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Hi, give me somebody an explanation? I simulate DS89c420 device on XTAL Freq 27MHz. After reset CPU clock is 6.75MHz and single cycle instuction's time is 148ns. I supposed 1 internal clock per 1 xtal clock. Thank you Petr
Dallas 8051 parts typically require 4 clocks to execute an instruction. 27 MHz / 4 = 6.75 MHz = 148 ns. The classic 8051 architecture requires 12 clocks per instruction. Other variants I've seen operate at 2 or 6 clocks per instruction. So, the only way to be sure is simply to check the data sheet for your particular part. Also, with parts with less than 12 clocks per instruction, keep an eye out for extra configuration bits that control the input to the timers, baud rate generators, and such periperhals. They'll often have a 12-clock-per compatibility mode switch for replacing older parts.