@Kiran,

4000RPM with an accuracy of 1RPM was absolutely no challenge for the 8051.

E.g. on an update time of minimal 500ms I count at 4000 RPM (=66.7Hz) a value of 34 revolutions (=510ms).
And at a crystal of 11.0592MHz I get a timer value of 470016. This value was 100 times more accurate as needed for the wanted accuracy of 1RPM.

So you can still allow an up to 100 cycle different delay from sensing the last revolution until reading the time base.

But if it was really needed, the time difference can be read out also with an accurcy of 1 cycle on using the idle mode.


Peter