So, Ive built a "final" version of a midi to CV circuit on proto breadboard and I am having trouble with the SPI communication between the Atmega and the DAC. Individually, the arduino part of the circuit appears to work perfectly: it is receiving Midi messages, flashing the appropriate led, etc and Ive successfully ran a few other test sketches like "blink." i did some basic testing with a multimeter, all the connections are good and wired correctly. The Dac seem to be getting something (voltage flickers with startup routine and midi keypress) but I assume it is garbled- the Dac doesnt respond.
To test the DAC and my layout, I removed the atmega chip , put it in a "real" arduino board, and then patched the arduino pin-outs back to the appropriate pin of the protoboard socket: arduino RX to socket pin 2; arduino pin3 to socket pin 5; 5v(from the protoboard power supply) to arduino 5v; etc. When running through an arduino and patched to the protboard, it works everytime. All connections are going through the same points on the protoboard, just with a mess of jumper wires added. I would assume that this would only add "noise" to the circuit, not reduce it... so I think that it is safe to assume that the arduino board is doing somthing that the scrath built versions lack.
Power filtration? Ive doubled up on the powersupply filter caps, have 100n across the rails of both the dac and the atmega. Could power supply noise still be a problem?
Furthermore I rebuilt a new "arduino" on a solderless board, and again while it run sketches, I cannot get the SPI to work. Only when connected to an uno or duemilanove does it run properly.
the only other difference( I can think of) between my circuit and the arduino board is when in the arduino, the atmega is using the on board crystal /cap resonator circuit, while I have single resonator components. I would think this would make not a difference, especially since the atmega appears to work.
Please, I am totally out of ideas and have gone through all the "SPI noise" or troubleshooting threads i could find. I havent tried pull-up resistors yet, but have never need them with this circuit before.
Any ideas or thoughts are greatly appreciated. thanks in advance.