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 - Uses clock stretching, violates I2C protocol timing in some cases, and sometimes needs to be reset. Does not work well on i.MX RT10xx chips. Does not work well with ESP32, ESP32-S3 before ESP-IDF 5.3.2. Use CircuitPython 9.2.2 or later.
• BNO085 - Uses clock stretching, violates I2C protocol timing in some cases, 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 v5.3.2 or later. When using versions of the Espressif ESP-IDF SDK older than v5.3.2, 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 9.2.2 and later use ESP-IDF 5.3.2 and later. Arduino esp32 v3.2.0 and later use ESP-IDF v5.4.0 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.