Introduction - If you have any usage issues, please Google them yourself
IC AT89C51 is a low-power, high-performance, 8-bit microcontroller with 8 kB of Flash programmable and eras-able read-only memory
(EPEROM),256 bytes of RAM,32 input/output (I/O)lines, three 16-bit timers/counters, a six-vector two-level interrupt architecture, a full-duplex serial port, on-chip oscillator and clock circuitry.
Reference: http://www.seminarprojects.com/Thread-microcontroller-based-dc-motor-speed-controller#ixzz1Nl1UtNYi