Raspberry Pi Interrupts vs. Rotary Encoder

Thinking about using a rotary encoder to focus a Raspberry Pi lens led to a testbed:

RPi knob encoder test setup
RPi knob encoder test setup

There’s not much to it, because the RPi can enable pullup resistors on its digital inputs, whereupon the encoder switches its code bits to common. The third oscilloscope probe to the rear syncs on a trigger output from my knob driver.

I started with the Encoder library from PyPi, but the setup code doesn’t enable the pullup resistors and the interrupt (well, it’s a callback) handler discards the previous encoder state before using it, so the thing can’t work. I kept the overall structure, gutted the code, and rebuilt it around a state table. The code appears at the bottom, but you won’t need it.

Here’s the problem, all in one image:

Knob Encoder - ABT - fast - overview
Knob Encoder – ABT – fast – overview

The top two traces are the A and B encoder bits. The bottom trace is the trigger output from the interrupt handler, which goes high at the start of the handler and low at the end, with a negative blip in the middle when it detects a “no motion” situation: the encoder output hasn’t changed from the last time it was invoked.

Over on the left, where the knob is turning relatively slowly, the first two edges have an interrupt apiece. A detailed view shows them in action (the bottom half enlarge the non-shaded part of the top half):

Knob Encoder - ABT - fast - first IRQs
Knob Encoder – ABT – fast – first IRQs

Notice that each interrupt occurs about 5 ms after the edge causing it!

When the edges occur less than 5 ms apart, the driver can’t keep up. The next four edges produce only three interrupts:

Knob Encoder - ABT - fast - 4 edges 3 IRQ
Knob Encoder – ABT – fast – 4 edges 3 IRQ

A closer look at the three interrupts shows all of them produced the “no motion” pulse, because they all sampled the same (incorrect) input bits:

Knob Encoder - ABT - fast - 4 edges 3 IRQ - detail
Knob Encoder – ABT – fast – 4 edges 3 IRQ – detail

In fact, no matter how many edges occur, you only get three interrupts:

Knob Encoder - ABT - fast - 9 edges 3 IRQ
Knob Encoder – ABT – fast – 9 edges 3 IRQ

The groups of interrupts never occur less than 5 ms apart, no matter how many edges they’ve missed. Casual searching suggests the Linux Completely Fair Scheduler has a minimum timeslice / thread runtime around 5 ms, so the encoder may be running at the fastest possible response for a non-real-time Raspberry Pi kernel, at least with a Python handler.

If. I. Turn. The. Knob. Slowly. Then. It. Works. Fine. But. That. Is. Not. Practical. For. My. Purposes.

Nor anybody else’s purposes, really, which leads me to think very few people have ever tried lashing a rotary encoder to a Raspberry Pi.

So, OK, I’ll go with Nearer and Farther focusing buttons.

The same casual searching suggested tweaking the Python thread’s priority / niceness could lock it to a different CPU core and, obviously, writing the knob handler in C / C++ / any other language would improve the situation, but IMO the result doesn’t justify the effort.

It’s worth noting that writing “portable code” involves more than just getting it to run on a different system with different hardware. Rotary encoder handlers are trivial on an Arduino or, as in this case, even an ARM-based Teensy, but “the same logic” doesn’t deliver the same results on an RPi.

My attempt at a Python encoder driver + simple test program as a GitHub Gist:

# Rotary encoder test driver
# Ed Nisley - KE4ZNU
# Adapted from https://github.com/mivallion/Encoder
# State table from https://github.com/PaulStoffregen/Encoder
import RPi.GPIO as GPIO
class Encoder(object):
def __init__(self, A, B, T=None, Delay=None):
GPIO.setmode(GPIO.BCM)
self.T = T
if T is not None:
GPIO.setup(T, GPIO.OUT)
GPIO.output(T,0)
GPIO.setup(A, GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.setup(B, GPIO.IN, pull_up_down=GPIO.PUD_UP)
self.delay = Delay
self.A = A
self.B = B
self.pos = 0
self.state = (GPIO.input(B) << 1) | GPIO.input(A)
self.edges = (0,1,-1,2,-1,0,-2,1,1,-2,0,-1,2,-1,1,0)
if self.delay is not None:
GPIO.add_event_detect(A, GPIO.BOTH, callback=self.__update,
bouncetime=self.delay)
GPIO.add_event_detect(B, GPIO.BOTH, callback=self.__update,
bouncetime=self.delay)
else:
GPIO.add_event_detect(A, GPIO.BOTH, callback=self.__update)
GPIO.add_event_detect(B, GPIO.BOTH, callback=self.__update)
def __update(self, channel):
if self.T is not None:
GPIO.output(self.T,1) # flag entry
state = (self.state & 0b0011) \
| (GPIO.input(self.B) << 3) \
| (GPIO.input(self.A) << 2)
gflag = '' if self.edges[state] else ' - glitch'
if (self.T is not None) and not self.edges[state]: # flag no-motion glitch
GPIO.output(self.T,0)
GPIO.output(self.T,1)
self.pos += self.edges[state]
self.state = state >> 2
# print(' {} - state: {:04b} pos: {}{}'.format(channel,state,self.pos,gflag))
if self.T is not None:
GPIO.output(self.T,0) # flag exit
def read(self):
return self.pos
def read_reset(self):
rv = self.pos
self.pos = 0
return rv
def write(self,pos):
self.pos = pos
if __name__ == "__main__":
import encoder
import time
from gpiozero import Button
btn = Button(26)
enc = encoder.Encoder(20, 21,T=16)
prev = enc.read()
while not btn.is_held :
now = enc.read()
if now != prev:
print('{:+4d}'.format(now))
prev = now
view raw encoder.py hosted with ❤ by GitHub

8 thoughts on “Raspberry Pi Interrupts vs. Rotary Encoder

  1. Hmm – There’s a encoder DT overlay (rotary-encoder.dtbo). I wonder if that would solve the delay issue?

    1. Wholly Smoke! Sounds like exactly what’s needed!

      Having used evdev events with a remote keypad, there’s a good chance I can (figure out how to) make encoder events pop up in a program.

      Bonus: I also found the gpio-key overlay to convert the encoder knob switch into a useful keycode event.

      Many, many thanks!

      1. You’re more than welcome – I’ve gotten LOTS of great ideas from this blog! (Not to mention many new rabbit holes to explore)

  2. I wonder if a i2c or spi chip with a quadrature decoder and up/down counter would be a useful thingy.

    1. Some preliminary screwing around shows rotary-encoder.dtbo elegantly solves the entire problem: no need for more hardware. Hooray!

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