State storage circuitry is described comprising a master-slave latch having tristate inverter circuitry 2 at its functional input and tristate scan signal insertion circuitry 12 for inserting scan data. The tristate scan signal insertion circuitry 12 is controlled by a first clock signal nclk and a second clock signal bclk. The tristate inverter circuitry 2 is controlled by a third clock signal nfclk and a fourth clock signal flck. The clock generating circuitry holds the third and fourth clock signals at fixed values which tristate the tristate inverter circuitry 2 when in scan mode. This moves scan control logic out of the function path comprising the tristate inverter circuitry into the clock control circuitry.