Fast DDS with ATmega88

I’m planning to make some RFID hacking in near future using 150 kHz tags. Since I don’t have a signal generator, I decided to go where quite many people have gone before and build myself one, more specifically a DDS. Instead of just taking a complete project from the net, I thought this would be a good way to learn a bit of AVR assembly programming, and manual D/A (digital to analog) conversion using R-2R ladders. Here’s what I built:

I’m skipping the schematic to save some time – basically it’s a ATmega88 with 6-pin programming header, power, a 16 MHz crystal (other frequencies also work, lfuse for this setup if 0xFF) and a red LED that is not used. The R-2R ladder is wired with white jumper wires to PB0-PB5 (it’s a 6-bit DAC) so that PB0 is the “least significant bit” and PB5 the most significant one. Read on for details.

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