EE 308: Microcontrollers

Transcription

1 EE 308: Microcontrollers Introduction to Communication USART Aly El-Osery Electrical Engineering Department New Mexico Institute of Mining and Technology Socorro, New Mexico, USA February 27, 2018 Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

2 Why? Need to communicate with other devices, microcontrollers, or computer Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

3 Types Serial vs parallel Synchronous vs asynchronous One-to-one vs bus Full-duplex vs half-duplex Single-ended vs differential Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

4 Serial vs parallel Serial interface transmits one bit at a time data throughput is low needs only one data line Parallel interface transmits several bits at a time higher throughput requires more data lines Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

5 Synchronous vs asynchronous Synchronous interface requires sharing the clock between the transmitter and the receiver Asynchronous interface does not require the same clock Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

6 One-to-one vs bus One-to-one is communication between only two devices Some communication protocols allows the use of bus topology to connect multiple addressable devices Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

7 Full-duplex vs half-duplex Full-duplex: both devices can transmit at the same time Half-duplex: only device can trasmit at any given time Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

8 Single-ended vs differential Single-ended: levels are in reference to ground Differential: levels are determined based on the voltage difference between signals transmitted on two lines Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

9 Serial Communication Interface (SCI) Asynchronous communication interface known as Universal Asynchronous Receiver Transmitter (UART) using two lines, i.e., TxD: trasmit line RxD: receive line Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

10 Parameters The various parameters to configure the UART module of a microcontroller are Start bit: the communication line idles at high, a transition to low is the start bit Number of data bits: varies, e.g., ATmega ranges from 5 to 9 Parity bit: Even parity: set to zero if the number of data bits is even and one otherwise Odd parity: set to zero if the number of data bits is odd and one otherwise None Stop bits: can be set as one or two bits Baud rate: bits per second (note: modern definition of baud rate is different) Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

11 Baud Rate As UART is asynchronous, there is no common clock 1 baud rate needs to be known to both transmitter and receiver Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

12 Baud Rate As UART is asynchronous, there is no common clock 1 baud rate needs to be known to both transmitter and receiver 2 receiver uses oversampling to overcome clock drift problems Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

13 Baud Rate As UART is asynchronous, there is no common clock 1 baud rate needs to be known to both transmitter and receiver 2 receiver uses oversampling to overcome clock drift problems For ATmega1284, the receiver oversamples by 16 Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

14 RS-232 Uses UART Found in PCs 25 or 9 lines Three lines minimum required: TxD, RxD and ground Voltage level between ±3-15V with max tolerance of ±25V Requires line driver, e.g., MAX232 Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

15 RS-422 Similar to RS-232 Uses differential lines (twisted-pair) Better in noisy environments Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

16 USART Universal Synchronous Asynchronous Receiver Transmitter Uses additional line for clock ATmega1284 supports both UART and USART Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

17 Registers UBRR USART Baud Rate Register UDR USART Data Register UCSRA, USCRB and UCSRC USART Control Status Register Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

18 UBRR calculation where BAUD baud rate in bps f OSC system oscillator frequency UBRRn = UBRRn (0-4095) stored in UBRRnH and UBRRnL f OSC 16BAUD 1 (1) Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

19 Initialize using assembly USART_ Init : ; Set baud rate to UBRR0 out UBRR0H, r17 out UBRR0L, r16 ; Enable receiver and transmitter ldi r16, (1<< RXEN0 ) (1 < < TXEN0 ) out UCSR0B, r16 ; Set frame format: 8data, 2stop bit ldi r16, (1<< USBS0 ) (3 < < UCSZ00 ) out UCSR0C, r16 ret Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

20 Initialize using assembly USART_ Init : ; Set baud rate to UBRR0 out UBRR0H, r17 out UBRR0L, r16 ; Enable receiver and transmitter ldi r16, (1<< RXEN0 ) (1 < < TXEN0 ) out UCSR0B, r16 ; Set frame format: 8data, 2stop bit ldi r16, (1<< USBS0 ) (3 < < UCSZ00 ) out UCSR0C, r16 ret doesn t work using ATmega1284!!, why? atmel atmega1284_datasheet.pdf Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

21 Transmit using assembly USART_ Transmit : ; Wait for empty transmit buffer in r17, UCSR0A sbrs r17, UDRE rjmp USART_ Transmit ; Put data (r16) into buffer, sends the data out UDR0, r16 ret Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

22 Transmit using assembly USART_ Transmit : ; Wait for empty transmit buffer in r17, UCSR0A sbrs r17, UDRE rjmp USART_ Transmit ; Put data (r16) into buffer, sends the data out UDR0, r16 ret doesn t work using ATmega1284!!, why? atmel atmega1284_datasheet.pdf Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

23 Receive using assembly USART_ Receive : ; Wait for data to be received in r17, UCSR0A sbrs r17, RXC rjmp USART_ Receive ; Get and return received data from buffer in r16, UDR0 ret Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

24 Receive using assembly USART_ Receive : ; Wait for data to be received in r17, UCSR0A sbrs r17, RXC rjmp USART_ Receive ; Get and return received data from buffer in r16, UDR0 ret doesn t work using ATmega1284!!, why? atmel atmega1284_datasheet.pdf Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

25 Initialize using C # define FOSC // Clock Speed # define BAUD 9600 # define MYUBRR FOSC /16/ BAUD -1 void main ( void ) {... USART_Init ( MYUBRR )... } void USART_Init ( unsigned int ubrr ) { / Set baud rate / UBRR0H = ( unsigned char )( ubrr > >8); UBRR0L = ( unsigned char ) ubrr ; Enable receiver and transmitter */ UCSR0B = (1<< RXEN0 ) (1 < < TXEN0 ); / Set frame format: 8data, 2stop bit / UCSR0C = (1<< USBS0 ) (3 < < UCSZ00 ); } atmel atmega1284_datasheet.pdf Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

26 Transmit using C void USART_ Transmit ( unsigned char data ) { / Wait for empty transmit buffer / while (!( UCSR0A & (1<< UDRE )) ) ; / Put data into buffer, sends the data / UDR0 = data ; } atmel atmega1284_datasheet.pdf Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22

27 Receive using C unsigned char USART_ Receive ( void ) { / Wait for data to be received / while (!( UCSR0A & (1<< RXC )) ) ; / Get and return received data from buffer / return UDR0 ; } atmel atmega1284_datasheet.pdf Aly El-Osery (NMT) EE 308: Microcontrollers February 27, / 22