MMA8451 on Raspberry Pi returns incorrect whoami

[tsnealon]

I'm currently working on connecting and reading an MMA8451 with a raspberry pi running C code. I've got the code up and running, and i can't find any reason why the accelerometer should be returning an incorrect whoami (currently sending 0xff). Here is the code i use to initialize the accelerometer:

mma8451 mma8451_initialise(int device int addr)
	{
	mma8451 handle;
	handle.file = -1;
	handle.address = addr;
	char buf[15];
	printf("Initalising MMA8451 sensor at address %#02x\n", addr);
	//Open /dev/i2c-x file without a buffer
	sprintf(buf, "/dev/i2c-%d", device);
	if ((handle.file = open(buf, O_RDWR)) < 0)
	{
	handle.file = -2;
	return handle;
	}
	//Configure slave i2c address via ioctl
	if(ioctl(handle.file, I2C_SLAVE, addr) < 0)
	{
	handle.file = -3;
	return handle;
	}
	//Check if we read correctly from the sensor
	char whoami = mma8451_read_byte(&handle, 0x0D);
	printf("whoami read %#02x\n", whoami);
	//Undefined behavior for the rest of device operation if the device is not returning hex 1A
	if(whoami != 0x1A) perror("mma451_pi warning: Device correctly intialized but not returning 0x1A at WHO_AM_I request.\n"
	"Are you sure you are using a MMA8451 accelerometer on this address?");
	//Send reset request
	mma8451_write_byte(&handle, 0x2B, 0x40);
	printf("Waiting for accelerometer to be reset\n");
	while(mma8451_read_byte(&handle, 0x2B) & 0x40); //reset done
	printf("Done\n");
	mma8451_set_range(&handle, 2);
	mma8451_write_byte(&handle, 0x2B, 0x02); //high resolution mode
	mma8451_write_byte(&handle, 0x2A, 0x01 | 0x04); //high rate low noise
	//Deactivate fifo
	mma8451_write_byte(&handle, 0x09, 0);
	//turn on orientation configuration
	mma8451_write_byte(&handle, 0x11, 0x40);
	printf("MMA8451 at address %#02x configured for real time sampling, in high rate, low noise mode, at high resolution, on a 2G max range\n", addr);
	return handle;
	}

This is directly from the mma8451_pi library found here: https://github.com/Ybalrid/mma8451_pi

Any ideas what could be causing it to return an incorrect whoami?

[TomasVaverka]

Hi Tim,

If you have an oscilloscope or a logic analyzer, can you please post here a plot illustrating what is going on the bus when you read the WHO_AM_I register?

Also please make sure you are using correct slave address depending on the connection of the SA0 pin.

Best regards,

Tomas