"but many of these routines impose heavy computational load on 8051 based micro I want a relatively simple routine."

That is a very controversial requirement. And can be a reality only for very slow responding systems with a very heavy inertia. Like for instance the temperature regulation of a large mass. Because of its large thermal inertia, the system response will be slow and you can get away with slow computations.

But if you doing a PID for a Servo valve spool position control where response times are of the order of a few milliseconds, simple or slow routines are of little use.

To be more specific what is your application ?

Even to date I find it is quite simple to implement PID in pure analog - process variables are anyway analog - so why digitize them and introduce quantization errors ? Maybe its me.

Raghu