writing UART TX

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Hi Jack,


I am trying unsuccessfully to run Hamster's cheapscope project.  I see no activity.  In an attempt to simplify things, I first wanted to see if I could just send something out the serial port.  Looking at the generic Papilio One constraint file, I would think I should send on TX, which maps to P88, but ISE won't generate the code when I do that (and also P88 has a pullup, which would indicate that it should be an input).  Should I actually be mapping UART TX to RX in the generic constraint file, which is P90?  When I created a simple program in DesignLabs with ZPuino transmitting out the serial port, I saw the RX LED blink at the same rate I was sending out messages.  However, when I sent out a 1 KHz waveform to P90 in straight VHDL, I didn't see the RX LED blink.  When I mapped the 1 KHz waveform to P86, I did see the waveform on an oscilloscope.




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Hello Blake,


With the conventional naming convention for serial ports it can get confusing as to which pin to use... It is not always clear what perspective is being used, is RX from the perspective of the FPGA or from the perspective of the computer?


In the Papilio One hardware guide I try to make it as clear as possible:


It looks like P90 is what you want to use for transmitting. The RX LED is driven by the FTDI USB chip and is not directly connected to P90, so I'm not surprised that putting a 1Khz waveform does not cause it to blink, it probably only blinks when valid serial data put into the USB chip.


Hope that helps,


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