A data storage apparatus is provided which has a plurality of data storage units, each respective data storage unit configured to store a respective data bit of a data word. Stored data value parity generation circuitry is configured to generate a parity bit for the data word in dependence on the data bits of the data word stored in the plurality of data storage units. The stored data value parity generation circuitry is configured such that switching within the stored data value parity generation circuitry does not occur when the data word is read out from the plurality of data storage units. Transition detection circuitry is configured to detect a change in value of the parity bit.