doc: add packet structure to README

README.md

@@ -13,3 +13,58 @@ 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>