Universal Asynchronous Receiver Transmitter Communication

Transcription

1 Universal Asynchronous Receiver Transmitter Communication 13 October 2011

2 Synchronous Serial Standard SPI I 2 C

3 Asynchronous Serial Standard UART

4 Asynchronous Resynchronization

5 Asynchronous Data Transmission Standard Data transmission uses 10 bits A start bit of 0 7 or 8 data bits transmitted from LSB to MSB An optional parity bit A stop bit Mark coding means the stop bit is a 1 Space coding means thhe stop bit is a 0 Parity Coding Even Parity Generation: P = ( d n) mod 2 Test: E = ( d n + P) mod 2 Odd Parity Generation: P = ( d n + 1) mod 2 Test: E = ( d n + P + 1) mod 2

6 ASCII Standards American Standard Code for Information Interchange 7-bits First 32 codes for printer control Last 96 printable characters IBM added 8th bit NUL DLE space 0 P p 1 SOH DC1! 1 A Q a q 2 STX DC2 2 B R b r 3 ETX DC3 # 3 C S c s 4 EOT DC4 $ 4 D T d t 5 ENQ NAK % 5 E U e u 6 ACK SYN & 6 F V f u 7 BEL ETB 7 G W g w 8 BS CAN ( 8 H X h x 9 HT EM ) 9 I Y i y A LF SUB * : J Z j z B VT ESC + ; K [ k { C FF FS, < L l D CR GS - = M ] m } E SO RS. > N n F SI US /? O o del

7 PIC24F UART Protocols EIA-232 (RS-232) EIA-485 (RS-485) LIN bus (Local Interconnect Network) IrDA (Infrared Data Association)

8 UART Drivers and Standards Use MC1488 and MC1489 drives with PIC for EIA232, requires 12 VDC, +12 VDC, and +5 VDC supply Use MC3487 and MC3486 drivers with PIC for EIA422 requires +5 VDC supply MAX232A provides two EIA232 outputs and two EIA232 inputs requires +5 VDC supply

9 EIA Terminology Standards DTE Data terminal equipment (computer) DCE Data communication equipment (modem) RxD Receiver data TxD Transmitter data DCD Data carrier detect (valid modem connection)

10 EIA Terminology (cont d) DTR Data terminal ready (Computer on and software is ready) DSR Data set ready (Modem on and software ready) RTS Request to send (DTE wants to send character) CTS Clear to send (DCE acknowledge to RTS from DTE) RI Ring indicator from telephone

11 DE-9 and DB-25 Connector Signal DE-9 DB-25 DCD 1 8 RxD 2 3 TxD 3 2 DTR 4 20 GND 5 7 DSR 6 6 RTS 7 4 CTS 8 5 RI 9 22

12 Baud Rate Standards Baud is state changes per second. When there are two states the baud equals bits per second or bps Standard baud rates for PIC: 300, 1200, 2400, 9600, 19200, 38400, 57600, 76800, 96000, and others The baud rate is specified in UxBRG and BRGH

13 Calculations with BRGH = 0 Baud rate = F CYC 16 (UxBRG + 1) Baud rate = F CYC UxBRG = 16 Baud rate 1 = = ( ) = 9615(0.16%error)

14 Calculations with BRGH = 1 Baud rate = F CYC 4 (UxBRG + 1) Baud rate = F CYC UxBRG = 4 Baud rate 1 = = ( ) = 9615(0.16%error)

15 Software Handshake

16 Hardware - UART Tx

17 Hardware - UART Rx

18 Hardware - UART (BRGH=1) Receive (4 (X + 1) T CYC = 1 bit period) Wait for 0 Wait 2 (X + 1) T CYC, If 0 continue else restart Wait 4 (X + 1) T CYC, get D 0 Wait 4 (X + 1) T CYC, get D 1. Wait 4 (X + 1) T CYC, get D 7 or parity Wait 4 (X + 1) T CYC, If 1 okay else frame error.

19 Hardware - UART (BRGH=0) Receive (16 (X + 1) T CYC = 1 bit period) Wait for 0 Wait 8 (X + 1) T CYC, If 0 continue else restart Wait 16 (X + 1) T CYC, get D 0 Wait 16 (X + 1) T CYC, get D 1. Wait 16 (X + 1) T CYC, get D 7 or parity Wait 16 (X + 1) T CYC, If 1 okay else frame error.

20 UART Definitions #include<p24fj128ga010.h> #define CTS RF12 // Clear To Send, input, HW handshake #define RTS RF13 // Request To Send, input, HW handshake #define TRTS TRISF13 // Tris control for RTS pin #define U ENABLE 0x8008 //enable UART, BREGH=1, 1 stop, no parity #define U TX 0x0400 //enable transmission, clear all flags

21 Configuring UART void initu2(int baudrate) { U2BRG=baudrate; // initialize the baud rate generator U2MODE=U ENABLE; //initialize the UART module U2STA = U TX; //enable the tansmitter TRTS=0; // make RTS an output pin RTS=1; //set RTS default status (not ready) }//initu2

22 Send (Transmit) a Character int putu2(int c) { while(cts); //wait for!cts (active low) Clear To Send while(u2stabits.utxbf); //wait while Tx buffer is full U2TXREG=c; return c; }//putu2

23 Receive a Character char getu2(void) { RTS=0; //asset Request To Send while(!u2stabits.urxda); //wait for a new character to arrive return U2RXREG; //read the character from the receive buffer RTS=1; }//getu2

24 Error Detection Standards Parity Bits Pro: Calculated by UART hardware Cons: Does not detect pair reversals Hamming Code Pro: Corrects errors Con: Requires 12-bits per byte and bit oriented Checksum Pros: Simple algorithm Cons: Does not detect reordered byte, inserted or deleted zero-value bytes, and multiple errors which sum to zero Cyclic Redundancy Check Pros: Detects All single bit errors, all two-bit errors within order of polynomial, all odd number bit errors, and all burst error less than order of polynomial Cons: Bit oriented. Does not detect strings of zeros

25 (11,7) Hamming Code (11,7) Hamming Code adds 4 parity bits to a 7-bit message Each parity bit covers certain message bits This code uses even parity

26 (11,7) Hamming Code (cont d) p 1 p 2 d 1 p 3 d 2 d 3 d 4 p 4 d 5 d 6 d 7 Data word p p p p Tx data

27 Checksum Standards Message M i of length n Standard ( n ) Checksum = M i mod 256 Flecher-32 A = ( 1 + ( B = n + i=0 ) n M i mod ( ) i=0 ) n (n 1)M i i=0 Checksum = B A mod ( )

28 Cyclic Redundancy Check All arithmetic is Modulo-2 By bit manipulation By byte-wise table look-up M(x) is the message Q(x) is the quotient result K(x) is the key polynomial (example CRC-16 IBM) R(x) is the remainder of result (CRC) M(x)x n = Q(x) R(x) + R(x) M(x)x n K(x) = Q(x) + R(x) K(x)

29 Common CRCs Standards Name Polynomial Use CRC-1 x + 1 Parity CRC-5 USB x 5 + x USB CRC-15 CAN x 15 + x 14 + x 10 + x 8 + x 7 + x 4 + x CAN CRC-16 IBM x 16 + x 15 + x Serial CRC-32 MPEG2 See internet MPEG2 CRC-32 IEEE See internet Ethernet 802.3