Patent Application
20080098244
For a more complete understanding of the present disclosure, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
In systems, including SoC designs, there are often many memories that are used in varied ways. Some require fast access and cycle time and do not need low leakage current. Others may need a low leakage current and can suffer slightly lower access and cycle times that are required to get the low leakage current. Still other memories need good leakage current reduction with the fastest possible access and cycle time. Embodiments of the present disclosure provide solutions that address these various memory requirements.
The plurality of memory blocks 105 employ static random access memory (SRAM) cells, in the illustrated embodiment. The power supply provides each memory block with positive and negative supply voltages. The positive and negative supply voltages are scalable in voltage value as directed by the power controller 120. This scalability allows each of the memory blocks to range between a fully powered supply voltage that is needed for memory access and a retention supply voltage employed to retain data storage while reducing active leakage current in the plurality of memory blocks 105.
The input module 121 receives the first and second input memory mode signals 101a, 101b, which determine an operating mode for the memory blocks 105. The input module 121 then provides mutually exclusive access, retain-till-access and expanded retain-till-access control signals for the power controller 120. The access module 122 is controlled by the access control signal and provides an active state of the memory blocks 105 that allows memory access, when enabled by the access control signal. In this operating mode, all of the memory blocks 105 are fully powered allowing minimum access and cycle times for memory reading and writing.
The retain-till-access module 123 is controlled by the retain-till-access control signal and, when enabled, cycles a memory block 105a between the active state and a low leakage data retention state that is employed for all of the memory blocks 105. In this operating mode, only the memory block 105a is allowed to be fully powered to provide minimum access and cycle times for memory reading and writing. The low leakage data retention state provides a minimum powered leakage for all of the memory blocks 105. The memory block 105a is a representative portion of the memory blocks 105 wherein it corresponds to a memory block controlled by a group of 32 word lines, in the illustrated embodiment. Of course, any grouping of word lines may define a memory block as deemed appropriate to a particular memory application.
The expanded retain-till-access module 124 is controlled by the expanded retain-till-access control signal. This operating mode extends the active state of all of the memory blocks 105 for a specified period of time before returning all of them to the low leakage data retention state. In one embodiment, the specified period of time restarts after each memory access that occurs during the extended active state. The specified period of time may correspond to a selected number of clock cycles employing a control counter. Alternatively, the specified period of time may be determined independent of the number of clock cycles. In either case, the specified period of time may be programmable. The output module 125 provides an output status signal that corresponds to extending the active state. This indicates that the memory blocks 105 are still in the active state that allows minimum access and cycle times for memory reading and writing.
A cycle of operating states for the entire memory is different than one employed when only a portion of the memory is accessed. A cycle path 206, 211, 216 is employed for the entire memory. Transitioning from the low leakage data retention state 205 to the active state 210 for the entire memory typically produces an access latency period of at least one clock cycle. This allows time for the entire memory to become fully active from its having been in a retention mode or “sleep mode”. After this latency period, the entire memory may be accessed at minimum access and cycle times for reading or writing. At the completion of a memory access, the entire memory transitions to the extended active state 215.
In the extended active state 215, the entire memory may still be accessed at minimum access and cycle times, without latency. The extended active state 215 continues until either another memory access occurs within a pre-selected, specified period of time, or the time period expires without memory access. When the time period expires without memory access, the entire memory returns to the low leakage data retention state 205, thereby completing a memory cycle.
A cycle path 206, 207 is employed when only a portion of the memory is activated for reading or writing. Transitioning the portion of the memory from the low leakage data retention state 205 to the active state 210 typically does not produce an access latency period large enough to prohibit the memory portion from being accessed during the same clock cycle. This occurs since the capacitances associated with the memory portion selected are typically small enough to allow recovery to full power quickly. The memory portion is returned to the low leakage data retention state 205 after each memory access. This allows the entire memory or a portion of the memory to be activated for another memory access.
If the first decisional step 310 determines that the access mode is not enabled, the method 300 continues to the third decisional step 325 wherein it is determined if a retain-till-access mode of the memory has been selected by an enabling retain-till-access control signal. If the retain-till-access mode has been selected, the fully active state is provided for a portion of the memory thereby allowing the memory portion to be accessed, in a step 330. Correspondingly, the remainder of the memory resides in a low leakage data retention state, which provides a minimum active leakage current for the remainder of the memory. In the fully active state, the portion of the memory is accessed only once before it cycles to the low leakage data retention state. This places all of the memory in the low leakage data retention state.
As before, the second decisional step 320 determines if the memory is still in the powered state. If the memory is still powered, and the first and third decisional steps 310, 325 determine that the access mode is not enabled and the retain-till-access mode is still enabled, the method 300 returns to the step 330 for the next access of the portion of the memory.
If the first and third decisional steps 310, 325 determine that the access and retain-till-access modes are not enabled, the method 300 continues to a step 335, which is an expanded retain-till-access mode. An expanded retain-till-access control signal enables the expanded retain-till-access mode, wherein all of the memory is initially placed in the low leakage data retention state thereby providing the minimum active leakage current for the memory. The fully active state is provided for all of the memory when it is accessed.
In this mode, the fully active state of the memory is extended for a specified period of time before the memory is returned to the low leakage data retention state. In one embodiment, extending the fully active state restarts after each memory access during the specified period of time. Additionally, the specified period of time may correspond to a preset timer or to a selected number of clock cycles that may be measured by a control counter. In either of these cases, the specified period of time may be programmable. A fully active state status signal is provided during the specified period of time to indicate that memory access is immediately available.
After the memory is returned to the low leakage data retention state, the second decisional step 320 again determines if the memory is still in the powered state. If the memory is still powered, and the first and third decisional steps 310, 325 determine that the access and the retain-till-access modes are not enabled, the method 300 returns to the step 335 for the next memory access. In this manner, the method 300 continues to select an appropriate mode as long as the memory remains in the powered state. If the second decisional step 320 determines that the memory is not in the powered state, the method 300 ends in a step 340.
While the method disclosed herein has been described and shown with reference to particular steps performed in a particular order, it will be understood that these steps may be combined, subdivided, or reordered to form an equivalent method without departing from the teachings of the present disclosure. Accordingly, unless specifically indicated herein, the order or the grouping of the steps is not a limitation of the present disclosure.
Those skilled in the art to which the disclosure relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described example embodiments without departing from the disclosure.
This application claims the benefit of U.S. Provisional Application No. 60/862,705 entitled “Leakage Reduction Modes for Semiconductor Memories” to Michael P. Clinton, et al., filed on Oct. 24, 2006, which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60862705 | Oct 2006 | US |
