PIO_4Intervals_Timer.py

'''
Measure time intervals on input pulses, using PIO state machine hardware
RP2040 CPU clock at 250 MHz
J.Beale 15-March-2021
'''

import utime
import rp2
from rp2 import PIO, asm_pio
from machine import Pin
    
'''
              ______       _____      _____
pin 1 _______/      \_____/     \____/    
             A      B     C     D    E
             
  Time A-B, A-C, A-D, and A-E intervals:
  Start counting on rising edge A
  report timer values at edges B, C, D, and E
  
'''    
    
# Assembly code for RP2040 PIO hardware state machine
@asm_pio()
def PULSE_LOW_DELTA():    
    
    # Initialize x to be -1
    set(x,0)
    jmp(x_dec,'START')
    label('START')

    # Syntax: wait(Polarity,Source,Index)
    # https://datasheets.raspberrypi.org/rp2040/rp2040-datasheet.pdf#section_pio
    wait(0,pin,0)     # wait for pin1 to be low (if isn't already)
    wait(1,pin,0)     # wait for pin1 to be high: first rising edge

    # Here, just had rising edge; pin1 is high
    # Now, dec x while pin1 remains high
    label('loop')
    jmp(x_dec,'continue')  # decrement X, jump if zero
    label('continue')
    jmp(pin, 'loop')       # jump when 'pin' (config as pin1=GPIO16) is high 

    mov(isr,x) # transfer X to input shift register
    push()  # transfer shift register to FIFO

    # here, just had falling edge. pin1 is low
    # dec x until pin1 goes high
    label('loop2')
    jmp(pin, 'exit')       # jump when 'pin' (config as pin1=GPIO16) is high        
    jmp(x_dec,'loop2')     # decrement X, jump if zero
    label('exit')

    mov(isr,x) # transfer X to input shift register
    push()  # transfer shift register to FIFO
    
    # Here, just had 2nd rising edge; pin1 is high
    # Now, dec x while pin1 remains high
    label('loop3')
    jmp(x_dec,'cont2')  # decrement X, jump if zero
    label('cont2')
    jmp(pin, 'loop3')       # jump when 'pin' (config as pin1=GPIO16) is high 

    mov(isr,x) # transfer X to input shift register
    push()  # transfer shift register to FIFO
    
    # here, just had 2nd falling edge; pin1 is low
    # dec x until pin1 goes high
    label('loop4')
    jmp(pin, 'exit2')       # jump when 'pin' (config as pin1=GPIO16) is high        
    jmp(x_dec,'loop4')     # decrement X, jump if zero
    label('exit2')

    # Now report value to CPU via FIFO, with push
    mov(isr,x) # transfer X to input shift register
    push()  # transfer shift register to FIFO
    
    label('End')
    jmp('End')  # stop everything

# regular Python code follows

class pulsedelay:
    
    def __init__(self,pin1,pin2, stateMachine=0):
        self.pin1 = pin1
        self.pin2 = pin2
        self.sm= rp2.StateMachine(stateMachine)

    def get(self):
        self.sm.init(PULSE_LOW_DELTA,freq=250000000,
                     in_base=(self.pin1),
                     jmp_pin=(self.pin2))
        
        ''' in_base declare the first Pin offset
            jmp_pin declare the pin to use for jmp (this is not an offset)
        '''
        self.sm.active(1)
        return (0xffffffff - self.sm.get()) # units of raw counter ticks, T = 1/freq

    def read_blocking(self, n):
        self.sm.init(PULSE_LOW_DELTA,freq=250000000,
                     in_base=(self.pin1),
                     jmp_pin=(self.pin2))
        
        ''' in_base declare the first Pin offset
            jmp_pin declare the pin to use for jmp (this is not an offset)
        '''
        self.sm.active(1)
        data = []
        for i in range(n):
            data.append(0xffffffff - self.sm.get() )
        return data

# -----------------------------------------
if __name__ == "__main__":

    import machine as m
    
    m.freq(250000000) # overclock to 250 MHz. Wheee!
    #m.freq(125000000) # standard clock speed

    # nominal timings from 1-PPS, 10% duty pulse
    nomInt = (12500000, 125000000, 137500000, 250000000)
    
    p1 = m.Pin(16,m.Pin.IN, m.Pin.PULL_UP) # Channel A input    
    led1 = m.Pin(25, m.Pin.OUT)              # set pin 25 (driving onboard LED) to output
    led2 = m.Pin(22, m.Pin.OUT)              # set external output pin (driving offboard LED) to output
    led1.off()
    led2.off()

    pulsein = pulsedelay(p1,p1) 
    
    rSum = 0  # running sum of interval deltas
    while True:
        newVals = pulsein.read_blocking(4)  # get interval times
        i = 0
        for n in newVals:
            delta = nomInt[i] - (n+i) # +i correction due to mov,push overhead
            print("%d" % delta,end="")
            if i < 3:
                i += 1
                print(", ",end="")
        print()
        led1.toggle()

# Note: Timer Period = 2/(250 MHz) so time in microseconds = newVal / (125) 

# Sample output with 1-PPS 10% duty cycle input:
# -1, 3, 1, 5
# -1, 3, 1, 6
# -1, 3, 2, 6
# -1, 3, 2, 6
# -1, 3, 1, 6
# -1, 3, 2, 6