I’m trying to read accelerometric data from the evaluation board EVAL-ADXL355-PMDZ. The board is connected to a raspberry pi 4 that runs on raspbian.

The circuit is cabled into the standard SPI pins (4-wires) in the raspberry pi but the chip selection (CS) is connected into one of the free pins (12).

To test the board I wrote the following code that uses spidev to make the comunication with the chip and RPi.GPIO to make the chip selection:

#!/usr/bin/env python3.7

import time
import spidev
import RPi.GPIO as gpio

pin = 12

gpio.setwarnings(False) # stop warnings when the script runs multiple times
gpio.setmode(gpio.BCM)
gpio.setup(pin, gpio.OUT)

spi = spidev.SpiDev()
spi.open(0,0)
spi.mode = 3
spi.max_speed_hz = 5000000

READBIT = 0x01
WRITEBIT = 0x00

def check_adxl355(pin):
    '''gets true if DEVID_MST is 0x1D'''
    address  = 0x01 << 1 | READBIT
    gpio.output(pin, gpio.LOW)
    id_ = spi.xfer2([address ,0])
    gpio.output(pin, gpio.HIGH)
    return id_[1] == 0x1D

def configure_adxl355(pin):
    '''configure the accelerometer '''
    RANGE = 0x2C << 1 | WRITEBIT
    gpio.output(pin, gpio.LOW)
    o_ = spi.xfer2([RANGE, 0x01]) # RANGE_2G
    gpio.output(pin, gpio.HIGH)

    POWER_CTL = 0x2D << 1 | WRITEBIT
    gpio.output(pin, gpio.LOW)
    o_ = spi.xfer2([POWER_CTL, 0x06]) 
    gpio.output(pin, gpio.HIGH)

print("ADXL355 : {}".format(check_adxl355(pin)))
configure_adxl355(pin)

# read data from the ADXL355
AXIS_START = 0x08 << 1 | READBIT
while 1:
    gpio.output(pin, gpio.LOW)
    axisBytes = spi.xfer2([AXIS_START, 0, 0, 0, 0, 0, 0, 0, 0, 0])[1:] # read 9 bytes
    gpio.output(pin, gpio.HIGH)

    X = (axisBytes[0] << 16 | axisBytes[1] << 8 | axisBytes[2]) >> 4
    Y = (axisBytes[3] << 16 | axisBytes[4] << 8 | axisBytes[5]) >> 4
    Z = (axisBytes[6] << 16 | axisBytes[7] << 8 | axisBytes[8]) >> 4

    print(">> {} {} {}".format(X, Y, Z))

    time.sleep(1)

 

In general what it does is to configure the communication between the accelerometer and the raspberry pi, check if the ADXL355 is connected to the pin 12 by locking at the the mems id register (check_adxl355), configure the range of the accelerometer (configure_adxl355) and reading samples from the FIFO register. When running the previous code I have as the results:

pi@raspberrypi:~/adxl355spi $ ./adxl355.py
ADXL355 : True
>> 0 0 0
>> 0 0 0
...

the first block looks ok as it reports that the accelerometer was found, after that the configuration of the accelerometer is not checked, and finaly the the block that read the accelerometric data (X, Y, Z) returns only 0 even though the accelerometer is moving.

Can anyone spot a problem when reading the accelerometric data?