Larry McGovern Posted April 8, 2016 Report Share Posted April 8, 2016 I've been making my way through Mike Field's terrific tutorial using the Papilio DUO and LogicStart Shield. I have to say, I've been nothing but pleased with the boards, the tutorial, everything! It has been the perfect way to jump start my knowledge of FPGAs. What a great education! I'm currently on the VGA chapter, and am successfully driving my LCD monitor with a VGA signal. However, there must be something I'm missing. The analog output is supposed to range from 0 to 0.7 volts, but as far as I can tell from the schematic, if I enable all four bits, then the analog output will go to 3.3V. With all four bits enabled, the "resistor ladder" does not act as a voltage divider anymore, and the output voltage is the same as the FPGA pin output. Indeed, I've confirmed this on my multimeter. What am I missing? Quote Link to comment Share on other sites More sharing options...
Larry McGovern Posted April 9, 2016 Author Report Share Posted April 9, 2016 Ok, I feel like a dummy. I'll answer my own question. It appears the resistor ladder on the LogicStart board depends on the monitor having a 75 Ohm impedance, providing the other half of the voltage divider. I had assumed the impedance would be much higher, but now understand 75 is the standard. VGA test circuit working now on my Papilio Duo. Quote Link to comment Share on other sites More sharing options...
Jack Gassett Posted April 11, 2016 Report Share Posted April 11, 2016 Larry, thank you for the question and the answer to the question! Jack Quote Link to comment Share on other sites More sharing options...
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