RCEN fuse bit blower for the AT90S1200 8Bit RISC CPU

Posted on Feb 5, 2014

The AT90S1200 RISC Microprocessor (AVR family) is a very neat component to program a PLL synthesizer or for doing other rather trivial tasks in a radio circuit. It is reasonably cheap, easy to program and a nice integrated development environment is available for free from the manufacturer ATMEL. And the best thing about it: It has an internal RC

RCEN fuse bit blower for the AT90S1200 8Bit RISC CPU
Click here to download the full size of the above Circuit.

clock generator so it does not need a crystal, in fact it does not need any additional components at all. The internal oscillators frequency is somewhat temperature dependent, but for most applications it doesn`t really matter how fast exactly the processor works. The only problem is that it is not possible to activate this internal oscillator with one of the (homemade or cheap-to-buy) serial in-circuit programmers. (The fusebits are only accessible in the parallel programming mode. ) Atmel sells a variant called AT90S1200A that has the internal oscillator activated by default. Unfortunately it seems to be impossible to get this variant here in Germany and from folks in the US I have learned that it is available but more expensive in the US. The circuit published here is able to activate the so called RCEN (RC oscillator ENable) fuse bit which is non-volatile and will be set forever unless you put it in the circuits with another firmware to deactivate it. It uses another AT90S1200 (oh boy I love this smallest part of the AVR family) as a master that sends all the necessary signals to the slave (the one that has the RCEN bit set). The hardest part was actually to extract all the necessary signal sequences from the datasheet. (It took me a full weekend to really understand everything and build the circuit. ) The master has a crystal attached so it works in no special mode or such. The master socket can also be used to plug in...

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