HI Vikrant,

the main question was the speed.

On low speed, the 89C52 can do the job without any additional hardware, e.g. to read a jog-shuttle-knob, moved from a human.

E.g. on using the P89C668 at 20MHz and T0 in mode3, the scan frequency was 13kHz. With an encoder of 40 steps per revolution it can count up to 19000 rpm.


On higher speed a small GAL22V10 can generate the two clock signals as counter input of the 89C52. And the 89C52 substract one counter value from the other.

If not the counters of the 89C52 want to be used, the GAL22V10 can drive some cheap CMOS-counters (e.g. 74HC590) or the counters can be included into a more complex CPLD.


On building CPLD projects, I prefer a synchronious design with a common clock signal. E.g. the ALE output can be used for this (2MHz on a 12Mhz 89C52).

The oscillation as Erik described should never be a problem, since any transition was clearly determined to count down or count up.
And the sample clock must not so high as this oscillation, only to catch any real movement.


Depending from the accuracy, which you want, you can decode all 4 phase changes or only a single phase change.


Peter