docs: add UART documentation

README.md

@@ -25,7 +25,7 @@ Everything is open source, so you can read, understand and modify the whole syst
 ## Demos
 ### Videos
 
-|Compiling and Running Hello-World|Moving Lines Demo|
+|Compiling and Running "Hello World"|Moving Lines Demo|
 |---|---|
 |<video src="https://insignificance.de/tridora/tdr-hello.mp4" controls></video>|<video src="https://insignificance.de/tridora/tdr-lines.mp4" controls></video>|
 
@@ -77,7 +77,8 @@ Everything is open source, so you can read, understand and modify the whole syst
 ## Documentation
 - [Instruction Reference](doc/tridoracpu.md)
 - [Memory Layout](doc/mem.md)
+- [UART](doc/uart.md)
 - [SD-Card controller](doc/spisd.md)
 - [VGA controller](doc/vga.md)
 
-More documentation is coming as time permits.
+More documentation is coming, as time permits.

doc/mem.md

@@ -31,6 +31,6 @@ Currently, only I/O slots 0-3 are being used.
 |I/O slot| Address | Controller |
 |--------|---------|------------|
 | 0      | $800    | UART       |
-| 1      | $880    | VGA        |
+| 1      | $880    | SPI-SD     |
-| 2      | $900    | SPI-SD     |
+| 2      | $900    | VGA        |
 | 3      | $980    | IRQC       |

doc/uart.md

@@ -0,0 +1,46 @@
+# UART
+The UART a single register at address $900.
+It uses a fixed serial configuration of 115200 bps, 8 data bits, 1 stop bit, no parity.
+
+## Reading the UART register
+
+|_bit_  |31|30|29|28|27|26|25|24|23|22|21|20|19|18|17|16|
+|-      |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
+|_Value_|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
+
+|_bit_  |15|14|13|12|11|10|09|08|07|06|05|04|03|02|01|00|
+|-      |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
+|_Value_|- |- |- |- |- |- |A |B |d |d |d |d |d |d | d | d |
+
+|Bitfields|Description|
+|---------|-----------|
+| _A_    | data available, at least one byte has been received
+| _B_    | TX busy, a byte is being transmitted and no data can be written
+| _d_    | 8 data bits
+| _u_    | unused
+
+## Writing the UART register
+
+|_bit_  |31|30|29|28|27|26|25|24|23|22|21|20|19|18|17|16|
+|-      |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
+|_Value_|- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
+
+|_bit_  |15|14|13|12|11|10|09|08|07|06|05|04|03|02|01|00|
+|-      |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |- |
+|_Value_|- |- |- |- |- |T |C |- |d |d |d |d |d |d | d | d |
+
+
+|Bitfields|Description|
+|---------|-----------|
+| _T_     | transmit enable, writes a data byte
+| _C_    | clear receiver, acknowledges a received byte
+| _d_    | 8 data bits
+| _u_    | unused
+
+
+## Notes
+A 16 byte FIFO is used when receiving data.
+
+When reading data, each byte needs to be acknowledged by writing the _C_ flag to the UART register.
+
+When the FIFO is empty, the _A_ flag in the UART register will be 0.