I2C Clock Stretching

Some I2C chips have slow processors or have a lot of work to do. As a result, they use a technique call 'clock stretching' that gives them more time to respond to hand shakes. This is fine except the Raspberry Pi has a hardware problem with its I2C implementation and, as a result, clock stretching does not work.

The YS-CH341T also does not support clock stretching. Unlike the Pi, slowing the baud rate often works. Luckily, there are a lot of I2C chips available and an alternative is usually available.

I2C chips known to have problems

SRF02 is poor
PCF8574 is poor - MCP23017 is better all round
PCF5991 does not work ?
PCF8591 is poor
All these chips work with the UMFT4222EV dongle. This list will be updated from time to time as information becomes available.