rp2: Crash with hard pin IRQ
Measuring hard IRQ latency with this script:
from machine import Pin, PWM
from time import sleep_ms
import math
# Link 16-17, time 16-18
pin18 = Pin(18, Pin.OUT)
def callback(_):
pin18(1) # Trigger
pin18(0)
pin17 = Pin(17, Pin.IN)
_ = pin17.irq(callback, trigger=Pin.IRQ_RISING, hard=True)
pwm = PWM(Pin(16))
pwm.freq(1000)
pwm.duty_u16(0xffff // 2)
while True:
y = 0
for x in range(100):
y += math.sin(x * 2 * math.pi/100) # 6.2ms of busywork
sleep_ms(10)
The usual latency was a creditable 25μs, but I have measured 100μs. It was not practicable to measure a worst case because the the machine suffers a hard crash usually almost immediately. If I comment out the busywork, leaving just the sleep_ms(10), it runs indefinitely with about 20μs latency.
port/rp2: Make tickless, remove 1ms timeout when idle.
The main motivation for doing this was to reduce the latency when the system is woken by a USB interrupt. The best_effort_wfe_or_timeout() function calls into the pico-sdk dynamic timer framework which sets up a new dynamic timer instance each time, and then has to tear it down before continuing after a WFE.
Testing Python interrupt latency it seems to improved by about 12us (from average of 46us to 34us running a Pin IRQ). C-based "scheduled nodes" should see even lower latency.
Test code was:
import machine
from machine import Pin
i = Pin(2, Pin.IN, Pin.PULL_UP)
o = Pin(3, Pin.OUT)
def handler(_):
o(1)
print('!')
o(0)
i.irq(handler, Pin.IRQ_FALLING)
while True:
machine.idle()
With latency measured using logic analyzer:
(Channel 0 is "i", Channel is "o". Please excuse noisy CH0, was triggering with a button. Time 0 is the first falling edge of channel 0.)
The risk with this change is that it exposes cases where a driver expects execution to continue without an interrupt. Alternative is to enable the standard ARM SysTick interrupt to wake the CPU at tick intervals. (There is a branch with SysTick enabled and working, but testing shows tickless is just as functional.)
This work was funded through GitHub Sponsors.