A fault detection system on a circuit can detect a fault on the same circuit without using a duplicate circuit. A test signal can be generated based on an input signal, and the input signal and test signal can be sent through the circuit and the resulting signals can be compared to determine if any fault is present. In an embodiment, a function can be applied to bits of the input signal and bits of the test signal such that when the signals are compared after passing through the processing block that is to be tested, if the variation between the signals is above a predetermined amount, it can be determined that a fault has occurred. In other embodiments, a first function can be applied to the input signal, and a second function can be applied to the test signal, and the resulting outputs can be compared.