In serial, recover poorly from glitch/error/...

[dglaude]

I tested this library with a serial sensor like this: https://www.adafruit.com/product/3686

Within one or two minutes, the code fail (mostly on if b != 0x42:) and does not seems to recover. Some tricks to empty the buffer, re-read or reset should be implemented for more reliability.

The code from the learn guide ( https://learn.adafruit.com/pm25-air-quality-sensor/python-and-circuitpython ) of the product behave much better, even when temporary removing the cable.

Here is the way it fail (and never seems to recover):

Concentration Units (standard)

PM 1.0: 10 PM2.5: 13 PM10: 13
Concentration Units (environmental)

PM 1.0: 10 PM2.5: 13 PM10: 13

Particles > 0.3um / 0.1L air: 1938
Particles > 0.5um / 0.1L air: 518
Particles > 1.0um / 0.1L air: 62
Particles > 2.5um / 0.1L air: 4
Particles > 5.0um / 0.1L air: 0
Particles > 10 um / 0.1L air: 0

Unable to read from sensor, retrying...
Unable to read from sensor, retrying...
Unable to read from sensor, retrying...

[ladyada]

@dglaude please try and submit a PR if you have some techniques that could be used!

[dglaude]

I proposed #6 and it does improve greatly the stability for me, and this can be tested by plugging and unplugging the cable.

My test code is based on the example but more verbose on exception.
I run it on EnviroPlus FeatherWing and would live to see this tested on some other hardware.

import time
import board
import busio
from digitalio import DigitalInOut, Direction, Pull
import adafruit_pm25

reset_pin = None
uart = busio.UART(board.TX, board.RX, baudrate=9600)
pm25 = adafruit_pm25.PM25_UART(uart, reset_pin)

print("Found PM2.5 sensor, reading data...")

while True:
    time.sleep(1)

    try:
        aqdata = pm25.read()
        # print(aqdata)
    except Exception as exception:
        print("Unable to read from sensor, retrying...", exception)
        continue

    print(
        "PM 1.0: %d\tPM2.5: %d\tPM10: %d"
        % (aqdata["pm10 standard"], aqdata["pm25 standard"], aqdata["pm100 standard"])
    )

And give this kind of result for various error.

main.py output:
Found PM2.5 sensor, reading data...
PM 1.0: 17 PM2.5: 26 PM10: 28
PM 1.0: 17 PM2.5: 26 PM10: 28
PM 1.0: 17 PM2.5: 24 PM10: 28
...
PM 1.0: 17 PM2.5: 26 PM10: 29
Unable to read from sensor, retrying... Invalid PM2.5 checksum
PM 1.0: 17 PM2.5: 26 PM10: 28
PM 1.0: 17 PM2.5: 26 PM10: 28
...
PM 1.0: 18 PM2.5: 25 PM10: 27
PM 1.0: 17 PM2.5: 22 PM10: 25
Unable to read from sensor, retrying... Unable to read from PM2.5 (no start of frame)
PM 1.0: 3 PM2.5: 3 PM10: 3
PM 1.0: 1 PM2.5: 1 PM10: 1
PM 1.0: 4 PM2.5: 7 PM10: 7

[dglaude]

Maybe @Gadgetoid will want to have a look to see if enhancement from https://github.com/pimoroni/pms5003-circuitpython can be introduced here. But #6 solve the issue for me.