US20070085585A1

US20070085585A1 - Data retention in operational and sleep modes

Info

Publication number: US20070085585A1
Application number: US11/249,135
Authority: US
Inventors: Marlin Frederick
Original assignee: ARM Ltd
Current assignee: ARM Ltd
Priority date: 2005-10-13
Filing date: 2005-10-13
Publication date: 2007-04-19
Also published as: TW200721674A; US20090033394A1; CN1991688B; TWI383586B; CN1991688A; US7616041B2; US20070103217A1; US7514975B2

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

1. Field of the Invention
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.
2. Description of the Prior Art
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 functional modes. This power gating is achieved by inserting power transistors between the targeted circuitry and Vdd creating a “virtual” Vdd rail, or by inserting power transistors between the targeted circuitry and Vss creating a “virtual” Vss rail. To enter a low leakage mode, the power transistors are turned off and the leakage of the design is limited by the leakage of the power transistors. Since the power transistors can be made to be high Vt (threshold voltage), and since the width of the power transistors can be much less than the width of the active devices in the circuit, leakage currents can be dramatically reduced. Thus, when the power transistors are turned off the virtual power rail at their output floats and the circuit is powered down.
Although this results in substantial power savings it also results in a loss of state within the targeted circuitry. If it is desired that the circuit retain state during sleep mode, data retention circuits such as special data retention flip-flops must be used within the design. Such a mode of operation allows the stored signal values to be securely held in a small portion of the circuitry whilst the remainder of the circuitry is powered down for leakage reduction purposes. When power is resumed, the saved signal value is restored and operation continues.
A common prior art approach to data retention is to provide an additional third storage or balloon latch that is not in the data pathway of the other two latches of a flip flop and to store data in this third latch during sleep mode. This latch has its own power supply and can be built of high threshold components. Such a system is described in “A 1-V High Speed MTCMOS Circuit Scheme for Power-Down Application Circuits” IEEE Journal of Solid-State Circuits, Vol 32, No 6, June 1997. A disadvantage of this approach is that the balloon latches consume considerable additional circuit area.
It has also been proposed for sense amplifier flip-flops and hybrid latch flip-flops which have associated scan cells that operate in accordance with the level sensitive scan design methodology to reuse the scan cells for data retention during a power down mode of operation. Whilst this approach reduces the increase in circuit overhead associated with providing the data retention capability, it does require control of the three clock signals of the sense amplifier flip-flops or hybrid latch flip-flops with their known disadvantages in terms of speed, power consumption and other factors. It is also only applicable to flip flops having dedicated scan latches.
“Lower Power Integrated Scan-Retention Mechanism” ISPLED August 2002, also addresses this problem.
Co pending U.S. application Ser. No. 11/088268 having the same assignee as this patent also addresses this problem.

SUMMARY OF THE INVENTION

A first aspect of the present invention provides a circuit 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 into said at least one latch and passes to said data output along said forward data path; wherein at least one of said at least one latch comprises a retention latch operable to retain a signal value during said sleep mode; and said circuit further comprises 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.
The arrangement of the present invention that provides a data retention device that can retain data in sleep mode and is not itself on the forward data path, is an effective way of retaining data without slowing the critical timing path, which comprises the forward data path. Furthermore, the use of a tristateable device placed between the forward data path and the retention latch which can selectively isolate the retention latch, is a convenient way of retaining data in sleep mode and again does not effect the critical timing path. Removing these devices from the critical timing path allows the designer greater freedom in his selection of components for these devices and therefore allows for the selection of, for example, low leakage components that may not have such high performance.
Although it is possible for there to be only one latch, in most embodiments the circuit comprises a plurality of latches clocked by said clock signal, said signal value passing from one of said plurality of latches to a subsequent one of said plurality of latches along said forward data path, at least one of said plurality of latches comprising said retention latch.
In some embodiments, said bidirectional tristateable device comprises two transistors operable to receive said first sleep signal and arranged such that in response to receipt of said first sleep signal said two transistors form a high impedance path and in response to not receiving said first sleep signal said two transistors form a low impedance path.
In other embodiments, said bidirectional tristateable device comprises four transistors, two of said four transistors forming said bidirectional tristateable device and two of said four transistors forming an inverter operable to invert said first sleep signal prior to inputting it to one of said two transistors.
Depending on whether an inverted sleep signal is available within the circuit, the bidirectional tristateable device may be provided by two or four transistors. In either case the provision of just a few transistors is sufficient to adapt a traditional latch into a retention latch in some embodiments. Thus, a retention latch is achieved with a small increase in circuit area.
Although any sort of bidirectional tristateable device may be used, in some embodiments the bidirectional tristateable device comprises a transmission gate. Any tristateable devices that provide a low impedance in response to one input and a high impedance in response to another would be appropriate, but a transmission gate is found to be particularly effective.
In some embodiments, said data retention latch does not receive said clock signal while in others said data retention latch is operable to receive said clock signal.
Although it is possible to provide a latch circuit that is not clocked, it is found that one that is clocked can switch between states more easily and thus, may be preferred in some embodiments.
In some embodiments, said circuit further comprises clock signal distribution means operable to distribute said clock signal to said retention latch.
Although the clock signal may be generated outside of the circuit in others it is generated within the circuit. Distributing the clock to the retention latch enables a clocked retention latch to switch state.
Although provision of the clocked signal to the retention latch enables it to switch state more easily, it does have a drawback in that power needs to be supplied to the clock distribution in sleep mode and this can add significantly to static power loss.
In some embodiments, said clock signal distribution means comprises a first sleep signal input operable to receive a first sleep signal; wherein in response to said first sleep signal said clock signal distribution means is operable to hold said clock signal at a predetermined value such that said retention latch retains state.
Providing a clock signal at a predetermined value ensures the latch is in its data retaining recirculating mode.
Although the clock distribution means may comprise a number of forms, in some embodiments, said clock signal distribution means comprises a logic gate having a clock signal input and a first sleep signal input.
Such an arrangement is simple and yet allows the clock signal as seen by the latches to be held at a predetermined value in response to the sleep signal.
In some embodiments said clock signal distribution means comprises a plurality of components through which a clock signal propagates, said circuit is operable to reduce a voltage difference across said components of said clock signal distribution means upstream in a clock signal propagation direction of said first sleep signal input such that said components are powered down in response to said first sleep signal, and to maintain a voltage difference across said components downstream of said first sleep signal input.
Power needs to be supplied to some of the clock distribution devices in order to ensure that the signal value is held at said predetermined value. The above arrangement allows much of the clock distribution devices to be powered down, while ensuring the clock signal is kept at the desired value.
In some embodiments, said retention latch comprises a clocked tristate inverter, and transistors arranged in parallel with a portion of said clocked tristate inverter and operable to receive said first sleep signal such that said retention latch is operable to retain state irrespective of a value of said clock during receipt of said first sleep signal.
One disadvantage with some embodiments of the present invention is that the clock signal to the retention latch needs to be maintained in order for the latch to remain in a recirculating mode. The clock distribution logic which distributes the clock signal to the different components comprises components that are often not low leakage components and which thus can use a lot of power. Thus, it is a considerable disadvantage to static power loss if power needs to be maintained to this part of the circuit during sleep mode. The provision of additional components which allow the clocked tristate inverter to retain state irrespective of the clock signal can therefore be very advantageous, as it allows the clock signal distribution logic to be powered down during sleep mode and therefore avoids or at least reduces power loss due to this part of the circuit.
In some embodiments, said transistors comprise two transistors in parallel with said two clocked transistors of said tristate inverter, said two transistors receiving said first sleep signal and an inverted first sleep signal respectively.
The advantages of reduced power loss due to enabling the clock circuit to be powered down in sleep mode can be achieved with just two additional transistors. Thus, a small increase in circuit area can lead to the advantages of a relatively high reduction in static power consumption.
In some embodiments, said circuit comprises at least one further latch, at least two of said latches comprising a master slave flip flop comprising a master latch and a slave latch, said retention latch comprising said slave latch.
Although the retention latch can comprise any latch, for example a glitching element, a latch within a master slave flip flop is found to be particularly advantageous. Furthermore, although the retention latch could be formed from the master latch of such a flip-flop, generally it is formed from the slave latch. It should be noted that whatever latch it is formed from it should be in “look aside” mode, i.e. it should not be located on the forward data path.
In some embodiments, said master slave flip flop comprises a reset master slave flip flop, said retention latch comprising two transistors operable to receive said first sleep signal and a reset signal and operable to block said reset signal and prevent it from resetting a state of said retention latch in response to receipt of said first sleep signal.
Although the retention latch may be formed within a reset flip flop, if it is there are additional potential problems that need to be addressed. The potential problems relate to the possibility of the reset signal being inadvertently activated on entering or leaving sleep mode such that the retention latch is reset and the data that it should retain is lost. Thus, in embodiments of the invention which utilise reset flops additional transistors may be used which block the reset signal from affecting the retention latch during sleep mode. Controlling the additional transistors with a first sleep signal which is activated before the second sleep signal ensures that the reset signal is held low while the portion of the circuit is powered down.
A similar problem may occur with set master slave flip flops and thus embodiments of the present invention provide a circuit wherein said master slave flip flop comprises a set master slave flip flop, said retention latch comprising two transistors operable to receive said first sleep signal and a set signal and operable to block said set signal and prevent it from setting a state of said retention latch in response to receipt of said first sleep signal.
Additional transistors can also be used to prevent these set signals from changing the data stored in the retention latch.
In some embodiments, the power supplied to the circuit is supplied from outside of the circuit, while in other embodiments the circuit comprises a voltage regulator operable to control a voltage level supplied to portions of said circuit, said voltage regulator being operable to receive said second sleep signal and in response to said second sleep signal to reduce a voltage difference across said portion of said circuit such that said portion of said circuit is powered down; and to maintain a voltage difference across said retention latch and said bidirectional tristateable device.
In some embodiments, said circuit is operable to be powered in response to a voltage difference applied across said circuit, said circuit further comprising a power transistor, said power transistor being arranged such that said voltage difference is applied across said power transistor and said portion of said circuit in series, said power transistor being operable to receive said second sleep signal and being operable to be turned off in response to said second sleep signal, such that a voltage difference across said portion of said circuit is reduced and said portion of said circuit is powered down in response to said second sleep signal.
Although the sleep state of the portion of the circuit that is powered down can be achieved in a number of ways, power transistors are simple and effective ways of achieving this sleep state which have very low static power loss.
In embodiments of the invention said retention latch and said bidirectional tristateable device comprise low leakage devices.
As the retention latch and tristateable device are continually powered, it is highly advantageous to make them from low leakage components such as devices having a high threshold voltage. This means that there is very little static power loss from these components. Furthermore, as these components are arranged to be not on the forward data path i.e. not on the critical timing path the provision of low leakage components in this pathway does not affect the performance of the circuit.
In some embodiments, the circuit comprises a plurality of retention latches.
A plurality of retention latches for storing a plurality of signals in sleep mode can be provided within the circuit.
In some embodiments, said circuit further comprises a plurality of portions each comprising at least one retention latch.
A device can comprise a plurality of portions each having its own retention latch. These can be controlled by the same sleep signals or the circuit can be controlled by a plurality of different sleep signals such that different portions of the circuit can enter sleep mode and be powered down at different times.
A further aspect of the present invention provides a method of storing a signal value within a circuit during a sleep mode while a portion of said circuit is powered down, said method comprising the steps of: distributing said clock signal to a clock input of at least one latch, said at least one latch being located between a data input and a data output such that a signal value received at said data input, is clocked into said at least one latch and passes to said data output along a forward data path, and at least one of said at least one latch is a retention latch operable to retain a signal value during said sleep mode; wherein in response to a first sleep signal: isolating said retention latch from said forward data path using a bidirectional tristateable device located between said forward data path and said retention latch; in response to a second sleep signal: reducing a voltage difference across said portion of said circuit such that said portion of said circuit is powered down; and maintaining a voltage difference across said retention latch and said bidirectional tristateable device.
A yet further aspect of the present invention provides A circuit for retaining a signal value while a portion of said circuit is powered down comprising: a clock signal input operable to receive a clock signal; a means for retaining data clocked by said clock signal and comprising: a forward data path such that a signal value passes from a data input and is clocked into a retention means and passes to a data output along said forward data path; said retention means being operable to retain a signal value during a sleep mode; and a bidirectional tristateable means for selectively isolating said retention means from said forward data path in response to receipt of a first sleep signal, said bidirectional tristateable means being arranged between said forward data path and said retention means; 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 means and said bidirectional tristateable means is maintained.
The above, and other objects, features and advantages of this invention will be apparent from the following detailed description of illustrative embodiments which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a master slave flip flop according to the prior art;
FIG. 2 schematically shows a master slave retention flip flop according to a first embodiment of the present invention and a timing diagram of its operation;
FIG. 3 shows the component transistors of transmission gate 50 of FIG. 2;
FIG. 4 shows the component transistors of an amended tristate inverter of the slave latch of a second embodiment of the present invention;
FIG. 5 schematically shows a master slave retention flip flop according to the second embodiment of the present invention and a timing diagram of its operation;
FIG. 6 schematically shows a reset master slave flip flop according to the prior art;
FIG. 7 schematically shows a reset master slave retention flip flop according to an embodiment of the present invention and a timing diagram of its operation;
FIG. 8 shows the component transistors of an amended tristate inverter of the slave latch of the reset master slave retention flip flop of FIG. 7;
FIG. 9 a shows the slave latch of a set master slave flop according to the prior art;
FIG. 9 b shows the slave latch of a set retention master slave flop according to an embodiment of the present invention;
FIG. 10 shows a single retention latch according to an embodiment of the present invention; and
FIG. 11 shows a master slave flop, wherein the retention latch comprises the master latch.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows in schematic form a basic master slave flip flop according to the prior art. This basic master slave flop 20 has a clock distribution means 10 which comprises a plurality of inverters operable to deliver different clock signals, clk, nclk an inverted form of clk, and bclk an inverted form of nclk. The basic master slave flop has a forward data path 23, between data input 21 and data output 29. This forward data path takes data from the input to a master latch 26 via transmission gate 22 and to slave latch 28 via transmission gate 24. Transmission gates 22 and 24 are tristateable devices able to provide a low impedance data path or a high impedance data path depending on the clock values at their inputs. Thus, they act to either isolate the latches or to allow transmission of data to them.
FIG. 2 shows a master slave retention flop 30 according to an embodiment of the present invention. This is an adaptation of the flop of FIG. 1 and is able to provide data retention within the slave latch during a “sleep mode” when a portion of the flop is powered down.
This embodiment comprises clock distribution logic 10 and additionally retention signal (ret) or first sleep signal distribution logic 12. These logic blocks are not powered down in sleep mode and this is indicated in the figure by the shading. Master slave retention flop 20 comprises a forward data path between data input 31 and data output 39. The forward data path 33 takes data from the input 31 to master latch 36 and then to slave latch 40. It comprises transmission gates 32 and 34 between the data input 31 and the master latch 36 and between the master 36 and slave latch 40 respectively.
Slave latch 40 also acts in this embodiment as the data retention latch and is selectively isolated from forward data path 33 by a bidirectional tristateable device 50. The slave lo latch 40 and bidirectional tristateable device 50 are not powered down in sleep mode and this is indicated in the figure by the shading. In this case bidirectional tristateable device 50 is a transmission gate, although any bidirectional tristateable device operable to selectively provide high or low impedance and operable to drive a signal in either direction would be suitable. Unlike transmission gates 32 and 34 transmission gate 50 receives the retention and inverted retention signals from retention or sleep signal distribution logic 12 as its control signals. Transmission gate 50 is responsive to these signals to either offer a low impedance state such that the slave latch 40 is in data communication with the forward data path 33 when the retention signal is low, or in response to the retention signal going high to offer a high impedance state such that the slave latch 40 is isolated from the forward data path 33.
Slave latch 40 comprises an inverter 42 and a tristate inverter 44. The tristate inverter 44 is clocked by clock signals sent from clock signal distribution logic 10. It is for this reason that clock signal distribution logic must always be powered up in this embodiment to ensure that the recirculating path within the slave latch 40 is driven and remains closed.
FIG. 2 also shows a timing diagram giving the values of the master clock, clk signal, the first sleep signal which corresponds to the retention signal ret and the second sleep signal which provides an indication to portions of the circuit to power down. It also shows the states that these signals put the flop into, that is the functional, the low leakage and the intermediate states of this master slave retention flop.
FIG. 3 shows transmission gate 50 of FIG. 2 in transistor form. This shows a preferred embodiment of the transmission gate. Clearly, any tristateable device that can selectively isolate the slave latch 40 from the forward data path 33 would be suitable. However, this preferred embodiment comprises just two transistors (four if the sleep signal, ret needs to be inverted and is not supplied to the circuit in inverted form) and thus, does not increase the circuit area of the flop by a large amount.
One drawback of the embodiment of FIG. 2 is that the clock distribution logic 10 needs to retain power in order for the slave or retention latch 40 not to lose state. FIG. 4 shows a second embodiment giving an alternative arrangement for the tristate inverter 44 of FIG. 2. In this alternative arrangement, an additional two transistors 46 and 48 have been added in parallel to the clocked transistors of the tristate inverter 44. These two transistors receive the retention signal and the inverted retention signal from retention signal logic 12. By placing these two transistors in this position, the retention of the data within slave latch 40 can be assured provided the retention signal is high whatever the values of the clock signals. Thus, data can be retained even if clock signal distribution logic is turned off. This ability to turn the clock signal distribution logic off can provide a large saving in power to the circuit as clock distribution signal logic is generally not made of high Vt (threshold voltage) devices and is quite large thereby consuming a relatively large amount of static power.
An alternative to this (not shown) would be to hold the clock signal of tristate inverter 44 using logic and the sleep signal ret, thereby ensuring that the circulating loop of the retention latch stays open and data is retained without the need to run the clock signal continuously. A disadvantage of this is that at least some of the clock signal distribution logic would need to be powered so there is more power loss than the embodiment of FIG. 4.
FIG. 5 shows the master slave retention form of the second embodiment comprising the tristate inverter 44 of FIG. 4. This diagram also shows a timing diagram relating to the operation of the master slave retention flop 30 of this second embodiment. This timing diagram shows how in the low leakage or sleep state the value of the clock signal is unimportant and thus, clock distribution logic can be turned off. In this embodiment it is just sleep signal distribution logic 12, slave latch 40 and transmission gate 50 which are powered up in sleep mode. This embodiment therefore allows a significant power saving at a cost in area of just two transistors compared to the embodiment of FIG. 2. This embodiment has an additional six transistors compared to the prior art master slave flop of FIG. 1 which had no retention capabilities.
FIG. 6 shows a reset master slave flop according to the prior art. Data retention during sleep mode can be particularly difficult if the retention latch is a latch within a set or reset flip flop. This is because when powering up great care must be taken that the latch storing the data is not set or reset before that data has been extracted, otherwise, the data could be lost on power up and its retention will then have been worthless. As can be seen from FIG. 6, the slave latch 60 comprises a NAND gate 63 in parallel with tristate inverter 64.
FIG. 7 shows a reset master slave retention flop according to an embodiment of the present invention. In this embodiment, slave or retention latch 60 comprises tristate inverter 64 adapted to retain state even when the clock signal is turned off, in a similar way to the retention latch shown in FIG. 5. Thus, clock distribution logic 10 does not have to be powered up during sleep mode. It should be clear to the skilled person that slave latch 60 could comprise a standard tristate inverter such as that shown in FIG. 2, in which case the clock distribution logic would need to retain power during sleep mode. In addition to clock signal distribution logic 10 and sleep signal distribution logic 12 there is reset distribution logic 14 that is operable to distribute a reset signal rst, and an inverted reset signal, nrst, to appropriate parts of the circuit. In addition to NAND gate 63 equivalent to NAND gate 63 of FIG. 6 there is additional logic 66 on the reset signal input to this NAND gate. This logic 66 ORs the inverted reset signal nrst with the sleep signal ret and thereby assures that the retention latch 60 is not accidentally reset either on entry into or exit from sleep mode.
FIG. 8 shows gate 66 in transistor form. Specifically, the addition of two sleep transistors 65 and 67 which have the ret signal on their inputs and thereby impede the reset signal from going high during sleep mode transform NAND gate 63 to OAI12 66.
FIG. 9 shows a corresponding embodiment for a set flop. FIG. 9 a shows the slave latch 70 of a conventional set flop. In this set flop, a NOR gate 73 is placed in parallel with the tristate inverter 74 of slave latch 70. Slave latch 70 corresponds to slave latch 60 of the reset flop of FIG. 6, for a set flop. FIG. 9 b shows how an addition of two nret FETs 75 and 77 can transform NOR gate 73 to an A0112 gate 70. These additional transistors 75 and 77 act like the transistors 65 and 67 of the reset flop to impede the set signal from being asserted during sleep mode.
FIG. 10 shows an embodiment of the present invention comprising a single retention latch 70 that is not clocked. It should be noted that although a tristate inverter is advantageous within the latch as it allows for the state of the latch to be easily switched, it is not essential and an unclocked latch comprising inverters arranged in a loop such as that shown as 70 in FIG. 10 is possible. In this embodiment the retention latch 70 and sleep signal distribution logic 12 retain power during sleep mode. The clock distribution logic, however, does not retain power in this embodiment.
FIG. 11 shows an alternative embodiment where retention latch 80 comprises the master latch of a master/slave flop. This embodiment corresponds to the clocked embodiment of FIG. 2 and clock distribution logic 10, sleep signal distribution logic 12 and retention latch 80 retain power during sleep mode.
It should be noted that all flops are shown as having inverters on the forward data path, but it should be clear to the skilled person that they could equally well be built with non-inverters in which case an additional inverter would be needed at the end of the forward data path 33. It should be clear to a skilled person that such alternative embodiments fall within this scope of the present invention as defined in the appended claims.
Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims

Claims

1. A circuit 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 into said at least one latch and passes to said data output along said forward data path; wherein

at least one of said at least one latch comprises a retention latch operable to retain a signal value during said sleep mode, and said circuit further comprises

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.

2. A circuit according to claim 1, comprising a plurality of latches clocked by said clock signal, said signal value passing from one of said plurality of latches to a subsequent one of said plurality of latches along said forward data path, at least one of said plurality of latches comprising said retention latch.

3. A circuit according to claim 1, wherein said bidirectional tristateable device comprises two transistors operable to receive said first sleep signal and arranged such that in response to receipt of said first sleep signal said two transistors form a high impedance path and in response to not receiving said first sleep signal said two transistors form a low impedance path.

4. A circuit according to claim 1, wherein said bidirectional tristateable device comprises four transistors, two of said four transistors forming said bidirectional tristateable device and two of said four transistors forming an inverter operable to invert said first sleep signal prior to inputting it to one of said two transistors.

5. A circuit according to claim 4, wherein said bidirectional tristateable device comprises a transmission gate.

6. A circuit according to claim 1, wherein said retention latch is operable to receive said clock signal.

7. A circuit according to claim 6, said circuit further comprising:

clock signal distribution means operable to distribute said clock signal to said retention latch.

8. A circuit according to claim 7, said clock signal distribution means comprising a first sleep signal input operable to receive a first sleep signal; wherein

in response to said first sleep signal said clock signal distribution means is operable to hold said clock signal at a predetermined value such that said retention latch retains state.

9. A circuit according to claim 7, wherein said clock signal distribution means comprises a logic gate having a clock signal input and a first sleep signal input.

10. A circuit according to claim 8, wherein said clock signal distribution means comprises a plurality of components through which a clock signal propagates, said circuit is operable to reduce a voltage difference across said components of said clock signal distribution means upstream in a clock signal propagation direction of said first sleep signal input such that said components are powered down in response to said first sleep signal, and to maintain a voltage difference across said components downstream of said first sleep signal input.

11. A circuit according to claim 6, wherein said retention latch comprises a clocked tristate inverter, and transistors arranged in parallel with a portion of said clocked tristate inverter and operable to receive said first sleep signal such said retention latch is operable to retain state irrespective of a value of said clock during receipt of said first sleep signal.

12. A circuit according to claim 11, wherein said transistors comprise two transistors in parallel with said two clocked transistors of said tristate inverter, said two transistors receiving said first sleep signal and an inverted first sleep signal respectively.

13. A circuit according to claim 1, said circuit comprising at least one further latch, at least two of said latches comprising a master slave flip flop comprising a master latch and a slave latch, said retention latch comprising said slave latch.

14. A circuit according to claim 13, wherein said master slave flip flop comprises a reset master slave flip flop, said retention latch comprising two transistors operable to receive said first sleep signal and a reset signal and operable to block said reset signal and prevent it from resetting a state of said retention latch in response to receipt of said first sleep signal.

15. A circuit according to claim 13, wherein said master slave flip flop comprises a set master slave flip flop, said retention latch comprising two transistors operable to receive said first sleep signal and a set signal and operable to block said set signal and prevent it from setting a state of said retention latch in response to receipt of said first sleep signal.

16. A circuit according to claim 1, said circuit further comprising a voltage regulator operable to control a voltage level supplied to portions of said circuit, said voltage regulator being operable to receive said second sleep signal and in response to said second sleep signal to reduce a voltage difference across said portion of said circuit such that said portion of said circuit is powered down; and to maintain a voltage difference across said retention latch and said bidirectional tristateable device.

17. A circuit according to claim 1, said circuit being operable to be powered in response to a voltage difference applied across said circuit, said circuit further comprising a power transistor, said power transistor being arranged such that said voltage difference is applied across said power transistor and said portion of said circuit in series, said power transistor being operable to receive said second sleep signal and being operable to be turned off in response to said second sleep signal, such that a voltage difference across said portion of said circuit is reduced and said portion of said circuit is powered down in response to said second sleep signal.

18. A circuit according to claim 1, wherein said retention latch and said bidirectional tristateable device comprise low leakage devices.

19. A circuit according to claim 1, comprising a plurality of retention latches.

20. A method of storing a signal value within a circuit during a sleep mode while a portion of said circuit is powered down, said method comprising the steps of:

distributing said clock signal to a clock input of at least one latch, said at least one latch being located between a data input and a data output such that a signal value received at said data input, is clocked into said at least one latch and passes to said data output along a forward data path, and at least one of said at least one latch is a retention latch operable to retain a signal value during said sleep mode; wherein

in response to a first sleep signal:

isolating said retention latch from said forward data path using a bidirectional tristateable device located between said forward data path and said retention latch;

in response to a second sleep signal:

reducing a voltage difference across said portion of said circuit such that said portion of said circuit is powered down; and

maintaining a voltage difference across said retention latch and said bidirectional tristateable device.

21. A circuit for retaining a signal value while a portion of said circuit is powered down comprising:

a clock signal input operable to receive a clock signal;

a means for retaining data clocked by said clock signal and comprising:

a forward data path such that a signal value passes from a data input, is clocked into a retention means and passes to a data output along said forward data path and;

said retention means being operable to retain a signal value during a sleep mode; and

a bidirectional tristateable means for selectively isolating said retention means from said forward data path in response to receipt of a first sleep signal, said bidirectional tristateable means being arranged between said forward data path and said retention means; 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 means and said bidirectional tristateable means is maintained.