Publisher Summary P89C66x Microcontroller is a member of the 80C51 family able to execute one instruction in six clock cycles; hence, providing twice the speed of a conventional 80C51. This device is a single-chip 8-bit microcontroller manufactured in an advanced CMOS process. The instruction set is 100% executing and timing compatible with the 80C51 instruction set. The use of high level (C) language programs is, in the main, left as an exercise for the reader; although, one example illustrating the use of a program written in C has been included. When an interrupt occurs, the processor pauses, saves the current program counter (PC) value into RAM designated by the stack pointer (SP), and then jumps to the interrupt vector address. The processor then carries out the instructions at the interrupt vector address and returns to the original program sequence, retrieving the previous PC data. The use of PWM allows a variable DC average voltage to drive small inductive loads such as a small DC motor. The PWM frequency has to be much faster than the movement of the application.