Sequential Delay Timer Circuit for Low-Voltage SRAM Read Windows
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Solution Overview
Problem
SRAM bitcells face challenges in low voltage operations due to insufficient read windows and voltage differentials, leading to invalid sensing and unsuccessful reads, as conventional timer circuits fail to provide the necessary delay for sense amplifier trigger signals at lower operating voltages.
Innovation Solution
The implementation of a sequential delay enabler timer circuit using a stacked NMOS pull-down structure and pre-charged gate capacitance to delay the discharge of bitlines, ensuring a sufficient voltage differential is reached for successful read operations, while minimizing circuit area and dynamic power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional timer circuits are used in SRAM bitcells for low voltage operation, then the circuit structure remains simple, but the read window becomes insufficient and voltage differential is inadequate leading to invalid sensing
Solution Approach 1:
The timer circuit performs preliminary action by pre-charging the bitline to a high logic value before the read operation begins. This pre-charging ensures that when the read operation starts, the bitline is already in the correct state to develop the necessary voltage differential, thereby extending the effective read window and improving reliability at low voltages.
Solution Approach 2:
The timer circuit is segmented into multiple components working in sequence: PMOS transistors for pre-charging, NMOS transistors for controlled discharge, and gate capacitance for timing delay. This segmentation allows each component to perform its function optimally, with the combined effect extending the read window while maintaining circuit reliability.
2Speed
If the sense amplifier trigger signal is asserted early to improve speed, then the operation speed increases, but the voltage differential is insufficient causing unsuccessful reads
Solution Approach 1:
The timer circuit acts as an intermediary between the read operation initiation and the sense amplifier trigger assertion. It introduces a controlled delay through the NMOS discharge path and gate capacitance, ensuring that the trigger signal is asserted at the optimal moment when the voltage differential is sufficient, thus maintaining both speed and reliability.
Solution Approach 2:
The circuit changes the timing parameter of the sense amplifier trigger signal dynamically. By controlling the discharge rate of the bitline through the NMOS transistors and adjusting the delay introduced by gate capacitance, the trigger signal is asserted at the precise moment when voltage differential conditions are met, optimizing both speed and sensing accuracy.
3Duration of action of moving object
If gate capacitance is added to extend the read window, then the timing control improves, but the area occupation increases
Solution Approach 1:
The gate capacitance serves multiple functions: it provides timing delay to extend the read window, it controls the discharge rate of the bitline, and it works in conjunction with the NMOS and PMOS transistors to achieve precise timing control. This multi-functionality maximizes the utility of the added capacitance while minimizing the overall circuit area.
4Loss of time
If NMOS pull-down structure is used to control bitline discharge, then the timing precision improves, but the dynamic power consumption increases
Solution Approach 1:
The NMOS pull-down structure operates in a periodic manner, controlling the bitline discharge in controlled stages rather than continuously. The gate capacitance charges and discharges in a periodic cycle, enabling precise timing control while allowing the circuit to enter low-power states when discharge is not actively occurring, thus reducing overall dynamic power consumption.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The proposed solution effectively extends the read window at low voltages, ensuring accurate bitcell reads by delaying the sense amplifier trigger signal, reducing area occupation and dynamic power leakage, and optimizing read cycle timing.
Implementation Method 1
a gate capacitance coupled to the NMOS pull-down structure such that the bitline is further discharged by the gate capacitance
Implementation Method 2
The NMOS pull-down structure may be structured to discharge the bitline based on a high logic value carried on the second wordline
Implementation Method 3
a gate capacitance coupled to the NMOS pull-down structure such that the bitline is further discharged by the gate capacitance. The gate capacitance may be adjusted to modify an amount of time to discharge the bitline
Data Source
AI summary
An integrated circuit includes a memory cell array coupled to a bitline and a first wordline and a negative-type metal-oxide-semiconductors (NMOS) pull-down structure coupled to the bitline and PMOS transistors. The positive-type metal-oxide-semiconductors (PMOS) transistors may be coupled to a second wordline, where a logic value carried on the second wordline is based on a logic value carried on the first wordline, and the PMOS transistors are structured to pre-charge respective drains of the NMOS pull-down structure to a high logic value based on a low logic value carried on the second wordline. The NMOS pull-down structure may be structured to discharge the bitline based on a high logic value carried on the second wordline.