USARTS CS423 Dick Steflik

USART ● Universal Synchronous Asynchronous Receiver Transmitter ● used to send and receive small packets over a serial line  full or half duplex ● typically asynchronously ● 5 – 8 bits of data ● 2 or 3 framing bits  start bit ● 1 or 2 stop bits ● 0 or 1 parity bits

Data Format ● Must be agreed on by sender and receiver before any exchanges can be made ● stop bit (1 to 0 transition) ● 5 – 8 data bits ● 0 or 1 parity bits (odd or even parity) ● 1 or 2 stop bits (logic 0)

Parity ● error detection mechanism ● odd parity  if an even number of data bits are 1 the parity bit is set to 1 to ensure an odd number of 1s ● even parity  if an off number of data bits are 1 the parity bit is set to 1 to ensure an even number of bits are 1s ● calculated and set by transmitter ● recalculated buy receiver and compared to transmitted value, if no match a bit was either picked or dropped in transmission.

Programming Model ● Data input register ● Data output register ● Control register  speed, data bits, parity, stop bits, start, stop ● Status register  data ready, transmitting ● interrupts  overflow, underflow, data ready, data sent

Sending data ● Remember synchronization is on a character by character basis ● check status ● load data register ● start transmit ● wait for transmission complete status or for interrupt ● repeat

Receiving data ● poll status register for data ready or wait for interrupt ● read data (save it) ● repeat

ATtiny2313 ● UDR – Data Register  read / write (depending on context) ● UBRRL & UBRRH – Baud Rate Register ● UCSRB – Control Register  writable ● UCSRA – Status Register  readable

UCSRA ● 7 – RXC – Receive Complete ● 6 – TXC – Transmit Complete ● 5 – UDRE – Data Register Empty ● 4 – FE – Frame Error ● 3 – DOR – Data Overrun Error ● 2 – UPE – Parity Error ● 1 – U2X - Double the speed (Async mode only) ● 0 – MPCM – Multi-processor Communication Mode

UCSRB ● 7 – RXCIE – Rx Comp Interrupt Enable ● 6 – TXCIE – Tx Comp Interrupt Enable ● 5 – UDRIE – Dara Reg Empty Interrupt Enable ● 4 – RXEN – Receiver Enable ● 3 – TXEN – Transmitter Enable ● 2 – UCSZ2 – Character size MSB ● 1 – RXB8 – 9 th bit if frame size is 9 ● 0 – TXB8 – 9 th bit if frame size is 9

UCSRC ● 6 - UMSEL – (0-async, 1-sync) ● 5,4 – UPM1, UPM0 USART Parity Mode  0 0 disabled  0 1 reserved  1 0 even parity enabled  1 1 odd parity enabled ● 3 – UCBS – Number of stop bits ( 0 – 1 bit, 1 – 2 bits) ● 2,1 – UCSZ1, UCSZ0 - USART Character Size used with UCSZ2 bit in UCSRB ● 0 – UCPOL – used in sync mode for clock polarity