Rejig serial periph

README.md

@@ -30,20 +30,19 @@ these headers.
 |--------|--------|----------|--------------|
 | TXBUF  | 0      | 32       | write        |
 | RXBUF  | 32     | 32       | read         |
-| TXDATA | 64     | 1        | atomic write |
-| TXHEAD | 65     | 1        | atomic read  |
-| TXTAIL | 66     | 1        | atomic write |
-| RXDATA | 67     | 1        | atomic read  |
-| RXHEAD | 68     | 1        | atomic write |
-| RXTAIL | 69     | 1        | atomic read  |
+| TXHEAD | 64     | 1        | atomic read  |
+| TXTAIL | 65     | 1        | atomic write |
+| RXHEAD | 66     | 1        | atomic write |
+| RXTAIL | 67     | 1        | atomic read  |
 
-For both sending (`TX`) and receiving (`RX`), there are four
-registers: `xBUF`, `xDATA`, `xHEAD` and `xTAIL`:
+For both sending (`TX`) and receiving (`RX`), there are three
+registers: `xBUF`, `xHEAD` and `xTAIL`:
 
 - `xBUF` registers are 32-byte FIFO ring buffers used for data
-- The `xDATA` registers indicate whether data is available (0 for
-  data, FFh for no data)
-- The `xHEAD` and `xTAIL` registers specify the start and end of
-  data in the FIFO, `xHEAD` being the offset of the first byte of
-  data, and `xTAIL` being the offset of the first byte after the
-  data.
+- The `xHEAD` and `xTAIL` registers specify the start and end of data
+  in the ring buffer, `xHEAD` being the offset of the first byte of
+  data, and `xTAIL` being the offset of the first byte *after* the data.
+
+In order to be distinguishable from the empty state, the ring buffers
+must never be completely full -- there must always be *at least one*
+unoccupied byte between the tail and the head.