Add images to docs

Author: matiasilva

docs/src/img/uart-rx-proto.svg

@@ -17,6 +17,7 @@ This documentation is also available to download as a PDF.
 motivation
 specification
 core
+uart
 fpgas
 testing
 log

docs/src/index.md

@@ -0,0 +1,30 @@
+# UART
+
+To communicate with the outside world, a memory-mapped UART module was added to
+the system. It contains a receiver and a transmitter.
+
+## Hardware block
+
+The receiver uses an oversampling technique to estimate transmitted bits on the
+data line. In the absence of a clock line, detection of a start bit (0 value)
+followed by sampling of data at the middle of a bit's transmission ensures
+accurate retrieval. A standard oversampling factor of 16 minimizes the error to
+1/16 from the middle point.
+
+```{figure} img/uart-rx-proto.svg
+:width: 200px
+:align: center
+
+Transmission of a byte
+```
+
+Specification:
+
+- 8 data bit
+- 1 stop bit
+- parity bit supported
+- AHB data interface
+
+The hardware has been tested at baud rates of 115200, 19200 and 9600.
+
+## Software interface