What is the most effecient way to read in data from a simple 2-wire (clock/data) source?

Question

I'm building a small device that will read in numerical data from a Mitutoyo indicator, record and chart this data on the screen. These indicators use an "SPC" protocol that has a single input "request" pin that once set low will cause the device to start sending a clock and data signals for a total of 52 bits. Explanation of the data stream can be found here

I've got a simple python script written that "waits" for the next pulse using a while loop waiting for the clock pin to go low before reading the next bit; similar to this arduino project I found. this sort-of works, but on occasion it seems as those it either misses a bit or accidentally reads a bit twice and then it gets out of sync and the whole thing stops working.

There must be a better way to do this. Is there a way to instruct the Pi that "this pin is a clock, expect 52 bits on the data line" and have it read in more efficiently. I need to read in pulses from a cycle counter and there are other functions that this Pi will need to perform in concert with taking the occasional readings from the indicator, I fear that once it starts doing other things at the same time the bad reads/sync issues will get even worse.

Answer 1

My pigpio library probably offers the most reliable way of reading such data.

Here is an example to monitor SPI traffic in Python.

http://abyz.me.uk/rpi/pigpio/examples.html#Python_SPI_mon_py

If you look at the callback function you could modify it for your SPC data stream.

#!/usr/bin/env python

# SPC.py
# 2016-12-04
# Public Domain

import time
import pigpio

REQ=5
CLK=6
DATA=7

# The callback implements the Mitutoyo SPC protocol
# 13 4-bit nibbles, least significant bit first.

def cbf(gpio, level, tick):
   global in_data, bit, nibble, nibbles, data_level
   if gpio == REQ:
      nibble = 0
      bit = 0
      nibbles = [0]*13
      in_data = True
   elif gpio == DATA:
      data_level = level
   else: # CLK
      if in_data:
         nibbles[nibble] |= (data_level << bit)
         bit += 1
         if bit > 3:
            bit = 0
            nibble += 1
            if nibble > 12:
               print(nibbles)
               in_data = False

pi = pigpio.pi()
if not pi.connected:
   exit()

in_data = False
bit = 0
nibble = 0
nibbles = [0]*13
data_level = pi.read(DATA)

cb1 = pi.callback(REQ, pigpio.FALLING_EDGE, cbf)
cb2 = pi.callback(CLK, pigpio.FALLING_EDGE, cbf)
cb3 = pi.callback(DATA, pigpio.EITHER_EDGE, cbf)

time.sleep(300)

cb1.cancel()
cb2.cancel()
cb3.cancel()

pi.stop()