SRAM access time matching

[big10p]

When replacing SRAM on game PCBs, how important is it to match the access time of the original memory being replaced?

I have a board that uses 120ns 2114s in one section and 150ns ones in another.

Access times of available 2114s seems to vary quite a bit.

I guess it depends on various things, but are there any general guidelines to follow when replacing SRAM/DRAM?

 

[tb2000]

Normally with memory chips, as a general rule, as long as your chips are the same speed or faster then you should be ok. Though i'm not 100% sure if that always applies with older stuff, like it might get confused with memory that refreshes quicker than expected. I've seen it happen once or twice with logic chips on older boards before but not sure about memory chips.

 

[NivagSwerdna]

It really only depends on one thing... Clock speed.

120ns would be good to 8.3 MHz but a design margin would make that nearer 4-6 MHz

150ns would be good to 6.7MHz but a design margin would make it typically 3-5 MHz

(OK I lied about clock speed being the only thing... You need to consider capacitance and length of traces etc hence the design margin above)

In practice a pixel clock if this is in the video section could be 6 MHz, the slow Z80 CPU could be 4 MHz hence you might have different requirements in different sections of the board.