The I²C modules can be placed into an internal loopback mode for diagnostic or debug work by setting the LPBK bit in the I²C controller configuration I2Cx.MCR register. In loopback mode, the SDA and SCL signals from the controller part of the I²C are tied to the SDA and SCL signals of the target part of the I²C module to allow internal testing of the device without having to connect the I/Os.

This loopback mechanism can be used to transmit known data from transmit to receive. The I2C configuration can be similar to the application configuration with respect to the bit rate, FIDO usage etc. The completion of the test can be timed to be within expected limits to detect any faults in the bit rate timing.