SRAM-Layout Ring Oscillator for Separate Rise and Fall Delay Measurement
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Solution Overview
Problem
Conventional ring oscillators formed by connecting the output terminal of an inverter element and the drain of a transmission transistor in SRAM cells cannot separately measure the rise delay time and fall delay time, as both delays are measured simultaneously.
Innovation Solution
A ring oscillator design with multiple delay circuits, where at least one delay element is laid out in the same layout region as an SRAM cell and a path circuit is connected in parallel, allowing the delay element to output signals based on either rise or fall transitions, enabling separate measurement of rise and fall delay times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a ring oscillator is formed by ring-connecting the output terminal of an inverter element and the drain of a transmission transistor in SRAM cells, then the speed characteristic of SRAM cells can be evaluated, but it is not possible to separately measure the rise delay time and fall delay time
Solution Approach 1:
The ring oscillator is divided into multiple delay circuits (first delay circuit and second delay circuit) that are ring-connected. Each delay circuit contains transmission transistors configured to handle specific signal transitions (rise or fall), enabling separate measurement of rise delay time and fall delay time while maintaining the overall ring oscillator structure for speed characteristic evaluation.
2Measurement precision
If analog signals are used to measure electrical characteristics of transistors, then the measurement can be performed, but the measurement is vulnerable to various kinds of measurement noises and it is not easy to perform measurement with high precision
Solution Approach 1:
The patent replaces analog signal measurement with digital signal measurement using a ring oscillator. Instead of measuring electrical characteristics as analog signals that are susceptible to noise, the invention uses oscillation frequency and delay time measurements from the ring oscillator, which provide digital-like precision and are less vulnerable to measurement noises.
Data Source
AI summary
There are provided a ring oscillator having a plurality of delay circuits to be ring-connected. At least one of the plurality of delay circuits has a delay element formed in a layout region including the same layout shape as the layout shape of an SRAM cell, and a path circuit connected in parallel to the delay element. The delay element outputs an output signal to a delay circuit in the next stage within the plurality of delay circuits in response to one of rise transition and fall transition of a signal input to the input terminal of the delay element from a delay circuit in the previous stage within the plurality of delay circuits. The path circuit outputs an output signal to the delay circuit in the next stage in response to the transition other than the one transition.