i2cioexpander inconsistencies and problems

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Évaluation

Difficulté
4/5
Temps estimé
3-5 jours
Accessibilité débutants
45/100
Type d'issue
Bug
Clarté
Plutôt claire
Activité
Calme
Stack technique
c
Domaine
embedded-iot

Piste de recherche

Start by reading reset_board_buses() in shared-module/board/__init__.c and compare its behavior with common_hal_i2cioexpander_ioexpander_construct() and common_hal_i2cdisplaybus_i2cdisplaybus_construct(). Then trace the CIRCUITPY_I2CIOEXPANDER defaults across ports. Done means the intended bus and pin reset behavior is consistent, including for external I2C devices and user-created expanders.

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Description

bug digitalio
CircuitPython version and board name
main
Code/REPL
n.a.
Behavior

There are a number of inconsistencies across ports regarding the i2cioexpander module. Also, the behavior is not as intended:

  1. CIRCUITPY_I2CIOEXPANDER defaults to 0 for all ports except raspberrypi. That port sets the value to =1 (instead of ?=1), so it is impossible to override it in a mpconfigboard.mk file. While this sounds like just another module for the raspberrypi port, it also changes behavior as a side effect. Setting CIRCUITPY_I2CIOEXPANDER prevents resetting the board I2C bus (see reset_board_buses() in shared-module/board/__init__.c. So none of the raspberrypi boards reset the board I2C bus, while all espressif boards except the yotas do the reset.
  2. The coding in reset_board_buses() has two additional problems. It silently assumes that the ioexpander is an internal peripheral and not just a normal external i2c device. This assumption can be wrong.
  3. Major problem: not resetting the bus in this way does not prevent resetting the SCL/SDA pins. So although the bus is not explicitely reset, it is still unusable after the cleanup routines from main.c.

(1) is easy to solve.
(2) is debateble: treating ioexpanders as something special is possible, but the current implementation treats them differently if they are not on the board I2C bus.
(3) common_hal_i2cioexpander_ioexpander_construct() could call common_hal_busio_i2c_never_reset(), like common_hal_i2cdisplaybus_i2cdisplaybus_construct() does. As an alternative, this can be added to all boards that create an ioexpander and treat all ioexpander instances created from user code as "normal" i2c devices.

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Additional information

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Langage dominant
C
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