[sdiy] external memory for microcontrollers

[Eric Brombaugh]

On 02/16/2017 12:48 PM, Ingo Debus wrote:
>
>> Am 16.02.2017 um 00:06 schrieb Eric Brombaugh <ebrombaugh1 at cox.net>:
>>
>> It's easy to set up and works without too much fuss. You'll generally find that you have to move to the 144-pin package to get sufficient pins for a memory connection though, and that will increase the cost of the MCU somewhat.
>
> 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.

Eric