Hi,

I have a problem getting SPI communication to work right on the ADXL375z breakout board: Sensor seems to communicate, but only delivers nonsense-values. Examples:
- When registers are written at setup, they all show an eqal, wrong value (most of the time 0xE2 or 0x03. Mostly 0x00 after board reset.)
- Sensor values show 0x00, with occastional jumps to random value (independent of accelleration)
See attachted Screenshot of serial trace of register readback and values (registers marked yellow).

Some more facts:
- Sparkfun ESP32 board with Arduino (using ESP32 SPI Lib)
- Electrical connection/soldering verified --> is OK. VCC and VIO are OK.
- Tried a second ADXL375z --> same problems occur (the first one showed weired SPI signals on the oscilloscope, so I thought it might be damaged and bought a second one).
- SPI frequency is 2 MHz
- Exact same setup (wiring and code) works perfectly with an ADXL345 connected!

Is there anything special, maybe in addition to what is listed in the datasheet, that causes this problem?
I am thankful for any hints or tips for further investigation!


The SPI setup code on Arduino looks like this:

void start_SPI_Measurement () { 
  delay(500);
  digitalWrite(CS, HIGH); //Before communication starts, the Chip Select pin needs to be set high (disable). 
  delay(10);
  pinMode(CS, OUTPUT); //Set up the SPI Chip Select pin to be an output from the Arduino. 
  delay(500);  
  //Initiate an SPI communication instance.
  SPI.begin();
  //Configure the SPI connection for the ADXL345.
  SPI.beginTransaction(SPISettings(i_SPI_freq, MSBFIRST, SPI_MODE3)); 
  delay(100);  
// pullup CS am Sender ca. 5-10K (Kein Low außer aktiv runter gezogen)
// delays nach jedem write, gemäß Spec
// daten mind. 5 ns vor Clock-Wechsel anliegend  Config SPI lib
  // Set data-format
  writeRegister(0x31, 0x0B); // #00001011 // volatile char DATA_FORMAT = 0x31;
  delay(10);
  writeRegister(0x2C, SAMPLE_RATE );  // Sample-rate of sensor  i_sample_rate //  1600 = 0x0E im Code gesetzt wegen Arduino
  delay(10);
  writeRegister(0x1E, 0); // i_x_offset
  delay(10);  
  writeRegister(0x1f, 0); // i_y_offset
  delay(10);  
  writeRegister(0x20, 0); // i_z_offset
  delay(100);  
  //Put the ADXL3x5 into Measurement Mode by writing 0x08 to the POWER_CTL register 0x2D
  writeRegister(0x2D, 0x08);  //Measurement mode   
  delay(100);
#ifdef DEBUG
  // Readback registers for test
  readRegister(0x31, 1, values);
  Serial.print("DATA_FORMAT: "); 
  Serial.println(int(values[0]), HEX);
  //Serial.print(" ");
  delay(1);
  readRegister(0x2c, 1, values);
  Serial.print("0x2C: ");
  Serial.println(int(values[0]), HEX);
  delay(1);
  readRegister(0x1e, 1, values);
  Serial.print("0x1E: ");  
  Serial.println(int(values[0]), HEX);
  delay(1);
  readRegister(0x1f, 1, values);
  Serial.print("0x1F: ");  
  Serial.println(int(values[0]), HEX);
  delay(1);
  readRegister(0x20, 1, values);
  Serial.print("0x20: ");  
  Serial.println(int(values[0]), HEX);
  delay(1);
  readRegister(0x2D, 1, values);
  Serial.print("POWER_CTL: ");  
  Serial.println(int(values[0]), HEX);
  delay(1);
#endif
  // Code for timer interrupt 
  /* Use 2st timer of 4 */
  /* 1 tick take 1/(80MHZ/80) = 1us so we set divider 80 and count up */
  timer_s = timerBegin(2, 80, true);
  /* Attach onTimer function to our timer */
  timerAttachInterrupt(timer_s, &onTimer, true);
  /* Set alarm to call onTimer function every second 1 tick is 1us
  => 1 second is 1000000us */
  /* Repeat the alarm (third parameter) */
  timerAlarmWrite(timer_s, TIMER_INTERVAL, true);  //initial 625 us = 1600 Hz sample rate   
  /* Start an alarm */
  timerAlarmEnable(timer_s);
  #ifdef DEBUG
  Serial.println("SPI initiated, Measurement started");  
  #endif
}


// ************************************
// *** Utilities
// ************************************

//This function will write a value to a register on the ADXL345.
//Parameters:
//  char registerAddress - The register to write a value to
//  char value - The value to be written to the specified register.
void writeRegister(char registerAddress, char value){
  //Set Chip Select pin low to signal the beginning of an SPI packet.
  digitalWrite(CS, LOW);
  //Transfer the register address over SPI.
  SPI.transfer(registerAddress);
  //Transfer the desired register value over SPI.
  SPI.transfer(value);
  //Set the Chip Select pin high to signal the end of an SPI packet.
  digitalWrite(CS, HIGH);
}

//This function will read a certain number of registers starting from a specified address and store their values in a buffer.
//Parameters:
//  char registerAddress - The register addresse to start the read sequence from.
//  int numBytes - The number of registers that should be read.
//  char * values - A pointer to a buffer where the results of the operation should be stored.
void readRegister(char registerAddress, int numBytes, volatile unsigned char * values){
  //Since we're performing a read operation, the most significant bit of the register address should be set.
  char address = 0x80 | registerAddress;
  //If we're doing a multi-byte read, bit 6 needs to be set as well.
  if(numBytes > 1)address = address | 0x40;
  //Set the Chip select pin low to start an SPI packet.
  digitalWrite(CS, LOW);
  //Transfer the starting register address that needs to be read.
  SPI.transfer(address);
  delayMicroseconds(10);
  //Continue to read registers until we've read the number specified, storing the results to the input buffer.
  for(int i=0; i<numBytes; i++){
  values = 0;
  values = SPI.transfer(0x00);
  //delayMicroseconds(10);
  }
  //Set the Chips Select pin high to end the SPI packet.
  digitalWrite(CS, HIGH);

Moderator edit: added code tags

 

Do you have access to an oscilloscope for trouble-shooting and debugging purposes?

 

Do you have access to an oscilloscope for trouble-shooting and debugging purposes?

I measured the first one: clear SCL, MOSI, CS at sensor side, with either short and long wires. MISO just produced some ultrashort pulses, this why I thought it was damaged.
I will measure the second 375 the next days and post some pictures.
What I find strange, that exact setup works with the 345 sensor. Could it be some startup thing, that is not dedcribed in the 375 data sheet?