Memory access circuitry controls access to multiple memory units with two access units. Arbitration circuitry forwards memory access requests for one memory unit to a first access unit, for a further memory unit to a second access unit, and for yet further memory unit to one of the first or second access units. The access units store requests in a queue prior to transmitting them to the respective memory unit. Tracking circuitry tracks requests and determines when to transmit subsequent requests from the queue. Control circuitry sets a state of each access unit to one of active, prepare and dormant, switches states of the two access units periodically, and does not set more than one access unit to the active state at the same time.