RAM is one big problem for "discrete projects". It must be fast, large and cost-effective. There are many types of discrete RAM but none that is based on discrete MOSFET (yet).
Integrated DRAM uses MOSFETs but they differ from the discrete form because of the intrinsic diode with the substrate : it takes two discrete MOSFET to make one normal MOSFET. This would not be cost-effective to use discrete MOSFET to implement classic DRAM circuits.
I found inspiration with the diode-capacitor cell described for the TIM computers :
I can't figure out what capacitance was used in the relay-based TIM8. However, with a MOSET-based circuit, the capacitance can be greatly reduced (to 100nF for example) because the required trigger energy is considerably lower. I even have a reel of dual Schottky diodes in SOT23. But I'm not sure about the leakage...
Now, a good MOSFET has a pretty low leakage. I have played around (on paper) and have come up with a topology that replaces the diode with a MOSFET. The gate capacitance is not used but the parasitic diode is !
Each access cycle takes 2 steps:
- Read and empty the storage capacitor: drive RD high => This pushes the capacitor's lower electrode high, which is then read on the Data line. Probably a threshold current is required to read a "1".
- Write the value: drive the desired level on the Data line, drive RD to 0V and WR to 1 (let the current flow to RD so the lower electrode is 0V).
It's funny that the read phase works a bit like a charge pump...
Data, RD and WR are totem-pole (complementary) drivers. I will have to try different circuits for the sense. The line and column select (demultiplexers) will need a lot of transistors too... But at least it's more compact than the only other MOSFET RAM that I know :-)
Now I wonder what should be the refresh frequency...
Any comment/remark/historical perspective ?