Doug, the P54CS memory design manager, stated numerous times to not fail a good cell. My mantra during the design of the very first weak write test mode circuit became “thou shalt not fail a good cell.” To address this requirement, the boundaries between a good and bad cell needed to be determined. Given the methodology of weakly overwriting a cell I had to determine “weak” and “good” cell boundaries.
One can look at an SRAM cell as a latch and all latches can be put into meta-stability
The following criteria guided the WWTM circuit design:
- Not overwriting a good cell
- Detecting open or weak connections in the p-devices
- Detecting weak p-devices
Simulating good cells during a weak write became my first task. I varied the WWTM
Summarizing the design exploration, I used five lines to delineate the boundaries for each criterion:
1 – Does not overwrite a good cell at worse case corner;
2 – Does not overwrite a good cell at typical process corner;
3 – Flip a symmetric defect within 50 ns;
4 – In flipping symmetric defects, recover a stable ‘1’ state at the end of the 50 ns weak write signal;
5 – Detect 250 kilo-ohm minimum resistance for asymmetric.
In responding to the “Don’t Fail Good Cells” mantra, I explored the difference between good and bad SRAM cell behaviors to set reasonable boundaries. I had to assure that the faulty cells would be detected. However, if the WWTM circuit had failed good cells I would not have an effective solution.
Have a Productive Day,
Anne Meixner
Dear Reader, What memory or question does this piece spark in you? How have you balanced multiple design criteria for your solution? What kind of trade-offs between efficiency and effectiveness of a solution have you made? Please share your comments or stories below. You, too, can write for the Engineers’ Daughter–See Contribute for more Information.
Additional Reading
For more information on metastability read this page.
To learn some basis on semiconductor manufacturing you can read this overview on wikipedia.
Anne, I’m not in your field, so the spelling you’ve chosen for “though” may be unique to engineering. In my world, “though” is most often used as a shorter form of “although” [these two words rhyme], and the spelling for the old formal way of saying “You” is “Thou” [as in “Thou shalt not worship graven images” or “Wherefore art thou, Romeo.”]. So I’m a little confused here, but I do enjoy learning more about your problem solving process!
Lili,
thanks for capturing the error in “Though” vs “Thou” I meant “Thou” and I’m going to fix this right now. I will claim it is due to aa dsylexic moment.
This is what happens when I don’t get a copy editor to look at this prior to publishing.
glad you liked learning about the problem solving methods, it’s one of the motivations I have in writing.
Anne