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doc: add packet structure to README

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Xiretza 2022-06-20 13:59:01 +02:00
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@ -13,3 +13,91 @@ Because blinkenwall v3 has `64 * 96 = 6144` pixels, driving it at 100 FPS requir
(typically SPI), so this would require coordinating several microcontrollers in parallel. A
much more integrated solution is to instantiate as many WS2812 drivers as desired in an FPGA,
then point the entire video firehose at the FPGA.
## Ethernet communication
The controller accepts UDP packets containing image data on port 61437 ("PIXEL"). Each packet
contains color data for one strand of LEDs.
The packet structure is as follows:
<table>
<tr>
<th></th>
<th>00</th>
<th>01</th>
<th>02</th>
<th>03</th>
<th>04</th>
<th>05</th>
<th>06</th>
<th>07</th>
<th>08</th>
<th>09</th>
<th>0a</th>
<th>0b</th>
<th>0c</th>
<th>0d</th>
<th>0e</th>
<th>0f</th>
</tr>
<tr>
<th>00</th>
<td colspan="4">magic (<code>0x5049_584c</code>)</td>
<td colspan="4">strand number</td>
<td colspan="4">frame number</td>
<td>0</td>
<td>px1 R</td>
<td>px1 G</td>
<td>px1 B</td>
</tr>
<tr>
<th>10</th>
<td>0</td>
<td>px2 R</td>
<td>px2 G</td>
<td>px2 B</td>
<td>0</td>
<td>px3 R</td>
<td>px3 G</td>
<td>px3 B</td>
<td>0</td>
<td>px4 R</td>
<td>px4 G</td>
<td>px4 B</td>
<td colspan="4">...</td>
</tr>
</table>
Every packet begins with a magic number `0x5049584c` (ASCII for "PIXL"), followed by the strand
number and the frame number. All numbers are big endian. After the header follows the pixel data,
separated into one 4-byte word per LED.
Once all strands for a given frame number have been received and the video page has been flipped,
the controller answers with a confirmation packet:
<table>
<tr>
<th></th>
<th>00</th>
<th>01</th>
<th>02</th>
<th>03</th>
<th>04</th>
<th>05</th>
<th>06</th>
<th>07</th>
<th>08</th>
<th>09</th>
<th>0a</th>
<th>0b</th>
<th>0c</th>
<th>0d</th>
<th>0e</th>
<th>0f</th>
</tr>
<tr>
<th>00</th>
<td colspan="4">frame number</td>
</tr>
</table>