I2C pins get in bad state if you read too quickly

[dhalbert]

(I am an Adafruit engineer.)

A user encountered a problem with the Tiny Code Reader, which seems to be caused by reading too fast. There is a long and somewhat confusing thread here: https://forums.adafruit.com/viewtopic.php?t=208488. However, I've managed to reproduce the problem, with the sample code below. If you do reads quickly, with no delays in between, and then hard reset the controlling microcontroller, the TCR is somehow leaving the I2C pins in a bad state, with at least one pulled low, or is continuing to send data which causes the pins to be intermittently low.

Here is a reproducer, which should work on pretty much any CircuitPython board. I tested with several QT Py's, including RP2040, ESP32-S2, ESP32-S3, and SAMD21. There will be no error when the board is first plugged in. Then, if you press RESET on the board, the second time through, board.I2C() will complain about "No pull up". The TCR is still running and is blinking the blue LED as usual.

import board
import time

# Print state several times with delay so we can see it even if we reset.
for i in range(1, 5):
    try:
        i2c = board.STEMMA_I2C()
        print(i, "i2c bus OK")
    except RuntimeError as e:
        print(i, e)
    time.sleep(1)

while not i2c.try_lock():
    pass

# DELAY of 1 second is OK; DELAY of 0 is not.
DELAY = 0

print('Press RESET to cause potential "No pull up" failure.')
print("Reading repeatedly with a delay of", DELAY, "seconds")

while True:
    print(".", end="")
    read_data = bytearray(256)
    i2c.readfrom_into(0x0c, bytearray(256))
    time.sleep(DELAY)

Sample output from a QT Py SAMD21, using tio as the terminal program, with annotations:

[tio 17:07:11] Disconnected
          [<------------------- Plug in board here]
[tio 17:15:43] Connected
2 i2c bus OK
3 i2c bus OK
4 i2c bus OK
Press RESET to cause potential "No pull up" failure.
Reading repeatedly with a delay of 0 seconds
..............................................
          [<------------------- Press RESET here]
[tio 17:15:47] Disconnected
[tio 17:15:50] Connected
3 No pull up found on SDA or SCL; check your wiring   [<---- board.I2C() is failing]
4 No pull up found on SDA or SCL; check your wiring
Traceback (most recent call last):
  File "code.py", line 14, in <module>
NameError: name 'i2c' is not defined     [<------- error due to board.I2C() instantiation failure]

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Adafruit CircuitPython 8.2.10 on 2024-02-14; Adafruit QT Py M0 with samd21e18
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