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Which Impedance?

williajl , 05-13-2021, 07:05 PM
On the microcontrollers that I design with, they do not specify any specific impedance for signals on the EMIF (Parallel Interface Bus), or for any signal. Therefore can we just selectively choose an arbitrary impedance such as 50 or 75 ohm and stick with that through out the layout? Does it really matter? I am asking because I am using a standard 6 layer stack where I want to use layer one to layer two interchangeably for my traces. Likewise I want to use layers 5 and 6. When I run tracks on layer one, I will always make sure their is a reference polygon below it entirely and when I switch to run tracks on layer two I will ensure that there is a complete reference polygon above it. The only difference is the copper weight. Layer one and six have 1oz and the inner layers have 1/2oz. At first I was going to rout with a 50 ohm, however, that would require the inner layer trace width to be at 0.12794mm which is at the limit of the particular board house (0.127mm) JLCPCB. Also, I chose layers three and four as a full ground on layer three and full power on layer four. I chose this because dielectric is 0.127mm for better coupling. Also it helps on the other two inner layers where the core separation is 0.565 from these two layers.

Again, does it really matter so long as it is consistent when the chip set does not specify anything?

Anyone's thoughts is appreciated.
robertferanec , 05-17-2021, 03:49 AM
Simple advice: if the tracks are very short, it is not so critical.

I would recommend to watch this my video

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