Variation Tolerant SRAM Write and Read Assist Technique

Abstract

In SRAM design, more challenging task to improve SNM (Static Noise Margin) and WM (Write Margin). In this thesis i used Read and Write Assist technique to improve my WM and SNM in memory. In our memory there is more challenges at lower technology. When we go at low technology at that time some margin voltage will change according to our requirement. So for that we used assist technique to improve this things in memory.

Basically we used assist technique when my read and write operation is fail. In my

memory during read operation my read is fail some time because of size and some param- eter like voltage etc. For that i have to use read assist technique because my stability is so

poor some time because of size problem so i got bad SNM in my memory. To reduce my SNM we changed in design like size change or Vt change and after that i used technique and got better SNM using WL Boosting technique. During write operation, i got bad WM or some time i got write data very late time so i got failure in write operation. For that i used technique which is Negative Bit-line technique. In this i give some dip voltage according to our requirement and got better WM (Write Margin).

Read failure occurs when the cell is not able to preserve a zero and that is possibly because the pass gate becomes too strong as compared to pull down and trip point of the opposite inverter shifts towards 0, because of the weak PMOS and strong NMOS.

Write Failure occurs when the cell is not able to create a regenerative effect and flip the state of the cell and that is possibly because the pass gate becomes weak as compared to the pull up which inhibits the node voltage to go below the switching threshold and flip the state. So, the current through the transistor decreases and the effects of threshold voltage fluctuation cannot be neglected at lower supply voltages. The limitation on the minimum operating supply voltage Vmin is critical and require circuit assist techniques to enhance the functional window of the cell.