Troublesome chips:

Some sensors or chips have non-standard behavior that causes issues when trying to use I2C. Here's a few of the ones to watch for

• AGS20MA - Use a bus speed of 20-30 kHz.
• ATECCx08 - Use slow-speed I2C to get out of sleep mode.
• BNO055, BNO085 - Uses clock stretching, violates I2C protocol timing in some caes, and sometimes needs to be reset. Does not work well on i.MX RT10xx chips, ESP32, ESP32-S3.
• CCS811 - Uses clock stretching.
• LC709203F - Repeated start, clock stretching, sleep mode
• MCP9600 (date codes 1845 or before) - bug: duplicate data from register. reads, perhaps due to clock stretching.
• MCP9600, MCP9601 - Repeated start, clock stretching. Often will not respond to zero-length writes, so scanning the I2C bus to find the device can fail.
• PN532 - Clock stretching.

If you're using Raspberry Pi with these chips, check out our guide on how to work-around clock stretching.

Troublesome microcontrollers:

• MicroChip Atmel SAMD21, SAMx5x - I2C bus frequency below 95 kHz not available in CircuitPython when I2CTarget is drive with a 48 MHz clock, which is typical. CircuitPython checks for out-of-range bus frequencies.

• Espressif ESP32-S3 - Use software that uses Espressif ESP-IDF v5.0.0 or later. When using versions of the Espressif ESP-IDF SDK older than v5.0.0, ESP32-S3 has problems with I2C devices that do clock stretching or have other unusual timing behavior. ESP-IDF is the underlying software used by CircuitPython and the Arduino esp32 board support package. CircuitPython 9.0.0 and later and Arduino esp32 v5.0.0 and later use ESP-IDF v5 and later. These versions support most I2C devices well. However, some of the troublesome chips listed above still don't work well or at all even with ESP-IDF v5.