2 How can a Stuck Bus Occur?

A stuck bus can occur in several different ways, the main offender for most cases is a false clock edge being generated. The reason why a false clock edge is dangerous on the I²C bus is because a false clock edge desynchronizes the I²C target’s clock relative to the I²C controller, who is responsible for generating the clock edges. Differences between the I²C target device ns deglitch filter and the I²C controller’s deglitch filter can contribute to one device seeing the false edge while the other does not. An example is if the I²C target has a 70ns deglitch filter while the controller has a 50ns deglitch filter and a false edge occurs with a 60ns window which makes the controller see the edge but the target ignores the false edge. Another potential case is if there is an I²C redriver (also known as a buffer) between the target and the controller. If a false edge occurs on one side, it cannot propagate through the redriver so either the controller or the target can see the edge while the other does not.

An example is shown in Figure 2-1. The first transaction in this example shows what is supposed to happen. The controller is sending data to the target with the data package of 0x0Ah and at the 9^th clock pulse the target ACKs to tell the controller that the target receives the data then a stop condition is generated by the controller. The second transaction is the same as the first, however a false edge is generated during the 5^th clock pulse of which the controller sees but not the I²C target. This means the controller is one clock ahead in the I²C transaction while the I²C target is one clock behind. The data the target sees is then shifted to the left by one so it receives bad data (0x05h) instead of seeing 0x0Ah. When the 8^th falling SCL edge comes (from the target’s perspective), the I2C target drives the SDA line low but never sees the 9^th falling SCL edge. This results in the SDA line getting stuck low indefinitely. The controller cannot issue a stop condition and depending on the controller's hardware and software, can keep SCL low or release SCL.

Root causes for a stuck bus can come from crosstalk, electromagnetic interference, hot-insertion events, or bad power on reset situations.