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fast comunication

Comunication between two MCUs

Hi all, this is the question: which is the fastest method to exchange informations between two 8-bit mcus?

I'm developing a tacho unit for a friend's go-kart. As long as I'm planning to use multiplexed 7-segment displays, I want to dedicate one micro to the multiplexing job and the other one will perform measurements and other operations ( the tacho is only a part of the things I want put on the kart )... maybe I'll use two PIC16f877a.

To have all finally working, I nedd them to comunicate, with the one telling the speed to the other. My ideas were: