Pulsed Ring Oscillator for Storage Cell Read Timing Evaluation
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Solution Overview
Problem
Current memory cell evaluation methods fail to accurately measure internal read timing and read strength independently of write timing, and cannot be integrated within production storage devices to assess actual array load conditions.
Innovation Solution
A ring oscillator circuit that includes a selected storage cell, where the bitline is stimulated by a pulse generator, allowing for independent measurement of read timing and read current by varying the wordline and measuring frequency differences with and without the wordline enabled, and using a pulse regenerator to sustain oscillation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If ring oscillator circuits are used to measure storage cell timing, then access time measurement is provided, but read timing cannot be measured independently of write timing
Solution Approach 1:
The measurement process is segmented into two independent phases: first measuring the oscillation period with the word line disabled (excluding cell read timing), then measuring with the word line enabled (including cell read timing). The difference between these two measurements isolates the cell read timing component, enabling independent measurement of read timing separate from write timing operations.
2Measurement precision
If storage cells are cascaded in delay circuits, then delay measurement is achieved, but cell read strength cannot be measured independently
Solution Approach 1:
The storage cell under test serves its own measurement purpose by being placed in the ring oscillator configuration where its read current directly influences the oscillation frequency. The cell's read strength is self-measured through the frequency shift it causes when enabled, eliminating the need for separate test circuits or external measurement apparatus.
3Measurement precision
If test circuits are built to simulate storage cell operation, then read timing can be measured, but the circuits cannot be integrated within actual array environment
Solution Approach 1:
The ring oscillator circuit is designed to serve multiple functions: it can measure read timing, measure read current strength, and be integrated within the actual production storage array. The same circuit structure performs both as a functional part of the array and as a measurement tool, eliminating the need for separate dedicated test circuits.
4Measurement precision
If word line is enabled during oscillation measurement, then storage cell read timing is included in measurement, but write operation timing is also affected
Solution Approach 1:
The measurement uses periodic oscillation at a frequency where the oscillation period is deliberately made longer than the cell read timing. This timing relationship allows the oscillation to complete its cycle after the cell read operation finishes, preventing write operation interference from affecting the measurement while still capturing the read timing information in the frequency shift.
Data Source
AI summary
A pulsed ring oscillator circuit for storage cell read timing evaluation provides read strength information. A pulse generator is coupled to a bitline to which the storage cell to be measured is connected. The storage cell thereby forms part of the ring oscillator and the read strength of the storage cell is reflected in the frequency of oscillation. A pulse regeneration circuit is included in the ring so that the storage cell read loading does not cause the oscillation to decay. Alternatively, a counter may be used to count the number of oscillations until the oscillations decay, which also yields a measure of the read strength of the storage cell. The pulse generator may have variable output current, and the current varied to determine a change in current with the storage cell enabled and disabled that produces the same oscillation frequency. The read current is the difference between currents.