Category

Functional

Function

SMBus High timeout feature fails at I2C clock less than 24KHz onwards.

Description

SMBus High timeout feature is failing at I2C clock rate less than 24KHz onwards (20KHz, 10KHz). From SMBUS Spec, the upper limit on SCL high time during active transaction is 50us. Total time taken from writing of I2C START bit to SCL low is 60us, which is >50us. It will trigger the timeout event and let I2C Controller go into IDLE without completing the transaction at the start of transfer. Below is detailed explanation.

For SCL is configured as 20KHz, SCL low and high period is 30us and 20us respectively. First, I2C START bit write at the same time high timeout counter starts decrementing. Then, it takes one SCL low period (30us) from the START bit write to SDA goes low (start condition). Next, it takes another SCL low period (30us) from SDA goes low (start condition) to SCL goes low (data transfer starts) which should stop the high timeout counter at this point. As a total, it takes 60us from counter start to end. However, due to the upper limit(50us) of the high timeout counter, the timeout event will still be triggered although the I2C transaction is working fine without issue.

Workaround

Do not use SMBus High timeout feature when I2C clock less than 24KHz onwards.