Dicussion 03 UART

Objectives

Install the appropriate program on your computer

COM or serial port uses as few as 3 wires, GND, RX and TX (named with respect to the master)
Slave Device (TX) -> Master Device (RX)
Slave Device (RX) <- Master Device (TX)
Data is transmitted without a clock and is instead transmitted at a rate predetermined or pre-negotiated rate known on both sides
The baud rate defines the length of each bit as $\frac{1}{\rm buadrate}$

data is framed by a start bit and one or more stop bits
start bit is typically a high signal to start the data frame
stop bit is typically a low signal, often 1, 1.5 or 2 times as long as the other bits
- serves as pause between the frames, allowing the next start bit

Typically, a FIFO buffer of 1 or more bytes is used for transmission and reception
The status of the buffers is probed internally by empty and possibly full status bits
If the read buffer is only 1 byte, NOT EMPTY is the same as DATA READY
If the write buffer is only 1 byte, NOT EMPTY is the same as NOT READY/FULL

What happens if the Master sends data to the slave receive buffer faster than it can be unloaded ?
- Data is lost
To prevent overflow, additional hardware lines can be used to communicate status and perform handshaking
A software handshaking can also be implemented
The configuration for stop/start bits, parity, hardware/software hand shaking, and baud rate must be set the same on both ends of the communication

The following set of registers are used to communicate over UART
- UCSRA
  - Flags for various errors that might occur during data transmission, e.g. parity error, frame error etc.
- UCSRB
  - Contains lot of enable bits. eg. different interrupt enable bits and the receiving and transmitting enable bits
- UCSRC
  - Set the parity mode, stop bits etc.
- UBRRH & UBRRL
  - The higher byte (UBRRH) and lower byte (UBRRL) is stored for generating the required baud rate
For Our AVR, the required registers have slightly different names and are in the EXTENDED I/O, meaning, we must access them as memory and not as registers

Realterm

Select the correct port, Baud(rate), #Stop Bits
Use the Open and/Change buttons to update settings
Status in displayed in the Bottom-Right Status Bar
- Initially it will read "Closed"
- Make sure that any changes are displayed

Download interfacing code from your instructor’s website (uart.s)