Direct digital synthesizer (DDS) with ATmega8515, Xilinx XC9572 CPLD and SDA5708-24B display. This is my home brew hobby DDS synthesizer consisting of different self-made components like a 16-bit R-2R digital to analog converter (DAC). Due to design failure in the output driver circuitry the output lacks of any low-pass filter, but it is interesting to see R-2R DAC raw output in action :)
ATmega8515 polls two buttons to increase/decrease frequency status word, computes resulting frequency and prints it on the display.
The EPROM is a 16-bit x 65536 words one containing full sine wave samples.
The Xilinx CPLD implements a 16-bit phase accumulator using frequency status word provided by the ATmega8515. It also includes a state machine for proper timings accessing EPROM (27C210-15) and latching samples for 16-bit DAC. The base clock of the CPLD is 16.257 MHz. Due to 150 ns access time of the EPROM, the state machine divides the clock by 6 resulting in 2.7095 Mega-samples per second. With 16-bit phase accumulator and frequency status word the lowest DDS frequency step is 41.343689 Hz.
Every bit of this DAC has a CMOS 4007 as a line driver, so there are total 16 CMOS
4007 ICs. There is no low-pass filter at the output, because I underestimated the constraints
of the operational amplifier.
It is an 8-digit LED matrix display.
