背景

在通過I2C讀SFP 模塊的時候發(fā)現(xiàn)讀到數(shù)據(jù)不對，現(xiàn)象是感覺丟掉了幾個bit。

調(diào)查

通過測試波形，發(fā)現(xiàn)異常。

原因

有經(jīng)驗的同事指出是I2C 的stretching。
簡單說：從設(shè)備跟不上主設(shè)備的速度，想要跑慢一點。就可以通過拉低住時鐘信號來實現(xiàn)。

In an I2C communication, the master device determines the clock speed. Unlike RS232 the I2C bus provides an explicit clock signal which relieves master and slave from synchronizing exactly to a predefined baud rate.
However, there are situations where an I2C slave is not able to co-operate with the clock speed given by the master and needs to slow down a little. This is done by a mechanism referred to as clock stretching.
An I2C slave is allowed to hold down the clock if it needs to reduce the bus speed. The master, on the other hand, is required to read back the clock signal after releasing it to the high state and wait until the line has actually gone high.

搜索相關(guān)資料：
I2C Clock Stretch
clock-stretching/