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Serial Communication Interface (SCI) 1 Ellenor Brown Howard Liles Algan Samur.

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Presentation on theme: "Serial Communication Interface (SCI) 1 Ellenor Brown Howard Liles Algan Samur."— Presentation transcript:

1 Serial Communication Interface (SCI) 1 Ellenor Brown Howard Liles Algan Samur

2 Presentation Outline Types of data transmission Parallel Serial Serial Communication Synchronous Asynchronous Baud and Bit Rates Asynchronous Serial Transmission Start Bit Data Bit Stop Bit Parity Bit Noise SCI Registers Show two examples of how data words are transmitted 2 Howard Liles Ellenor Brown Algan Samur

3 Data Transmission 3 Electronic devices communicating with each other Desktop computer Turbine generators Hard drive Printer Lights Power supply circuit

4 Types of Data Transmission 4 Serial Data Transmission One bit at a time Parallel Data Transmission One word (N bits) at a time Transmitter (microprocessor) Receiver (printer) Word Receiver (Monitor) Transmitter (CPU) = bits

5 Parallel vs. Serial 5 Parallel requires more transfer lines Bits have to be synchronized Fast, but expensive Examples: Printer cables Serial requires less transfer lines Transfers one bit at a time Slow comparatively, but less expensive Examples: USB, Firewire, ethernet

6 Receiver Transmitter Serial Communication 6 Synchronous Serial Communication Transmitter and Receiver have synchronized clocks Data must be sent constantly in order for them to stay synchronized Any data not sent on a regular clock cycle is considered noise Transmission parameters are set up before sending data 30% faster than asynchronous transmission for large continuous blocks of data Clock rate determines data transfer rate Asynchronous Communication Transmitter and receiver do not have synchronized clocks and act independently Simpler and less expensive than synchronous Start, Stop and Parity caution bits are sent with each word of data Word

7 Comparisons Andrew Byrley SYNCHRONOUSASYNCHRONOUS How It WorksClockStart and Stop bits AdvantagesLower overhead Greater throughput Faster Simple Cheap hardware DisadvantagesMore complex hardware More expensive Large overhead Slower 7

8 Baud and Bit Rates 8 Baud Rate (Bd) is the rate at which Symbols are transferred. A symbol is a given signaling event Symbols/second Number of bits per Symbol is Hardware Specific (our hardware uses 1 baud/bit) Conversion factor:1 bit = 1 Symbol = 1 baud Bit rate (bps) - the rate at which bits are transmitted or bits/second Conversion Equation: Bit rate (bps) = baud rate × number of bits per baud

9 Baud and Bit Rates Cont.d 9 Some bits are data and some are not! – Remember the start, stop, and parity bits which are also known as overhead bits Data throughput can be determined by Characters per second (cps) Cps = actual rate of data being sent 1 Standard Character = 1 Bit Characters per second (cps)=Bit rate*(character bits/total bits) *Remember CPS is not same as bytes/second. CPS does not include overhead!

10 Baud and Bit Rates Example 10 Example 1: You have an asynchronous serial connection. Assuming 2 bits per symbol, 9600 bd line speed, 8 bit data format with no parity, 1 start bit and 1 stop bit, calculate the throughput in cps. Bit rate (bps) = baud rate × number of bits per baud bps= 9600x2=19200 (cps)=Bit rate*(character bits/total bits) cps=19200*(8/10)= 15360

11 Presentation Outline Types of data transmission Parallel Serial Serial Communication Synchronous Asynchronous Baud and Bit Rates Asynchronous Serial Transmission Start Bit Data Bit Stop Bit Parity Bit Noise SCI Registers Show two examples of how data words are transmitted Howard Liles Ellenor Brown Algan Samur 11

12 Asynchronous Serial Communication Transmitter and Receiver are independent Transmitter sends Start, Parity and Stop bits with each word of data Data received between a Stop bit and the next Start bit is ignored ParityStart Transmitter Receiver Data Stop 12

13 Data Frame Format Start bit – Indicates beginning of data Data bit – Data being transmitted Parity bit – Integrity check Stop bit – Indicates end of data word Data frame size: 10 or 11 bits ParityStart Transmitter Receiver Data Stop 13

14 Start Bit One bit Indicates the beginning of word Opposite polarity from idle bit state – Idle state for HCS12 is 1s – Start bit = 0 14

15 Data Bits Actual data being transmitted plus a parity bit Most common mode: – 8-bit transmission – Used for ASCII character transmission ( 7-bit ASCII + 1-bit parity = 8-bit) Less common mode: – 9-bit transmission – Can be used to send a full byte of data + parity bit HCS12 sends least significant bit (LSB) first 15

16 Parity Bit Used to determine if an error occurred during data transmission Error Detection – Transmitter calculates proper parity bit – Receiver calculates parity bit based on data it received – Receiver compares its parity bit to the one it received Evan Johnson 16

17 Parity Bit 2 types of Parity functionality Even Parity – Parity bit is set to 1 if there is an odd number of 1s in data bits # of 1s becomes even Odd Parity – Parity bit is set to 1 if there is an even number of 1s in data bits # of 1s becomes odd) Even/Odd Parity is set by user on HCS12 Evan Johnson 17

18 Stop Bit 1 or 2 bits Used due to asynchronous nature Directly after the parity bit Stop bit is the same as the polarity of the data-lines idle state – Idle state for HCS12 = all 1s – Stop bits = 1 18

19 Asynchronous Data Transmission Example 1: – Hex# 4A 16 is to be sent with one start bit, even parity, 8-bit data length and one stop bit – 4A 16 = Start Bit Data Bit 0 Data Bit 1 Data Bit 2Data Bit 3Data Bit 4 Data Bit 5 Data Bit 6Data Bit 7 Parity BitStop Bit LSB 19

20 Asynchronous Data Transmission Example 2: – Hex# B4 16 is to be sent with one start bit, even parity, 8-bit data length and one stop bit – B4 16 = Start Bit Data Bit 0 Data Bit 1 Data Bit 2Data Bit 3Data Bit 4 Data Bit 5 Data Bit 6 Data Bit 7 Parity BitStop Bit

21 Asynchronous Data Transmission Example 3: – Hex# B4 16 is to be sent with one start bit, odd parity, 8- bit data length and one stop bit – B4 16 = Start BitData Bit 0Data Bit 1 Data Bit 2Data Bit 3Data Bit 4Data Bit 5Data Bit 6Data Bit 7 Parity BitStop Bit

22 Error sources Frame shift – Detected when a logic 0 is accepted as the stop bit Overrun – Software fails to read the SCI data register before the shift register receives the next frame Noise – SCI detects noise on the receiver input 22

23 Framing Error Occurs when stop bit is not where receiver expects it to be Detected when a logic 0 is accepted as the stop bit Ex: 4 bit is skipped and stop bit is one bit before it should be

24 Overrun Software fails to read the SCI data register before it receives the next frame Data in the shift register is lost Data already in the SCI data registers is not affected RECEIVER REGISTER SOFTWARE TRANSMITTER 24

25 Noise Detection SCI detects noise on the receiver input 25

26 Presentation Outline Types of data transmission Parallel Serial Serial Communication Synchronous Asynchronous Baud and Bit Rates Asynchronous Serial Transmission Start Bit Data Bit Stop Bit Parity Bit Noise SCI Registers Coding example Howard Liles Ellenor Brown Algan Samur 26

27 SCI Baud Rate Registers SCIBDH & SCHBDL - $00C8-$00C9 13-Bits register determines SCI Baud rate Baud rate generator is disabled until TE or RE bit is set for the first time after reset. Baud rate generator is turned off when this register contains $0000 Note: Writing to SCIBDH has no effect w/out writing to SCIBDL 27

28 Implementation Specific Features (S12SCIV2) 13-bit baud rate selection 8- or 9-bit data format Separately enabled transmitter and receiver Programmable transmitter output parity Interrupt driven operation with 8 flags 8 registers used to control SCI ($00C8- $00CF) Uses Port S pins 0 & 1 for RXD and TXD respectively 28

29 SCI Control Register 1 SCICR1 - $00CA M (data format mode) - 0: 8 data bits, - 1: 9 data bits. - Both use 1 start bit and 1 stop bit PE (parity enable) – 0: Off, 1: On PT (parity type) – 0: Even, 1: Odd 29

30 SCI Control Register 2 SCICR2 - $00CB TIE (transmit interrupt enable) – 0: disables interrupts for transmit data register empty, 1: enables TCIE (transmit complete interrupt enable) – 0: disables interrupts for transmit complete, 1: enables RIE (receiver interrupt enable) – 0: disables interrupts for receiver full and overrun, 1: enables ILIE (idle line interrupt enable) – 0: disables interrupts for idle line, 1: enables TE (transmit enable) – 0: disable transmitter, 1: enable RE (receiver enable) – 0: disable receiver, 1: enable 30

31 SCI Status Register 1 SCISR1 - $00CC Read only register Can be used to provide input to the microcontroller for generation of SCI interrupts TDRE (transmit data register empty) – 0: No byte transferred,1: byte successfully transferred to transmit shift register TC (transmit complete flag) – 0: transmission in progress, 1: no transmission in progress RDRF (receive data register full) – 0: no data in data register, 1: data in data register IDLE (idle flag) – 0: receiver input is active, 1: receiver input has become idle 31

32 OR (overrun) – 0: no overrun, 1: overrun (overrun happens when new data is received before old data is read) NF (noise flag) – 0: disable, 1: enable FE (framing error flag) – 0: disable, 1: enable PF (parity error) – 0: No parity error, 1: parity error SCI Status Register 1 SCISR1 - $00CC 32

33 SCI Status Register 2 SCISR2 - $00CD BK13 (break transmit character length) – 0: 10 or 11 bit, 1: 13 or 14 bit TXDIR (transmitter pin direction) – 0: TXD pin used as input, 1: TXD pin used as output. (used only in single wire mode) RAF (receiver active flag) – 0: no reception in progress, 1: reception in progress 33

34 SCI Data Registers SCIDRH &SCIDRL - $00CE - $00CF SCIRDL contains incoming bytes of data from serial port R8 – bit 8 of received 9-bit data T8 – bit 8 of transmitted 9-bit data 34

35 SCI is easy SCI module makes it easy to send/receive data SCI module encodes data into standard NRZ format! Hardest part is setting up baud rate Can use either polling or interrupt based logic to drive SCI SCIDRH/SCIDRL are like two registers in one. Read this register to receive data Write to this register to send data 35

36 Example First, calculate baud rate. Assume 8MHz bus and desired baud rate is 9600 SCI module runs at bus speed Desired value for SCIBR is 52 You will have some error margin – Exact solution is – Actual baud rate is (0.160% error) 36

37 Example Write SCIBR ($0034) to SCIBDH/SCIBDL For 8-bit, no parity, no interrupts, default values will work Simply enable transmit and receive in SCICR2 Read from SCIDRL to receive 8-bit data Write data to SCIDRL to send 8-bit data 37

38 Code Example 38

39 Code Example 39

40 #include /* common defines and macros */ #include /* derivative information */ #pragma LINK_INFO DERIVATIVE "mc9s12c32" void SCI_init(void){ int BR = 0x0034; SCIBDH = BR>>8; //stores high Byte SCIBDL = BR; //stores low Byte SCICR2 = 0x0C; //sets TE and RE to 1 } char SCI_getByte(void){ while (!(SCISR1_RDRF)) ;//waits FOREVER until receive register is full return SCIDRL; } void SCI_sendByte(char data){ while (!(SCISR1_TDRE)) ;//waits FOREVER until transmit register is empty SCIDRL = data; //return void; } void main(void) { //variable declarations must go at beginning /* put your own code here */ EnableInterrupts; //required code as per instructions MISC = 0x03; PEAR = 0x0C; MODE = 0xE2; //Call function to setup SCI SCI_init(); //Main loop while(1) { char data = SCI_getByte(); SCI_sendByte(data); } /* loop forever */ /* please make sure that you never leave this function */ } 40


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