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Using the USART

Using the USART. Configuration and Programming. Configuring the USART. Several I/O registers are used to: Control the configuration of the USART Baud rate (bits/s) Parity (even/odd/none) Number of data bits (4-9) Framing ( 1 or 2 stop bits)

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Using the USART

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  1. Using the USART Configuration and Programming CE-2810 Dr. Mark L. Hornick

  2. Configuring the USART Several I/O registers are used to: • Control the configuration of the USART • Baud rate (bits/s) • Parity (even/odd/none) • Number of data bits (4-9) • Framing ( 1 or 2 stop bits) • Represent the status of a recent transmission or reception • Frame errors • Data overrun errors • Tx complete • Rx complete CS-280 Dr. Mark L. Hornick

  3. Lots of I/O registers make configuring the USART a bit complicated: UCSRA – USART Control/Status Register A Mainly status flags UCSRB – USART Control/Status Register B Flags for enabling transmitting, receiving, interrupts UCSRC – USART Control/Status Register C Mode selection UBRRH, UBRRL– USART Baud Rate Registers (16 bit combined) Baud rate selection UDR– USART Data Register Does two things at once: • Contains data to be transmitted, or • Data that is received Full documentation on the USARTcan be found (around p 146) of theAtmega32.pdf reference manual CE-2810 Dr. Mark L. Hornick

  4. UBRRL/UBRRH are used to set transmit and receive speed URSEL – Register select UCSRC I/O address overlaps with address of UBRRH. Set to 0 if writing this Register as UBRRH; otherwise UCSRC UBRR11:0 – Baud Rate select 12-bit register specifying the USART transmit/receive baud rate. Use the following table for calculating the value to place in these bits. BAUD is in bits/sec, fosc is the Atmega32 clock frequency. CE-2810 Dr. Mark L. Hornick

  5. UCSRB/C serve multiple purposes UCSZ2, UCSZ1, UCSZ0 – Character size select Together, these set the number of data bits being transmitted or received Set these to (0:1:1) for 8 data bits; see reference manual for other settings URSEL– Register select UCSRC I/O address overlaps with address of UBRRH. Set to 1 when using this Register as UCSRC; otherwise UBRRH UMSEL– Mode select Set to 0 for Asynchronous mode UPM1, UPM0 – Parity mode select Parity disabled (0:0), Even (1:0), Odd (1:1), Reserved (0:1) USBS– Stop bit select Set to 0 for framing using a single Stop Bit UCPOL– Clock polarity (do not use) CE-2810 Dr. Mark L. Hornick

  6. UCSRB/C continued RXCIE – Receive complete interrupt enable Set to enable Receive Complete Interrupt. TXCIE – Transmit complete interrupt enable Set to enable Transmit Complete Interrupt. UDRIE – Data Register Empty interrupt enable Set to enable UDRE Interrupt. RXEN– Receiver Enable Set to enable the USART Receiver TXEN– Transmitter Enable Set to enable the USART Transmitter UCSZ2 – see previous slide explaining UCSRC bits • RXB8 – contains the 9th data bit when receiving 9-bit data • TXB8 – contains the 9th data bit when transmitting 9-bit data CE-2810 Dr. Mark L. Hornick

  7. UCSRA contains status bits you read RXC – Receive complete Set when data is received in the Receive Buffer (RXB) TXC – Transmit complete Set when data in the Transmit Buffer has been completely transmitted and no new data in UDR; manually cleared by writing a 1 to this bit UDRE – Data Register Empty Set when Transmit Buffer is ready to accept new data to be transmitted FE – Frame Error Set when Data in Receive Buffer had a frame error detected DOR – Data Overrun Set when Receive Buffer is full (but unread), and new data is coming in PE – Parity Error Set when Data in Receive Buffer had a parity error detected (if parity checking was enabled) U2X – Transmission speed doubler Leave at 0 (default). When explicitly set to 1, doubles the transmission speed of the USART MPCM– Multiprocessor Comm mode Leave at 0 (default) for normal communication CE-2810 Dr. Mark L. Hornick

  8. The UDR register contains the value the USART transmits or receives • UDR– USART Data Register • TXB and RXB I/O addresses overlap • When UDR is written, the contents are shifted to the Transmit Buffer (TXB). This takes a short period of time • When UDR is read, the contents of the Receive Buffer (RXB) are first shifted into UDR, so that UDR contains the contents of RXB CE-2810 Dr. Mark L. Hornick

  9. How to use the USART • Set baud rate via UBRRH and UBRRL • Enable Transmit and Receive via UCSRB • Disable all USART interrupts via UCSRB • Set Parity, #Data bits, #Stop bits, Async via UCSRC • To Transmit: • Poll UDRE bit (bit 5) in USCRA until set (ready to transmit) • Once set, place character to be written in UDR register, which causes the USART hardware to transmit that character • To Receive: • Poll RXC bit in UCSRA until set (character was received) • Once set, received char will be in UDR register, and the UDR register will be able to be read. CS-280 Dr. Mark L. Hornick

  10. The USART can receive while it is transmitting But the TXB and RXB buffers can each hold only one character • So you must keep polling RXC to avoid dropping received characters • And you must poll UDRE before sending another character to ensure that TXC has been emptied CS-280 Dr. Mark L. Hornick

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