[sdiy] external memory for microcontrollers

[Veronica Merryfield]

>> I’ve heard that the PCB traces for the address/data lines have to be matched in length. Is this really true?
> 
> I made no special effort to match lengths of the interfacing signals and things work fine, but my SDRAM is < 3cm from the MCU and the length mismatch between any two signals wouldn't be more than ~2cm. Given that the FMC pins on the STM32 FMC come out on all sides of the 144-pin LQFP, it's hard to imagine how matching would help in any case.
> 
> The rule of thumb I've always used is that signal speed on a PCB is ~1ns/foot. At that rate a 2cm mismatch amounts to about 66ps delta. On a ~100MHz SDRAM that's probably not going to be noticeable.

This rule of thumb is a pretty good one. But even so, matching is not that hard to do, just a little tedious.

Veronica