Polling MMA8452Q Accelerometer Data over I2C

Question

I am a newbie at using i2C and working with registers and reading and writing bits. I am trying to create python program that will use an MMA8452Q accelerometer to measure the vibration produced by a device I am testing. I have managed to get a program that spits out some data, but I am not getting all the data. I am pretty sure I am being limited by how I have the code set up.

After reading through the spec sheet, I know I can set the data rate up to 800Hz and that I can poll the accelerometer to tell me when new data is available. When the status of the ZYXDR bit changes new data is available but I don't know how to monitor the status of that bit.

The read commands for I2C over SMBus seem to be reading the whole register at one time.

read_byte_data(addr,cmd)

How do I know that just one bit has changed?

Answer 1

Bit-Masking

For an explanation see: https://stackoverflow.com/questions/10493411/what-is-bit-masking

If we assume that the ZYXDR is bit 2 then we construct a mask in python by defining a binary literal, then we use the bitwise-and (python wiki) to extract that bit of data.

ZYXDR_MASK = 0b00000100
val = read_byte_data(add,cmd)
if (val & ZYXDR_MASK): 
  print "zyxdr bit set"
else:
  print "zyxdr bit not set"

Typically, when writing a driver or interface code, one defines many constants representing the masks and bit shifts

There are alternatives which require other libraries, you may consider using bitstring