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And you will find that syncing that with the 1 GHz clock is more accurate than a countdown. There may be some 1 clock 1/2 period jitter because there will be some sort of error in 80Mhz oscillator wrt the 1 Ghz rate. The overall rate will be as accurate as the 80 Mhz oscillator. It also means you only need 1 D flip-flop that can clock at 1 Ghz.