF 7 Z
6. 128 128 8g 1001 0000 0x90 112 144 7g 1010 0000 OxA0 96 160 6g 1011 0000 0xBO 80 176 5g 1100 0000 0xCO 64 192 4g 1101 0000 0xDO 48 208 3g 1110 0000 OxEO 32 224 2g 1111 0000 OxFO 16 240 1g 1111 1111 OxFF 1 255 0g Optimal Settings for Freefall using Level Detection For optimal settings for the freefall using Level Detection choose the following settings 1 Absolute Logic THOPT 2 X Y and Z must be enabled ZDA YDA XDA 3 Negative AND Logic LDPL 1 4 LDTH Level Detection Threshold 0x03 0 19g 1 THOPT 0 Absolute Condition 2 ZDA 0 Enable Z YDA 0 Enable Y XDA 0 Enable X RegisterWrite 0x18 regs 0x18 amp 0x87 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 1 0 0 0 0 1 1 1 18 result DFBW 0 ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 3 Negative AND Logia Set LDPL RegisterWrite 0x19 regs 0x19 0x01 19 CTL2 zx ec x DRVO PDPL LDPL 19 mask 0 0 0 0 0 0 0 1 19 Result zn B DRVO PDPL 1 AN3468 Sensors Freescale Semiconductor 20 4 Set Threshold 0 19 g RegisterWrite 0x1A 0x03 1A LDTH LDTH 7 LDTH 6 LDTH 5 LDTH 4 LDTH 3 LDTH 2 LDTH 1 LDTH 0 Set the threshold to 3 counts 3 16 0 1875 g Optimal Settings for Motion using Level Detection For optimal settings for the motion using Level Detection choose the following settings 1 Absolute Logic THOPT X Y enabled YDA XDA with Z disabled 2 3 Positive OR Logic LDPL 0
7. General Pseudo Code using IIC The following is general IIC code based on work by V Himpe n x general BYTE SIZE max number DATA SIZE an array holding up to SIZE number of bytes This will contain the data transmitted and store received data BUFFER a byte value holding immediate received or transmit data SSSSSSSSSSSSSSSS SSSSS S S S S S SSS S SS SSSSSS SSsSSSSsSSsSsSsSsssssssssss I2C Driver General I2C Pseudo Code Released as Public Domain SESSSSSSSSSSS SSSS S SSSS SSS S SSS S SSS S SSsS sSSssSsSssssssssssssssssss DECLARE N SIZE BUFFER X Byte DECLARE DATA Array of SIZE elements SUBroutine I2C INIT call this immediately after power on SDA 1 SCK 0 FOR n 0 to 3 CALL STOP NEXT n ENDsub SUBroutine START SCK 1 SDA 1 SDA 0 SCK 0 SDA 1 ENDsub SUBroutine STOP SDA 0 SCK 1 SDA 1 ENDsub SUBroutine PUTBYTE BUFFER FOR n 7 TOO SDA BIT n of BUFFER SCK 1 SCK 0 NEXT n SDA 1 ENDsub SUBroutine GETBYTE FOR n 7 to 0 1 http www esacademy com faq i2c general i2cpseud htm Sensors Freescale Semiconductor AN3468 ENDsub SCK 1 BIT n OF BUFFER SDA SCK 0 NEXT n SDA 1 SUBroutine GIVEACK ENDsub SDA 0 SCK 1 SCK 0 SDA 1 SUBroutine GETACK ENDSUB this SDA 1 SCK 1 WAITFOR SDA 0 SCK 0 concludes the low level set of instructions for the I2C driver The next functions will handle the telegram formatting on a higher level SUBroutine READ Device address N
8. of using measurement mode level mode and pulse detection AN3571SW shows different settings for using either the level or the pulse detection mode Connecting the MMA745xL to an MCU using SPI Communication When connecting the sensor to the MCU using SPI communications there are 6 connections into the MCU These connections are power ground the clock SPSCK MOSI data MISO data and CS which is the slave select line CUSPC SDA SDI SDO SDO N C NC A INT2 DANTZ INT1 DRDY JANTI lt MISO K CS Figure 4 SPI Communication Sensor Connections AN3468 Sensors Freescale Semiconductor 10 lt Read Operation gt 4 wire mode ESI ser SEDED MME MAG URES MP ER OE EA FZ SDI SDO orm orsi ec osos oforo 3 wire mode JEN sec TEPPRPEBPEBBEBRPPEHPEDBPED ETE it SD SDO wwjas nte ac omo ago ore ore ees od ecce ec ory lt Write Operation gt Cs sc UUUUUUUUUUUUU UU soi Hras aoaea aE or oeoo Figure 5 SPI Timing Diagrams SPI communication can be implemented using 4 lines MOSI master output slave input MISO master input slave output SS slave select and SPSCK SPI serial clock The master device initiates all SPI data transfers During a transfer the master shifts data out on the MOSI pin to the slave while simultaneously shifting data in on the MISO pin from the slave SDO The transfer effectively exchanges the data that was in the SPI shift registers of the two SPI systems The SPSCK sign
9. 0 bit mode and the calibrated values are greater than 8 bits then there is another register for up to 3 more bits Register 10 and 11 are for X Register 12 and 13 are for Y Register 14 and 15 are for Z These registers follow signed byte data using 2 s complement Reading or writing low byte X Y or Z OUTL latches high byte data X Y or Z OUTH to allow coherent 10 bit reads or writes X Y or Z OUTH should be read written directly following X Y or Z OUTL Step 4 After this compensation process is complete you can continue to modify the values by overcompensating until you get the final output to be right at 0 for X and Y and at 64 for Z Note this is an iterative process AN3468 Sensors Freescale Semiconductor 15 Step 5 Now the calibrated values are stored in the Offset Drift Registers To avoid the values being erased when the power is turned off it is recommended to store these values in flash or a nonvolatile memory in the main processor or external to the processor It is also recommended to include a short start up sequence to write the compensation values stored in flash or nonvolatile memory to the registers 10 15 in the accelerometer on start up If a microcontroller is used a recursive program is available to auto calibrate the device To set the 0 g offset the following recursive program can be written as a routine in the software Place the part flat in the x 0 g y 20g z 1 g orientation 1 Set t
10. 00 08 The first 6 registers of the 9 are 10 bit XYZ output values LSB first MSB second Please verify with the data sheet for detailed register information Step 2 In this step the offset compensaiion is calculated to shift the offset to zero For example if the Og offset is calibrated flat where X 0 g Y 0 g and Z 1 g in 2 g mode 64 LSB g sensitivity the outputs from Registers 00 08 might be the following X 18 Y 20 Z 44 In this case X must be shifted by 18 to get X back to zero Y must be shifted by 20 to get to zero Zmust be shifted by 64 44 20 to get to 64 since Z is in the 1 g orientation Step 3 These compensation values can be written in hexadecimal into the Offset Drift Registers 10 15 The Offset Drift Registers require each value to be LSB therefore the calibration values calculated in Step 2 must be multiplied by two Note that there will still be a bit of offset shift and you may need to multiply by a bit more than two to exactly subtract the offset If the register values in the X 0 g Y 0 g and Z 1 g orientation are the following X 18 Y 20 Z 44 Write 36 DC Hex into the X drift Register 10 Write 40 28 Hex into the Y drift Register 12 Write 40 28 Hex into the Z drift Register 14 If the compensation requires negative values remember that 2 s complement is always used in hexadecimal for storing the signed value as was done for the X axis above If using 1
11. 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 Write function SPI 3 Wire or SPI 4 Wire Mode void spi write byte reg byte data byte x CS 0 SPI2D reg amp 0x3F lt lt 1 0x80 while SPI2S SPRF wait for transmission complete x SPI2D dummy read SPI2D data while SPI2S SPRF wait for transmission complete x SPI2D CS 1 General Pseudo Code using SPI This is pseudo code for general communication 1 SS 0 Slave Select is cleared 2 Assert MOSI most significant bit 3 Toggle the clock 4 Shift the next data bit onto the MOSI pin 5 Repeat 3 and 4 until done 6 SS 1 Slave Select is set For further examples using the SPI SW protocol please refer to AN3468SW has examples of using measurement mode level mode and pulse detection AN3480SW shows the programming required to communicate to the device using the SPI bus AN3571SW shows different settings for using either the level or the pulse detection mode Sensors Freescale Semiconductor AN3468 13 PROGRAMMING THE MMA745XL This section focuses on explaining all the available features of the device which are all accessible by reading and writing to the 32 registers of the MMA745xL Figure 6 is a screen shot of an evaluation interface for this device This GUI uses the MMA745xL with the MC9S08QG8 microcontroller on a small 1 inch by 1 inch board which connects through SCI to USB to the computer It displays the 3 axis output on a scope screen to vie
12. 4 LDTH Level Detection Threshold 20x20 2 g 1 THOPT 0 Absolute Condition 2 ZDA 1 Disable Z YDA 0 Enable Y XDA 0 Enable X Se regs 0x18 amp 0x87 0x20 3 Positive OR Logic Clear LDPL RegisterWrite 0x19 regs 0x19 amp 0xFE 19 CTL2 DRVO PDPL LDPL 19 mask 1 1 1 1 1 1 1 0 19 result DRVO PDPL 0 4 Set Threshold to 2 g RegisterWrite 0x1A 0x20 1A LDTH LDTH 7 LDTH 6 LDTH 5 LDTH 4 LDTH 3 LDTH 2 LDTH 1 LDTH 0 Set the threshold to 32 counts 32 16 2 g Pulse Detection Mode Pulse detection mode is capable of detecting both motion and freefall There are timers associated with the pulse detection The OR logic is used for motion detection where either a single or a double pulse can be detected For motion detection the logic is set up to detect an event when a programmed Threshold has been reached for TimeWindow It is not possible to set up motion detection for Threshold reached for Time Window The AND logic is used for freefall detection In this case when below the set Threshold for Latency Timer the Pulse Detection flag is set AN3468 Sensors Freescale Semiconductor 21 MMA745x1 GUI rev18 Connected to COM2 at 38 400 HWID 490 1 SWID 490E r OPERATION MODE INT1 and INT2 registers INT1 register
13. 468 Sensors Freescale Semiconductor 23 Optimal Settings for Double Pulse Detection 1 Positive OR Logic PDPL 0 2 Zaxis enabled only 3 Pulse Threshold set to 4 g 4 Pulse Duration 3C 60 counts 30 ms 5 Latency Time 0x5A 90ms 6 2 Time Window 0x82 130 ms 1 Positive OR Logic Enabled PDPL 0 RegisterWrite 0x19 regs 0x19 8 0xFD 19 CTL2 DRVO PDPL LDPL 19 mask 1 1 1 1 1 1 0 1 19 result DRVO 0 LDPL 2 Zonly enabled RegisterWrite 0x18 regs 0x18 amp 0x98 0x18 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG O INTPIN 18 mask 1 0 0 1 1 0 0 0 18 result DFBW 0 0 1 1 INTRG 1 INTRG O INTPIN 3 PDTH Pulse Threshold set to 4 g RegisterWrite 0x1B 0x40 1B PDTH PDTH 7 PDTH 6 PDTH 5 PDTH 4 PDTH 3 PDTH 2 PDTH 1 PDTH 0 1B result 0 1 0 0 0 0 0 0 4 Pulse Duration for first pulse 30 ms RegisterWrite 0x1C 0x3C NECEERL INE MES EDI OENU ee ae eee RFN FZ EE EUN RN FEN ee 5 Latency time between the pulses 90 ms RegisterWrite 0x1D 0x5A 1D LT LT 7 LT 6 LT 5 LT 4 LT 3 LT 2 LT 1 LT 0 1D result 0 1 0 1 1 0 1 0 6 Time Window for Second Pulse 130 ms ReigsterWrite 0x1E 0x82 1E TW TW 7 TW 6 TW 5 TW 4 TW 3 TW 2 TW 1 TW 0 1E result 1 0
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15. ADDRO pin is enabled on the MMA7458L which will be available in Q4 2008 AN3468 Sensors Freescale Semiconductor 4 The following are four simple rules of the IIC bus to be aware of 1 4 The SDA data and SCL clock cannot actively be driven high by any I C device I C devices must use open drain drivers The logic is high by using the recommended external pull up resistors The information on the data line is only read on the high phase of the clock Changing the level of the data is only allowed in the low phase of the clock except during start or stop conditions and this is how these events are signified When the bus is not busy SDA and SCL lines are pulled back to logic 1 For more detailed information on the IIC protocol please refer to the NXP Semiconductor IIC bus specification and user manual available online BASIC START UP PROCEDURE The following are some simple steps to set up the accelerometer using IIC 1 2 3 5 6 Set up the microcontroller hardware shown above for IIC communication Configure the clock speed and all the pins required The speed is set by adjusting the baud rate Write simple single byte Read and Write command to communicate to the device The example below was done using the MC9S08QE hardware controller for the IIC Write to the MMA745xL Register 16 sending in a value of 0x05 to set up the device for measurement mode with 2g dynamic range Read the Control Reg
16. C2Cl TX 1 IIC2C1 MST 1 Sends Start IIC2D IIC WriteAdr 1 send the accelerometer address Delay 0x50 while IIC2S IICIF 0 Waiting for transmission to complete IIC2S IICIF 1 if IIC2S RXAK 1 IIC2C MST 0 send stop ACK 0 while 1 else ACK 1 IIC2D reg write in the register address Delay 0x50 Check_ACK check ACK while IIC2S IICIF 0 Waiting for transmission to complete IIC2S8 IICIF 1 if IIC2S RXAK 1 IIC2C MST 20 send stop ACK 0 while 1 else ACK 1 Sending a Restart the device Address with a Read IIC2C1_TX 1 IIC2C1 RSTA 1 Sends Start IIC2D IIC_ReadAdr lt lt 1 0x01 Send the accelerometer device address with the Read Bit Delay 100 Check_ACK check ACK while IIC2S IICIF 0 Waiting for transmission to complete IIC2S IICIF 1 if IIC2S RXAK 1 1 IIC2C MST 0 send stop ACK 0 while 1 Sensors Freescale Semiconductor AN3468 else ACK 1 IIC2C1 TX 0 Change over to Receiver Mode IIC2C TXAK 1 send NACK in the next read data IIC2D dummy read while IIC25S_IICIF 0 IIC2S IICIF 1 wait until the transmission ends data IIC2D This is the real Read command while IIC2S IICIF 0 wait until the transmission ends IIC2S IICIF 1 Clear the interrupt flag IIC2C1 MST 0 send a stop leave master mode return data 2 2 2 2 2 2 2 2
17. Freescale Semiconductor AN3468 Application Note Rev 0 08 2008 The MMA745xL Digital Accelerometer by Kimberly Tuck Inertial Applications Engineer Tempe AZ The MMA745xL digital accelerometer is a 3x5x1 mm product that can communicate using both IC and SPI This device has both threshold and pulse detection interrupts There are 2 sampling rates available at 125 Hz using the 62 5 Hz digital filter and 250 Hz using the 125 Hz digital filter There is a self test function to verify the integrity of the MEMS sensor and the ASIC signal path Figure 1 shows the simple evaluation board for the MMA745xL which is available online This board contains the accelerometer with all pins mapped out to a 14 pin header The image on the left shows the ground plane which ties the digital and analog ground pins together This is necessary to improve the noise performance of the part based on internal trim and is the recommendation for this part The image in the center is the front side of the board which shows all the other connections This board can be wired to a Freescale MCU evaluation or demo board and programmed for various application functions which are described in detail below a o d Back View Front View Actual Image of the Board Figure 1 MMA745xL Accelerometer Simple Evaluation Board APPLICATIONS AND SENSING CAPABILITIES OF THE MMA745XL The MMA745xL is the first family of digital 3 axis consumer accelerometers that Freescale has designed
18. H MCTL INTRST Cni CTL2 LDTH PDTH LT TW 00 OxFF 01 0x03 02 0x00 03 0x00 04 0xCO 05 0x03 06 0x00 07 0x00 08 OxFO 09 0x00 0A 0x00 05 0x00 0D 0x1D 10 0x52 11 0x00 12 0x2D 13 0x00 14 0x37 15 0x00 16 0x02 17 0x00 18 0x00 19 0x00 SI 0x20 18 0x00 1C 0x00 1D 0x00 1E 0x00 NWM lt Regs Figure 6 MMA745XL Evaluation Software Interface OPERATION MODES OF THE MMA745XL ACCELEROMETER OPERATION MODE c C Measurement Mode Level Detection Mode C Pulse Detection Mode Figure 7 Operation Modes of the MMA745XL All modes of the device are controlled in the mode control register MCTL Figure 7 displays the different modes of the MMA745XL The following are the bits in the register and the different modes that the device can be set 16 MCTL LPEN DRPD SPISW STON GLVL 1 GLVL O MODI1 MOD O Standby mode is a low power mode consuming less than 5 uA This mode is used when the sensor is not needed to take new data In standby mode the sensor can read and write to the registers and the other sensor detection modes are easily enabled Sensors Freescale Semiconductor AN3468 14 Standby Mode RegisterWrite 0x16 0x00 In measurement mode the device can be set up for 3 different dynamic ranges 2 g 4 g and 8 g With the 8 g dynamic range 8 bit or 10 bit data is available The 2 g and 4 g ranges are only 8 bit data
19. OFF 6 ZOFF 5 ZOFF 4 ZOFF 3 ZOFF 2 ZOFF 1 ZOFF 0 15 ZOFFH ZOFF 10 ZOFF 9 ZOFF 8 16 MCTL LPEN DRPD SPI3W STON GLVL 1 GLVL 0 MOD I1 MOD 0 17 INTRST CLRINT2 GLRINT1 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG 0 INTPIN 19 CTL2 DRVO PDPL LDPL 1A LDTH LDTH 7 LDTH 6 LDTH 5 LDTH 4 LDTH 3 LDTH 2 LDTH 1 LDTH O 1B PDTH PDTH 7 PDTH 6 PDTH 5 PDTH 4 PDTH 3 PDTH 2 PDTH 1 PDTH 0 1C PW PD 7 PD 6 PD 5 PD 4 PD 3 PD 2 PD 1 PD 0 1D LT LT 7 LT 6 LT 5 LT 4 LT 3 LT 2 LT 1 LT 0 1E TW TWI7 TW 6 TW 5 TW 4 TW 3 TW 2 TW 1 TW 0 1F NOTES 1 http www esacademy com faq i2c general i2cpseud htm AN3468 Sensors Freescale Semiconductor 25 How to Reach Us Home Page www freescale com Web Support http www freescale com support USA Europe or Locations Not Listed Freescale Semiconductor Inc Technical Information Center EL516 2100 East Elliot Road Tempe Arizona 85284 1 800 521 6274 or 1 480 768 2130 www freescale com support Europe Middle East and Africa Freescale Halbleiter Deutschland GmbH Technical Information Center Schatzbogen 7 81829 Muenchen Germany 44 1296 380 456 English 46 8 52200080 English 49 89 92103 559 German 33 1 69 35 48 48 French www freescale com support Japan Freescale Semiconductor Japan Ltd Headquarters ARCO Tower 15F 1 8 1 Shimo Meguro Meguro ku Tokyo 153 0064 Japan 0120 191014 or 81 3 5437 912
20. T ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 1 1 1 1 1 0 1 1 18 result DFBW THOPT ZDA YDA XDA 0 1 INTPIN INT1 reg Single Pulse and INT2 reg Pulse RegisterWrite 0x18 regs 0x18 amp 0xFD 0x04 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 1 1 1 1 1 1 0 1 18 result DFBW THOPT ZDA YDA XDA 1 0 INTPIN INT1pin assigned to DRDY This pin can be set to allow INT1 to be used to indicate when data is ready from the sensor RegisterWrite 0x16 reg 0x16 0x40 16 MCTL LPEN DRPD SPI3W STON GLVL 1 GLVL O MOD 1 MODJ O0 16 mask 0 1 0 0 0 0 0 0 16 Result LPEN DRPD SPI3W STON GLVL 1 GLVL O MOD 1 MODJO AN3468 Sensors Freescale Semiconductor 17 INT1pin assigned to INT1 Reg INT2pin INT2 Reg RegisterWrite 0x18 regs 0x18 amp FE 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 1 1 1 1 1 1 1 0 18 result DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 0 INT1pin assigned to INT2 Reg INT2pin z INT1 Reg RegisterWrite 0x18 regs 0x18 0x01 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG 0 INTPIN 18 mask 0 0 0 0 0 0 0 1 18 result DFBW THOPT ZDA YDA XDA INTRG 1 INTRG 0 1 LEVEL DETECTION MODE Level detection mode is capable of detecting both motion and freefall There are no timers associated with the level detection The OR logic is used for motion detection and the AND logic is used for freefall detection T
21. There are several sensing functions that accelerometers are capable of detecting These are motion freefall shock vibration and tilt The advantage of a digital accelerometer is that the A D converter is implemented within the sensor and an MCU can be used to accompany the sensor that does not require the A D converter The sensor can connect directly to the SPI or IIC into the MCU The disadvantage is that there are selected digital filters within the sensor which limits the sampling rate for certain applications Motion Detection Typically motion detection is used to identify if an object is in use or not based on change in acceleration output The purpose of power cycling is to try to minimize the power consumption for the application By moving into a standby low power mode when the device is not in use then switching to measurement mode active mode which is a full power mode the overall power consumption can be decreased The power consumption is the same regardless of the two sampling rate options in the active modes The MMA745xL can be put into standby mode for a set period of time consuming only 5 HA Then the device must be programmed to switch over to measurement mode to monitor the X Y Z outputs to determine if a significant change in acceleration has occurred This can be done using a timer function in the MCU The device will not auto wake from standby mode When in an active mode the device consumes 400 450 pA of current The devic
22. ad byte reg void spi write byte reg byte data void main void MCU init call Device Initialization PTDD PTDD3 1 CS Pin description PTDDD PTDDD3 1 Xdata 0 CTLReg 0 Set up sensor for SPI 3 Wire Mode or 4 Wire Mode by writing to SPI3W in Register 16 Spi write 0x16 0x25 3 Wire Mode Set up in the Accelerometer Spi write 0x16 0x05 4 Wire Mode Set up in the Accelerometer CTLReg spi read 0x16 Read back the value that has been set for the Mode for 77 Xdata spi read 0x06 Read X Y Z outputs from Sensor Ydata spi read 0x07 Zdata spi read 0x08 for RESET WATCHDOG by default COP is disabled with device init When enabling also reset the watchdog loop forever please make sure that you never leave main Function Definitions for Read and Write Read function SPI 3 Wire Mode or 4 Wire Mode byte spi read byte reg byte x CS 0 x SPI28 X SPI2D while SPI2S SPTEF SPI2D reg amp 0x3F lt lt 1 write in the register address with the read command while SPI2S SPRF wait for transfer AN3468 Sensors Freescale Semiconductor 12 x SPI2D SPI2C2 BIDIROE 0 MOSI become input when 3 wire mode SPI2D 0x00 send 2nd byte while SPI2S SPRF wait transfer done x SPI2D SPI2C2 BIDIROE 1 change direction back to output when in 3 wire mode CS 1 return x 2 2 2 2 2 2 2 2 2 2
23. al is a clock output from the master and an input to the slave The slave device must be selected by a low level on the slave select input CS pin The MMA745xL is in SPI 4 wire mode by default Register 16 contains a bit labeled SPI3W which is set to 0 by default for 4 wire mode SPI communication can also be implemented using 3 lines MIMO master in master out SS slave select SPSCK SPI serial clock The SDA I O line on the MMA745xL changes from being an input into the accelerometer and becomes a bidirectional line To set up the sensor for 3 wire SPI communication bit SPI3W in Register 16 Control Register 1 must be set to 1 For four wire SPI mode SPI3W is set to 0 The following example shows how to set up the register in either mode and into 2 g measurement mode WriteRegister 0x16 0x25 to set the accelerometer into SPI 3 wire mode 2 g measurement mode WriteRegister 0x15 0x05 to set the accelerometer into SPI 4 wire mode 2 g measurement mode When using the SPI bus the register value is sent with the first value being a 1 to indicate write or O to indicate a read command Note this is opposite from the IIC R W bit The address is 6 bits followed by a Don t Care bit which has no meaning to the slave For example to send a read command the six bit register data is simply shifted left 1 bit SPI2D reg amp 0x3F lt lt 1 For a write command the first bit is a 1 SPI2D reg amp 0x3F lt lt 1 0x80 The SPI clock speed o
24. ating parameters including Typicals must be validated for each customer application by customer s technical experts Freescale Semiconductor does not convey any license under its patent rights nor the rights of others Freescale Semiconductor products are not designed intended or authorized for use as components in systems intended for surgical implant into the body or other applications intended to support or sustain life or for any other application in which the failure of the Freescale Semiconductor product could create a situation where personal injury or death may occur Should Buyer purchase or use Freescale Semiconductor products for any such unintended or unauthorized application Buyer shall indemnify and hold Freescale Semiconductor and its officers employees subsidiaries affiliates and distributors harmless against all claims costs damages and expenses and reasonable attorney fees arising out of directly or indirectly any claim of personal injury or death associated with such unintended or unauthorized use even if such claim alleges that Freescale Semiconductor was negligent regarding the design or manufacture of the part Freescale and the Freescale logo are trademarks of Freescale Semiconductor Inc All other product or service names are the property of their respective owners Freescale Semiconductor Inc 2008 All rights reserved Z freescale semiconductor
25. d 7 1 W Write 0 Master Master Master Figure 3 IIC Read and Write Protocol Format The MMA745XL IIC communication protocol follows the Philips Semiconductors now NXP Semiconductors standard In this interface only two bus lines are required a serial data line SDA and a serial clock line SCL Serial 8 bit oriented bidirectional data transfers can be made at up to 100 kbit s in the Standard mode and up to 400 kbit s in the fast mode These modes are adjustable by changing the clock frequency The maximum allowable bus capacitance is 400pF Both SDA and SCL are bidirectional lines connected to a positive supply voltage via a pull up resistor The recommended value is between 2 2 kQ 4 7 kQ The accelerometer is always considered the slave and the MCU is always considered the master The accelerometer is not capable of stretching the clock The benefits of the IC Communication interface is that many ICs can be added to this bus The only limitation is the bus capacitance The simple 2 wire serial I C bus minimizes interconnections so ICs have fewer pins and there are not as many PCB traces resulting in smaller and less expensive PCBs Each device is recognized by a unique address whether it is a microcontroller memory or an accelerometer The MMA7455L has an extra address bit to allow for two different addresses available but the address pin on this sensor has been disabled This device is addressed by 1D only The I
26. double pulse The threshold and the time of the pulse must be set In pulse mode the detection occurs when X or Y or Z is greater than the set threshold within the set time window less than the time window It is not able to detect motion for a time period greater than a set time window Motion detection can also be done in measurement mode sampling the X Y and Z outputs with a set timer This method would require programming the algorithm with the MCU This last method would be necessary to detect a motion for greater than a set time period Freefall Freefall is a sensing function that can be used to identify that a large impact is highly probable This is useful in notebook computers to park the drive heads before impact This is useful in general for many types of electronic equipment to shut down before impact Freefall can also be used for warranty protection along with shock to identify how high an object has fallen to determine the approximate resultant force For a robust algorithm there are various different freefall conditions that should be considered These are linear freefall projectile fall and rotational fall Cheaper freefall solutions typically only consider linear freefall Linear Freefall using the MMA745xL Logic Interrupts The MMA745xL has internal logic to detect linear freefall using either the Level detection or the Pulse Detection modes The level detection is not as robust because it does not have any timers associated with
27. e XDA bit to enable or disable the X axis RegisterWrite 0x18 regs 0x18 0x08 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG 0 INTPIN 18 mask 0 0 0 0 1 0 0 0 18 result DFBW THOPT ZDA YDA 1 INTRG 1 INTRG 0 INTPIN This is an example of how to toggle the YDA bit to enable or disable the Y axis RegisterWrite 0x18 regs 0x18 0x10 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG 0 INTPIN 18 mask 0 0 0 1 0 0 0 0 18 result DFBW THOPT ZDA 1 XDA INTRG 1 INTRG 0 INTPIN This is an example of how to toggle the ZDA bit to enable or disable the Z axis RegisterWrite 0x18 regs 0x18 0x20 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG 0 INTPIN 18 mask 0 0 1 0 0 0 0 0 18 result DFBW THOPT 1 YDA XDA INTRG 1 INTRG 0 INTPIN This is the pulse detection threshold value that can be set according to In Threshold Detection Mode the sensitivity is 16 counts g The threshold is set in the LDTH 1A 8 bit Register 1 AN3468 Sensors Freescale Semiconductor 22 This register defines the pulse width of the first pulse window The pulse duration increments in v2 ms values so every count is Ve ms The total range is from 0 127 5ms 1C PW PDI7 PD 6 PD 5 PD 4 PD 3 PD 2 PD 1 PD 0 This register defines the latency time which is the time in betwe
28. e can be power cycled between standby mode and measurement mode minimizing the overall current consumption for the application This is useful in a wide variety of applications It is particularly important in handheld devices which require batteries to operate Z freescale semiconductor Freescale Semiconductor Inc 2008 All rights reserved Hints for a Power Cycling Algorithm During a power cycling routine the device will be set to standby mode until the MCU timer triggers Then the MCU will switch the sensor into measurement mode and take one reading from X Y and Z If the RMS value is a certain threshold greater than 1gorless than 1 g then motion is detected and the device will stay in measurement mode Otherwise the MCU will switch accelerometer back into standby mode The timing will be somewhat dependant on the circumstances of the application and the tradeoff between the reaction time vs the consumed current Motion Detection using the MMA745xL Logic Interrupts The Level Detection mode can be used to detect motion with an interrupt The threshold level can be set and the interrupt will occur when a motion greater than the threshold occurs There are no timers in the Level Detection mode and false readings are possible It is recommended to set the threshold level to 2 g or greater for motion detection using the Level Detection to minimize false readings The Pulse Detection mode can detect motion as a single pulse or as a
29. en pulses for double pulse or the amount of time to trigger a freefall condition The scale increases from Oms 255ms 0x00 0xFF Every count is equal to 1ms 1D LT LT 7 LT 6 LT 5 LT 4 LT 3 LT 2 LT 1 LT 0 This is the timing window for the second pulse when setting up the double pulse condition The scale is in ms increments from Oms 255ms 0x00 0xFF 1E TW TWIT TW 6 TW 5 TW 4 TW 3 TW 2 TW 1 TW 0 Optimal Settings for Single Pulse Detection 1 Positive OR logic PDPL 0 2 X Y Z enabled 3 PDTH 0x40 4g 4 PD 0x10 1 Positive OR Logic PDPL 0 RegisterWrite 0x19 regs 0x19 amp 0xFD 19 CTL2 DRVO PDPL LDPL 19 mask 1 1 1 1 1 1 0 1 19 result DRVO 0 LDPL 2 X Y Z enabled RegisterWrite1 0x18 regs 0x18 amp 0x87 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG O INTPIN 18 mask 1 0 0 0 0 1 1 1 18 result DFBW 0 0 0 0 INTRG 1 INTRG 0 INTPIN 3 PDTH Pulse Threshold set to 4 g RegisterWrite 0x1B 0x40 1B PDTH PDTH 7 PDTH 6 PDTH 5 PDTH 4 PDTH 3 PDTH 2 PDTH 1 PDTH O 1B result 0 1 0 0 0 0 0 0 4 PD Pulse Duration set to 8 ms RegisterWrite 0x1C 10 1C PW PD 7 PD 6 PD 5 PD 4 PD 3 PD 2 PD 1 PD 0 1C result 0 0 0 1 0 0 0 0 AN3
30. f the sensor can go up to 4 MHz which is configured by the baud rate chosen in the MCU This is the maximum achievable speed when DVdd is less than 2 4 V When DVdd is greater than 2 4 V up to 8 MHz is achievable AN3468 Sensors Freescale Semiconductor 11 BASIC START UP PROCEDURE The following are some simple steps to set up the accelerometer with an MCU for the SPI communication 1 Set up the microcontroller hardware shown in Figure 4 for SPI communication 2 Configure the clock speed and all the pins required The speed is set by adjusting the baud rate 3 Write simple single byte Read and Write command to communicate to the device The example below was done using the hardware controller for the SPI 4 Write to the MMA745xL Register 16 sending in a value of 0x05 to set up the device for measurement mode with 2g dynamic range 5 Read the Control Register 16 to ensure that the value is correct 0x05 6 Read the X Y and Z registers and watch the outputs change Example Code for Start up using SPI The following example code was developed with a Freescale S08 MCU using the embedded SPI module This software is for the host side This example shows the code for both 4Wire Mode and 3Wire Mode The MCU can be initialized for 3 Wire Mode or for 4 Wire Mode and the following code will execute define CS PTDD PTDD3 byte Xdata Ydata Zdata CTLReg void MCU init void Device initialization function declaration byte spi re
31. he Accelerometer in measurement Mode 2g RegisterWrite 0x16 0x01 2 Initialize the X Y and Z calibration variables to O Xcal 0 Ycal 0 Zcal 0 3 Read X Y and Z data Xdata RegisterRead 0x00 While Xdata gt 1 Xdata lt 1 f Xcal 2 Xdata Delay OxFF Xdata RegisterRead 0x00 Ydata RegisterRead 0x02 While Ydata gt 1 Ydata lt 1 f Ycal 2 Ydata Delay OxFF Ydata RegisterRead 0x02 Zdata RegisterRead 0x04 While Zdata gt 65 Zdata lt 63 f If Zdata gt 65 Zcal Zdata 64 2 Else Zcal 64 Zdata 2 Delay 0xFF Zdata RegisterRead 0x04 SETTING UP THE SAMPLING FREQUENCY AND SELF TEST To set the bandwidth filter to 62 5 Hz the sampling frequency will be 125 Hz Clear DFBW in Control Register 1 RegisterWrite 0x18 regs 0x18 amp OxEF 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG O INTPIN 18 mask 0 1 1 1 1 1 1 1 18 result 0 THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN To set the bandwidth filter to 125 Hz the sampling frequency will be 62 5 Hz Set DFBW in Control Register 1 RegisterWrite 0x18 regs 0x18 0x80 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG O INTPIN 18 mask 1 0 0 0 0 0 0 0 18 result 1 THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN AN3468 Sensors Freescale Semiconductor To verify the self test Set STON in Mode Control Register 16 Clear STON to turn off self test Regis
32. hen there is signed threshold detection or or absolute and OPERATION MODE C standby Made Level Detection C Measurement Mode Mode C Pulse Detection Mode r INT1 and INT2 registers INT1reg Level C INT1reg Pulse INT2 reg Pulse INT2 reg Level C INTireg Single Pulse INT2 reg Pulse M INT1 register M INT2 register M INT1 and INT2 pins INTipin DRDY INT1pin INT ireg INT2pin INT2reg C INT 1pin INT2reg INT2pin INT ireg r LEVEL DETECTION MODE M PULSE DETECTION MODE MEASUREMENT MODE Absolute OR gt c Pulse Duration Oms Latency Time Oms C 2g Xcal 82 82 C Pos Neg AND lt C AND C 4g Yeal 45 45 Threshold 32 E gg Zcal 55 55 SRR EUN 2nd Pulse Oms write XYZ Outputs ar ee tag Tat et E Tee Met at ipa TC Ye at Pat a Lac Lat Tact Vest at Met tet TAC at I X8 0 X10 1 FI Y8 0 Y10 O Disable X Level Detect Condition nis Z8 16 Z10 64 DisableY X AbsLevel or Y gt AbsLevel or Z AbsLevel Disable Z Figure 9 Level Detection Settings These are the following available options for Level Detection Mode and how they can be enabled For motion or for freefall detection there is an absolute condition that can be used RegisterWrite 0x18 regs 0x18 amp 0xBF 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRG 0 INTPIN
33. ister 16 to ensure that the value is correct 0x05 Read the X Y and Z registers and watch the outputs change Example Code for Start up using IIC The following code has been written with a Freescale S08 MCU using the embedded IIC module This software is for the host side HW I2C driver embedded I2C module using FSL MC9SO8QE K Tuck define IIC WriteAdr 0x1D 1D shifted in with a 0 had been 3A define IIC ReadAdr 0x1D 1D shifted in with a 1 had been 3B Function Definitions void MCU init void From Device initialization void Delay byte count void IIC SingleByteWrite unsigned char reg unsigned char val char IIC SingleByteRead unsigned char reg global variables char Xdata 0 char Ydata 0 char Zdata 0 char ControlRegVal 0 unsigned char ACK writel Main void main void MCU init IIC2C1 MST 0 Delay 0xFF IIC2C1 MST 1 create start Delay 0xFF IIC2C1 MST 0 create stop Delay 0xFF IIC SingleByteWrite 0x16 0x05 Put ION into Measurement Mode Delay 0xFF ControlRegVal 0 AN3468 Sensors Freescale Semiconductor 5 Xdata 0 Ydata 0 Zdata 0 ControlRegVal IIC SingleByteRead 0x16 Read back out the value of Register 16 make sure it is 5 Read X Y and Z forever for Delay 0xFF Xdata IIC SingleByteRead 0x06 Read X Register Delay 0xFF Ydata IIC SingleByteRead 0x07 Read Y Register Delay 0xFF Zdata IIC SingleB
34. it It simply detects any resultant output of X amp amp Y amp amp Z Set Threshold The pulse detection Freefall condition is more robust because it has a timer The pulse detection freefall algorithm looks at X amp amp Y amp amp Z Threshold for Latency Timer The timer in the pulse detection helps avoid false readings Advanced Freefall Algorithm Hint Using a microcontroller to store some of the past history and to analyze the outputs linear projectile and rotational falls can be detected using the MMA745xL to detect if any of these different conditions are occurring Shock Shock is a sensing function of the accelerometer that is useful for warranty protection shipping and handling and to detect the end of a fall condition It is also used to detect tapping Shock is a sensing function that can be difficult to detect with the consumer low g accelerometers because shocks are typically high accelerations The MMA745xL is capable of detecting up to 8g of acceleration In some cases freefall can be used to determine the height of a fall using the standard Newtonian equations of motion and then back calculating for the distance Detecting Shock using the MMA745xL Logic Interrupts The Level Detection interrupt can be used to detect shock in the same manner it is done for motion detection The single and double pulse interrupts are the most useful for shock Vibration Vibration sensing is limited by the digital filtering in the accelerome
35. lines in AN3484 It is important to not place the sensor near an edge where it may be knocked around or touched by people s hands Also avoid bending the PCB as the PCB stress is transferred to the accelerometer Temperature can also be an issue It is good to avoid placing the sensor far away from components that may have large temperature variations or that are constantly very hot as this will also affect the offset of the sensor For optimal motion detection place the sensor away from the center of the device This will ensure better acceleration readings and make them more significant to detect smaller motions from a higher moment of inertia than if placed right on the center of movement Connecting the MMA745xL to an MCU using I C Communication Connecting this evaluation board to an MCU using IC communication is simple Connect power and ground SDA and the SCL lines appropriately to the MCU YIYNTZ SINT Figure 2 I C Communication Sensor Connections AN3468 Sensors Freescale Semiconductor 3 lt Single Byte Read gt Te Eg pepe gsm Te Ene so lt a ra lt Multiple Bytes Read gt EE S7 sive srs waster memes see rn JH sen T seca Single Byte Write eer Te me mm ED sens Eje siave mp qum Multiple Bytes Write master Te TE Te ET ED mm Eo Efe zs mq mme mme ST Start condition SR Repeated Start condition SP Stop condition AK Acknowledge NAK No Acknowledge R Rea
36. ter The MMA745xL has a maximum sampling rate of 250 Hz Therefore it is capable of detecting from DC to 125 Hz of vibration The MMA745xL is suitable for these lower frequencies AN3468 Sensors Freescale Semiconductor 2 Tilt Tilt is used for a lot of different applications The cell phone and PMP market has exploded with opportunities for accelerometers to perform tilt functions The most popular features are portrait landscape orientation detection scrolling and menu selection The two main challenges of tilt are to determine the required resolution and the required accuracy for the application The MMA745xL has a maximum sensitivity of 64 counts g The resolution is the smallest detectable change in acceleration which is 16 mg per count This corresponds to about 1 5 degrees of resolution using two axes The accuracy is how closely the true value is equal to the measured value from the accelerometer This is dependant on the sum of all errors from the accelerometer Typically after calibration the accuracy is about 4 degrees SENSOR PLACEMENT Sensor placement is very important and is often overlooked The MEMS sensor inside the package is very sensitive to stresses Small deflections inside the MEMS sensor on the order of 10 nm correspond to a change in acceleration of 1 g Care must be taken to ensure that the package is not stressed by holes or components on the PCB placed too closely to the accelerometer Please review the mounting guide
37. terWrite 0x16 regs 0x16 0x10 16 MCTL LPEN DRPD SPI3W STON GLVLI 1 GLVL O MODI1 MODI O0 16 mask 0 0 0 1 0 0 0 0 16 result LPEN DRPD SPI3W STON GLVLI 1 GLVL O MODI1 MOD 0 SETTING UP INT1 AND INT2 INTERRUPT REGISTERS This section explains how to set up the interrupt pins in all possible configurations The interrupt registers are set up in the Control 1 CTL1 Register to determine which interrupt is assigned to Level detection and which is assigned to Pulse detection Figure 6 and Figure 8 displays the different options for setting up the interrupt pins INT1 and INT2 registers INTireg Level INT2 reg Pulse C INT1reg Pulse INT2 reg Level C INTireg Single Pulse INT2 reg Pulse jj INTiregister INT2 register 5 LL Figure 8 Setting up the Interrupt Registers for Level and Pulse Detection INT1 and INT2 pins INT1pin DRDY INT1pin INT ireg INT2pin INT2reg C INT1pin INT2reg INT2pin INT ireg INT1 reg Level and INT2 reg Pulse RegisterWrite 0x18 regs 0x18 amp 0xF9 18 CTL1 DFBW THOPT ZDA YDA XDA INTRG 1 INTRGI0 INTPIN 18 mask 1 1 1 1 1 0 0 1 18 result DFBW THOPT ZDA YDA XDA 0 0 INTPIN INT1 reg Pulse and INT2 reg Level RegisterWrite 0x18 regs 0x18 amp 0xFB 0x02 18 CTL1 DFBW THOP
38. umber of bytes ENDsub Device adress Device adress OR 0000 0001 b This sets the READ FLAG CALL START CALL PUTBYTE Device adress CALL GETACK FOR x 0 to Number of bytes CALL GETBYTE DATA x BUFFER Copy received BYTE to DATA array IF X Number of bytes THEN Not ack the last byte CALL GIVEACK END IF NEXT x CALL STOP SUBroutine WRITE Device address Number of bytes ENDsub Device adress Device adress AND 1111 1110 b This clears READ flag CALL START CALL PUTBYTE Device adress CALL GETACK FOR x 0 to Number of bytes CALL PUTBYTE DATA x CALL GETACK NEXT x CALL STOP SUBroutine RANDOMREAD Device adress Start adress Number of bytes ENDsub Sensors Device adress Device adress AND 1111 1110 b This clears READ flag CALL START CALL PUTBYTE Device adress CALL GETACK CALL PUTBYTE Start adress CALL GETACK CALL START create a repeated start condition Device adress Device adress OR 0000 0001 b This sets the READ FLAG CALL PUTBYTE Device adress CALL GETACK FOR x 0 to Number of bytes CALL GETBYTE DATA x BUFFER CALL GIVEACK NEXT x CALL STOP AN3468 Freescale Semiconductor 9 For further information and examples using the QE128 Freescale S08 with the MMA745xL there is an example with a SW implementation of the driver available in the following software application notes on line AN3479SW shows the programming required to communicate to the device using the IIC bus AN3468SW has examples
39. w motion of the different axes All the different settings and operation modes of the device are displayed on the menu buttons or sliders On the right hand side all the useful registers of the device are displayed This allows the user to view how the registers change when different settings are changed MMA745x1 GUI rev18 Connected to COM2 at 38 400 OPERATION MODE standby Mad C Measurement Mode Level Detection Mode C Pulse Detection Mode MEASUREMENT MODE za Xel 82 s 49 Yelle 45 45 6 Zcal 55 55 XYZ Outputs X8 0 X10 1 Y8 0 Y10 0 Z8 16 Z10 64 HWID 4901 SWID 490E B INT1 and INT2 registers INT1reg Level C INT1reg Pulse C INT1reg Single Pulse INT1 register INT2 register INT1 and INT2 pins INTipin DRDY INT 1pin INT treg INT2pin INT2reg C INT ipin INT2reg INT2pin INT reg INT2 reg Pulse INT2 reg Level INT2 reg Pulse LEVEL DETECTION MODE p PULSE DETECTION MODE Absolute OR gt C PosNeg C AND lt AND Pulse Duration Oms Latency Time ms Level Threshold 32 Pulse Threshold 0 2nd Pulse Oms L Flags DisableX Level Detect Condition Disable Y Disable Z IX AbsLevel or Y gt AbsLevel or Z gt AbsLevel xl Digital Filter Band 62 5Hz C 125Hz SelfTest XOUTL XOUTH YOUTL YOUTH ZOUTL ZOUTH XOUTS YOUT8 gt ZOUTS STATUS DETSRC TOUT I2CAD XOFFL XOFFH YOFFL YOFFH ZOFFL ZOFF
40. yteRead 0x08 Read Z Register for RESET WATCHDOG by default COP is disabled with device init When enabling also reset the watchdog loop forever please make sure that you never leave main Simple Delay Function void Delay byte count byte i for i 0 i lt count i 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 2 This is the I2C byte write function void IIC SingleByteWrite unsigned char reg unsigned char val IIC2C1 IICEN 1 IIC2Cl TX 1 IIC2C1 MST 1 Sends Start IIC2D IIC_WriteAdr lt lt 1 send the accelerometer address while IIC2S IICIF 0 Waiting for transmission to complete Delay 100 IIC2S8 IICIF 1 Delay 100 if IIC2S RXAK 1 IIC2C MST 20 send stop ACK 0 while 1 else ACK 1 IIC2D reg write in the register address while IIC2S IICIF 0 Waiting for transmission to complete IIC2S8 IICIF 1 if IIC2S RXAK 1 IIC2C1_MST 0 send stop ACK 0 while 1 else ACK 1 AN3468 Sensors Freescale Semiconductor 6 IIC2D val send the data to be written while IIC2S IICIF 0 Waiting for transmission to complete IIC2S IICIF 1 if IIC2S RXAK 1 IIC2C MST 0 send stop ACK 0 while 1 else ACK 1 IIC2C1 MST 0 writel 1 This is the I2C byte read function char IIC SingleByteRead unsigned char reg char data IIC2C1 IICEN 1 II
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