On Friday 23 July 2010 05:53:43 Adam Wang wrote:
> How long of the cable length you tested?
> We need prepare a shielded twisted pair cable around 15 meters
Only about 2m. So let's test it again with the long cable. I will include a
6N138 with the board I'm shipping you.
For the waveform test, I used a 24.4kHz squarewave generator inside the FPGA
(design files attached, just create a Spartan6 ISE project with those) and
looped back MIDI OUT to MIDI IN with the 2m cable, then measured the signal at
the output of the optoisolator (btw: it might be a good idea to include a test
point there on the next PCB run, even though this signal is already easy to
access due to the big size of the components). I got a square signal
oscillating between 0.3V and 3.0V approximately. The rise time was pretty
slow, but most of the "slowness" occurs above 2V which is the threshold of the
FPGA input, so it should be fine. Falling edges are OK.
I also performed communication tests using real MIDI data between the board
and a computer using a USB/MIDI adapter.
S.
module top(
input clk50,
output midi_tx
);
reg [10:0] counter;
assign midi_tx = counter[10];
always @(posedge clk50) counter <= counter + 11'd1;
endmodule
NET "clk50" LOC = AB11 | IOSTANDARD = LVCMOS33;
NET "midi_tx" LOC = AA21 | IOSTANDARD = LVCMOS33 | SLEW = SLOW;
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