10T SRAM Using Half-VDD Precharge and Row-Wise Dynamically Powered Read Port for Low Switching Power and Ultralow RBL Leakage
We present, in this paper, a new 10T static random access memory cell having single ended decoupled read-bitline (RBL) with a 4T read port for low power operation and leakage reduction. The RBL is precharged at half the cell’s supply voltage, and is allowed to charge and discharge according to the stored data bit. An inverter, driven by the complementary data node (QB), connects the RBL to the virtual power rails through a transmission gate during the read operation. RBL increases toward the VDD level for a read-1, and discharges toward the ground level for a read-0. Virtual power rails have the same value of the RBL precharging level during the write and the hold mode, and are connected to true supply levels only during the read operation. Dynamic control of virtual rails substantially reduces the RBL leakage. The proposed 10T cell in a commercial 65 nm technology is 2.47×the size of 6T with β=2, provides 2.3×read static noise margin, and reduces the read power dissipation by 50% than that of 6T. The value of RBL leakage is reduced by more than 3 orders of magnitude and (ION/IOFF) is greatly improved compared with the 6T BL leakage. The overall leakage characteristics of 6T and 10T are similar, and competitive performance is achieved. The proposed architecture of this paper analysis the logic size, area and power consumption using Tanner tool.
SRAM cell must robustly operate under hold, read, and write mode. An SRAM cell uses the positive feedback of cross-coupled inverters (INVs) to store a single bit of information in a complementary fashion. Access transistors provide the mechanism for the read and write operation. Before every access, column BL pair (BL and BLB) is precharged to the supply voltage. For the write operation, one of the precharged BLs is discharged through the write driver.
Figure 1: Conventional 6T SRAM read. (a) Column of M bit-cells during read. (b) Top: hold and read SNM butterfly curve (with worst case noise polarity during hold). Bottom: transient behavior showing read disturbance
Fig. 1(a) shows a single column ofM6T SRAM cells, where one cell is accessed in read mode with data=0(Qa=0), while other M−1 cells are in the hold mode. Leakage components are labeled, and for the worst case leakage, all M−1 cells store data=1(Qu=1).I read flows from BL to the VSS through AL and NL of the accessed cell, and the BL voltage is decreased. The unaccessed cell on the BL exhibits BL leakage.IuLeak0 is the main component of BL leakage whileIuLeak1 is negligible, as VDS of AR of the unaccessed cell is large, while VDSof its AL is very small (varies from 0 to VBL). These leakage components decrease the differential BL voltage development. As there are a large number of cells in a single column, the worst case BL leakage can decrease BLB voltage enough to make an erroneous read. Thus, I read must be greater than (M−1)×IuLeak0,whereMis the number of cells in a single column.
Figure 2: SRAM read ports (a) 6T. (b) 8T.(c) 9T.(d) 9T.(e) 10T.
In essence, 6T SRAM has conflicting read and write requirements and transistor sizing cannot be done independently. Also, 6T has inherit RSNM problem as the read current passes through the cell internal node, and it further degrades with VDD scaling. Also, being considered as baseline design, 6T has overall a higher power dissipation, and higher BL leakages, as the low power techniques employ a certain mechanism to lower the dynamic power dissipation, e.g., charge sharing and hierarchical BL and the leakages (by employing virtual rails). The read port of 6T SRAM cell is shown in Fig. 2(a) that highlights the internal node Q in the read current path. Many alternative bit cells and techniques have been proposed in the literature to improve SRAM cell stability, reduce the leakage currents, and achieve low power operation compared with the conventional6T design.
An 8T SRAM cell adds a separate 2T read port, shown in Fig. 2(b), and necessarily solves the problem of read stability. Internal nodes are isolated from the read current path, and thus a high RSNM is achieved. Also, sizing of 8T read port can be done independently without affecting the write operation.
In 6T SRAM read operation, one of the BL stays at the VDD while the other decreases by VBL amount. However, in the case of 8T SRAM, there is only one BL (RBL) and it either decreases or stays at the VDD level depending on the bit read. Now, the sensing of SE BL can be done using different circuits such as: 1) domino sensing that requires full VDD swing ON the local-BL; 2) psuedo-differential that requires a reference signal; and 3) ac coupled sensing that requires the use of capacitors. Using a reference-based sense amplifier, only a small voltage difference is required.
- Power consumption is high
We present our half VDD precharge and charger cycling technique for low power read operation. A 4T read port is designed to employ the proposed technique. ReadBL (RBL) is charged and discharged through the read port according to the state of stored bit. Read port is powered by virtual power rails that run horizontal and are shared bythe cells of a word. The dynamic control of read port power rails reduces the RBL leakage substantially.
Figure 3: Proposed 10T SRAM cell with row-wise read port dynamic power lines
Proposed cell and low power technique:
The proposed 10T SRAM cell with SE RBL is shown in Fig. 3. We have added a 4T read port to the 6T cell to decouple the internal nodes during the read operation. Read port consists of an INV P1-N1 driven by node QB, and a transmission gate (TG) P2-N2. The output (Z) of the INV is connected to RBL during the read operation through TG, which is controlled by (read) control signals. Furthermore, read port is powered by virtual power rails, VVDD and VVSS, which are dynamically controlled. These virtual power rails (control signals) run horizontally, and have the true rail values only during the read operation. For the RBL leakage reduction, both the virtual rails have the same level as the precharge level of RBL.
- The 10T SRAM cell using an INV and a TG has been proposed earlier. However, our proposed 10T scheme is different from the previous design in the following aspects. The previous INV+TG-based 10T cell was application specific, while our proposed design is generic.
- We have used the dynamically controlled power rails for the read port.
- We precharge RBL at VDD/2, while the previous 10T design eliminated the precharge phase, and used INV to fully charge or discharge the RBL.
- The basic read technique of both the designs is completely different. The main idea of the proposed design is “the charging or the discharging of the read BL from VDD/2 for every read operation.” The previous design either discharges from VDD to VSS, or charges from VSS to VDD.
- A powerful INV was used previously to produce full VDD swing on the RBL. In the proposed design, RBL is precharged at VDD/2, and only a small voltage difference (comparable with 6T) is produced for every read cycle.
- In the proposed design, for every read cycle the RBL will exhibit some change (positive or negative) from its precharged value of vdd/2. However, the RBL would not change for consecutive similar bit reads. RBL would change only if consecutive read bits are different.
- Power consumption is low
- Tanner tool