Data retention in operational and sleep modes

Abstract

A circuit is disclosed for retaining a signal value during a sleep mode while a portion of said circuit is powered down comprising: a clock signal input operable to receive a clock signal; at least one latch clocked by said clock signal; a data input, a data output and a forward data path therebetween, wherein a signal value is operable to be received at said data input, is clocked through said at least one latch and passes to said data output along said forward data path; at least one of said at least one latch comprises a retention latch operable to retain a signal value during said sleep mode, said retention latch not being located on said forward data path; and a bidirectional tristateable device, said bidirectional tristateable device being arranged between said forward data path and said retention latch and being operable to selectively isolate said retention latch from said forward data path in response to receipt of a first sleep signal; wherein in response to receipt of a second sleep signal, said second sleep signal being received after said first sleep signal, said circuit is operable to enter said sleep mode such that a voltage difference across said portion of said circuit is reduced such that said portion of said circuit is powered down, and a voltage difference across said retention latch and said bidirectional tristateable device is maintained.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates to the field of data processing systems. More particularly, this invention relates to circuits and methods of operating circuits that allow for the storage of a signal value in both operational and sleep modes. [0003] 2. Description of the Prior Art [0004] In many circuits, particularly those that run off remote power supplies such as batteries, it is important to keep the power consumption of the circuits low. As well as addressing the issue of operational circuit efficiency attention is also being turned to reducing static power consumption, i.e. power loss due to leakage currents. One way of addressing this is to provide the circuit with a sleep mode so that it is in effect powered down during these non-operational periods. To reduce static power during these sleep periods, many circuit designs are now making use of on-chip power gating which allows rapid transitions between sleep and funct...

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