As you know the AT90S1200 microcontroller includes an internal RC oscillator that is disabled by default. If you want to change it (enable or disable) you must to program it with parallel mode. The most programmers work on serial mode that is not possible to program this fuse bit (RCEN).

With this stand-alone programmer you can enable or disable the internal RC oscillator that works at ~1MHz.


The circuit:

The circuit is based, on the most of the parts and source code, to Mr Herbert Dingfelder's rcen_blower  (

Thanks to him, for his great job, i have made some modifications to the source code and to the hardware and I made a new one programmer that can enable or disable the internal RC oscillator, dependents on the position of the JP jumper. The old rcen_blower can only enable this fuse bit.

Download the schematic, source code and hex file of the RCEN programmer Ver 0.3 : .

Download the schematic and PCB in pdf format RCEn1200Prog.pdf


       - V 0.3     By Kyriakos Kontakos. He changed the LED indication. When you enable the RCEN bit the LED is flashing slow and when you disable the RCEN bit the LED is flashing fast. Moreover, he added a PCB .

       - V 0.2     By Serasidis Vasilis. Now you can enable or disable the RCEN bit of AT90S1200(A).

       - V 0.1     Created by Herbert Dingfelder. This programmer can only enable the RCEN bit of AT90S1200(A)

 My email