initial commit: figuring out SPI on the Tang Primer 20K, already 3 devlogs, will commit on a per-devlog/document change basis

docs/specifications.md

@@ -0,0 +1,21 @@
+# Protocol Specifications for ROSE
+
+## Protocol header
+
+Rose will use a 20-byte header for its packets. The headers contain
+the following components:
+
+1. *CMD* (8 bits): Contains the command to the fabric or
+   interconnected devices, see *Commands* for details.
+2. *SRC* (16 bits): Contains the packet source address.
+3. *DEST* (16 bits): contains the packet destination address.
+4. *ID* (32 bits): contains a packet identifier unique to the source
+   in a large time frame.
+5. *RESERVED* (64 bits): reserved for future iterations.
+6. *CRC* (8 bits): reserved for future iterations incorporating CRC
+   checksums.
+7. *LEN* (16 bits): length of payload.
+
+## SPI Specifications
+We will be using SPI in mode 0, where data is sampled on the falling
+edge of the clock pulse and shifted out on the rising edge.