Self-timed High Temperature ROM

This is the 4th article about the design of the Tekmos high temperature ROM.

The Tekmos ROM is based off a read cycle. On a clock edge, the precharge on the bit lines is turned off, and the word line is turned on. If a zero is being read, the bit will pull the bit line low. And after sufficient time has passed, the sense amps will be clocked, latching in the read data. Then the word lines will be turned off, and the precharge turned back on again. Then the ROM waits for the next read cycle.

The challenge is setting the delay between turning on the word lines, and clocking the sense amps. If the sense amps are clocked too soon, then bad data will be clocked in, and the ROM will fail. But delaying the sense am clocking increases the ROM access time, which is the key parameter for the ROM. If the access time is too long, then the processor may read in bad data, which is a ROM failure.

To make the design work, we need a delay that exactly tracks the delays of the ROM read cycle. And the way we get that delay is to add another bit line to the design. The increase from 568 to 569 bit lines is a very small increase in ROM area, but provides a lot of benefits. This additional bit line is pre-programmed with zeros. Because every bit is set, and has added it's capacitance to the bit line, this additional bit line is guaranteed to be slower than any other bit line. The new bit line then goes through an 8:1 mux so that the total delay matches the other bit lines. But instead of a sense amp, this bit line goes to a ratioed inverter. Since the bit line is always going to read a zero, the inverter will always switch after every other bit line has valid data on it. We then use this inverter to clock the sense amps for the other bit lines. And since the sense amps are inverters too, as long as the reference has an equal or lower threshold, the design will work every time, regardless of voltage, temperature, or process variations.

Next month I will discuss how the ECC circuit works.

Date

2018-01-22