Don't go back to analog when you have it in digital
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Are you trying to build a test bed for PID controllers? If so, you don’t need to create a PID controller with 8051, but you need to simulate process with 8051.
Whether it is 'Process' or 'Controller' to be built with 8051, the parameters with-in it needs frequent adjustment. When you go for a digital (8051 based) system, better to have these parameters too get adjusted digitally. The system should have interface to change it digitally through a keypad or through the interfaces you use for programming 8051.