This application note illustrates the in-circuit programmability of the AtmelAT89C51 Flash-based microcontroller. Guidelines for the addition of in-circuit programmability to AT89C51 applications are presented along with an applicationexample and the modifications to it required to support in-circuit programming.A method is then shown by whichthe AT89C51 microcontroller in theapplication can be reprogrammedremotely, over a commercial telephoneline. The circuitry described in this application note supports five volt programming only, requiring the use of anAT89C51-XX-5. The standard AT89C51requires 12 volts for programming.The software for this application may beobtained by downloading from Atmel’sBBS: (408) 436-4309.