README.md

Universal Asynchonous Receiver/Transmitter (UART)

This directory contains assembly language examples related to UART.

UART Transmit example

This example shows how UART can be used to transmit.

Building

$ make uart-tx-sync.elf

Flash and Run the peripheral

Start openocd:

$ make openocd

Start a telnet session:

$ telnet localhost 4444

Flash the program:

> reset halt 
> flash write_image erase uart-tx-sync.elf.hex
> reset run

Start minicom:

$ minicom -D /dev/ttyUSB0 -b 115200 -8 

4. Run the executable

> reset run

This should now output a number of As in the minicom terminal window:

Welcome to minicom 2.7.1

OPTIONS: I18n
Compiled on Jan 26 2021, 00:00:00.
Port /dev/ttyUSB0, 05:25:03

Press CTRL-A Z for help on special keys

AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

Using an oscillooscope I've been able to capture the square wave for this transmission:

Notice that the signal is at 5V to start with when there is no trasmission and then a start bit. The start bit is followed by the first bit of the data which is high which represents a 1, this is then followed by 5 low signals, and finally another high signal which produces 1000001 which is 41h, 65d, and 1000001b and the the stop bit:

UART Receive Example

This example is similar to the previous one but instead of synchronously transmitting we will be receiving data.

Building

$ make uart-rx-sync.elf

Flash and Run the peripheral

Start openocd:

$ make openocd

Start a telnet session:

$ telnet localhost 4444

Flash the program:

> reset halt 
> flash write_image erase uart-rx-sync.elf.hex
> reset run

Start minicom:

$ minicom -D /dev/ttyUSB0 -b 115200 -8 

4. Run the executable

> reset run

If you type a character in the openocd terminal it will be echoed back, and the blue led on the board will light up, plus the rx led on the serial board will also indicate the transmission (and the sending too).