A storage circuit 2 in the form of a master slave latch includes a slave stage 6 serving as a bit storage circuit. The slave stage 6 includes an inverter chain which when operating in a normal mode includes an even number of inverters 10, 12 and when operating in an random number generation mode includes an odd number of inverters 10, 12, 14 and so functions as a free running ring oscillator. When a switch is made back from the random number generation mode to the normal mode, then the oscillation ceases and a stable pseudo random bit value is output from the bit value storage circuit 6.