Struggling to use SCD41 sensors with ESP32 and an I2C multiplexer

[graceful-failure]

Hello there, as my first micropython project I'm trying to create a set of sensors to monitor my building ventilation system.
I've got a pair of I2C SCD41 sensors (one for air intake, one for exhaust) which I'm trying to run on a firebeetle ESP32-E v1.0 using micropython V1.20.0.
I've found this driver for SCD4x sensors which works fine with them on their own but they both have the same address. The firebeetle only supports one I2C bus and won't allow I2C on other pins than the default, so I've got to use a multiplexer and I just can't get it going.

I have been able to test the sensors individually and they work OK. I've also been able to plug them into the multiplexer in different pins and detect them so I'm happy that's working. I've re-done the breadboard wiring a few times now...

Attempt 1 at multiplexing was with this method, sending the channel instructions to the multiplexer then trying to talk to the SCD41.

from machine import Pin
from machine import I2C
from time import sleep

import scd4x

i2c = I2C(0, sda=Pin(21), scl=Pin(22), freq=100000)
i2c.writeto(0x70, b'\x80') #setting the multiplexer channel here
sensor = scd4x.SCD4X(i2c)
print("Sensor detected: ", [hex(i) for i in sensor.serial_number])
print("Resetting sensor.")
sensor.factory_reset()
#print("Reset complete")
#sensor.self_test() #Gives OSError: [Errno 116] ETIMEDOUT
print("Starting measurement.")
sensor.start_periodic_measurement()
while True:    
    if sensor.data_ready:
        print("Result")
        print(sensor.temperature)
        print(sensor.relative_humidity)
        print(sensor.CO2)
    else:
        try:
            print(sensor.serial_number)#This function won't work if the sensor is on and running in periodic_measurement mode
            print("Sensor has switched off. Switching on.")
            sensor.start_periodic_measurement()
            print("Sensor restarted")
            i2c.scan()
        except:
            pass
    sleep(1)

In the REPL I get:

Sensor detected:  ['0xa1', '0x85', '0xb7', '0x7', '0x3b', '0xc1'] #so it can interrogate the sensor.
Resetting sensor.
Starting measurement.
Warning: exception chaining not supported #Expecting this, means the sensor is doing periodic measurement and won't give a serial number
Warning: exception chaining not supported
Warning: exception chaining not supported
Warning: exception chaining not supported
(161, 133, 183, 7, 59, 193) #Being able to output a serial number means measurement has stopped?
Sensor has switched off. Switching on.
Sensor restarted
Warning: exception chaining not supported #Back in business...
Warning: exception chaining not supported
(161, 133, 183, 7, 59, 193) #Again?
Sensor has switched off. Switching on.
Sensor restarted
Warning: exception chaining not supported #And this repeats. Every 3 loops the sensor stops measuring.
Warning: exception chaining not supported
(161, 133, 183, 7, 59, 193)

Attempt 2 was with the code from the last post in this thread on the old micropython forum.

import tca9548a_test #the I2C gets started up in this thing
import scd4x
from time import sleep

tca=tca9548a_test.TCA9548A()

tca.switch_channel(7)
sensor=scd4x.SCD4X(tca.bus)
print("sensor detected: ", [hex(i) for i in sensor.serial_number]) #check we're seeing a sensor
sensor.factory_reset()
#sensor.self_test() #Gives OSError: [Errno 116] ETIMEDOUT
sensor.start_periodic_measurement()

while True:
    if sensor.data_ready:
        print("Temperature", sensor.temperature)
        print("Humidity", sensor.relative_humidity)
        print("CO2", sensor.CO2)
    else:
        print("Waiting")
    sleep(0.5)

And in the REPL I get:

sensor detected:  ['0xa1', '0x85', '0xb7', '0x7', '0x3b', '0xc1'] #OK we can see the sensor then.
Waiting
Waiting
Waiting #But we can't read values from it?

Help! I've run out of ideas. There's a bit more example code for this multiplexer in C, but I'd set out to use this as an opportunity to learn python and I'd like to stick to that.

[2dof]

Try perform some tests using directly I2C commands (i2c.writeto() and i2c.readfrom() etc like from micropython-tca9548a without using lib/driver. It allow directly test hardware , then if all OK , You will know that driver fails.

Based in linked example You can write You own driver , also remember about physical limitation of i2C bus length vs speed and at beginning test all in shot length wiring (to eliminate/minimize physical limitation errors)

[graceful-failure]

I can do writeto/readfrom on the multiplexer with good results but the sensors are more difficult than I can manage.
That said, using code from attempt 1 (i.e. writeto/readfrom on multiplexer to select channel rather than using its driver) I can successfully select channels then use the SCD4x driver to carry out writeto/readfrom commands and get the serial numbers of connected sensors.

So I can happily work the SCD4x sensor through the multiplexer for short commands like serial_number (1ms) or factory_reset (1200ms), but nothing long duration like a self test (10secs) or ongoing temperature monitoring.
I can do those long duration commands using the SCD4x driver when it's not on the multiplexer.
It's on a breadboard with short wiring at the mo.

[robert-hh]

You could use SoftI2C, which works with every GPIO pin pair.

[graceful-failure]

I think I've tried SoftI2C on the firebeetle earlier and I couldn't get that or HardI2C to work on any pins other than default 21 and 22. My guess was that there's other outputs on the board labelled SCL and SDA. If they're connected to 21 and 22 then changing the I2C pins would make the other outputs behave in an odd way.

Either way, SoftI2C worked where HardI2C did not. I can now read one of the two SCD4x sensors via the multiplexer, which is something to build on. Thank you.

[robert-hh]

b.t.w: Did you close the solder bridges at the multiplexer breakout for the pull-up resistors on SDA and SCL? Otherwise I2C is not reliable.

[graceful-failure]

The pull-up resistors for the adafruit multiplexer are enabled by default and the sensors I have are on breakout boards with their own pull-ups.