Use Python to get state of combustion engine based

Question

Working to create a data acquisition system for a custom off-road vehicle. Using Raspberry Pi and a custom tachometer (tested and confirmed working) to measure RPM. Using interrupts in the following code to get RPM value.

def get_rpm():                                         
    GPIO.wait_for_edge(17, GPIO.FALLING)
    start = time.time()
    GPIO.wait_for_edge(17, GPIO.FALLING)
    end = time.time()
    duration = end - start
    rpm = (1/duration)*60
    return rpm

This code only works if the engine is running and producing a spark. If there is no spark, the code sits waiting for that edge and does not proceed. When calling get_rpm(), if the code is waiting for an edge, this causes other processes to hang.

My intended workaround for this is to get the state of the engine in another process. I think it will work best in two parts.

Part 1, running (looped) in a separate thread:

GPIO.wait_for_edge(17, GPIO.RISING)
last = time.time

Part 2, running called as a function as needed:

def get_state():
    while time.time - last < .5:
        engine_state = true
    else:
        engine_state = false
    return engine_state

With Part 1 saving last to memory accessible to Part 2, Part 2 will determine whether or not the car is running based on the last time the spark plug sparked. Using engine_state as a comparator, the data acquisition system will get and store the RPM value from get_rpm() only when engine_state is true.

How can I implement Part 1 in such a way that I can use the last variable in Part 2? last will be changing very, very quickly. I don't want to store it to a text file on the Raspberry Pi's SD card every time last is updated. I want to store last in RAM.

Thanks so much!

Answer 1

This is just for inspiration. I don't have my Pis at hand so this is written blindly from memory.

import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN)
# the line below is important. It'll fire on both rising and falling edges
# and it's non-blocking
GPIO.add_event_detect(17, GPIO.BOTH, callback=callback_func)

length_of_last_high = 0
length_of_last_low = 0

last_rise = 0
last_fall = 0
last_callback = 0

def callback_func(pin):
    # all of these global variables aren't needed 
    # but I left them in for demo purposes
    global last_rise
    global last_fall
    global last_callback
    global length_of_last_high
    global length_of_last_low
    last_callback = time.time()
    if GPIO.input(17):
        print "Pin 17 is rising!"
        length_of_last_high = last_callback - last_rise
        last_rise = last_callback
    else:
        print "Pin 17 is falling!"
        length_of_last_low = last_callback - last_fall 
        last_fall = last_callback


# last passed as parameter (the preferred solution if it works).
# You test it - I can't at the moment
def get_state(last=last_rise):
    engine_state = False
    if time.time() - last < .5:
        engine_state = True
    return engine_state

# last as global variable. 
# This works too but global is bad practice if it can be avoided     
# (namespace littering)
def get_state2():
    global last_rise
    engine_state = False
    if time.time() - last_rise < .5:
        engine_state = True
    return engine_state


def main():
    while True:
        print "Not blocking! This loop could sleep forever. It wouldn't affect the readings"
        time.sleep(1)
        print get_state()

"""
GPIO.cleanup()
"""