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Serial Communications Interface (SCI)

Serial Communications Interface (SCI). Michael Lennard Zachary Peters Bao Nguyen. Overview. Types of Data Transmission Michael Lennard Parallel Serial Serial Communication Synchronous Asynchronous Baud and Bit Rates Asynchronous Serial Transmission Zachary Peters

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Serial Communications Interface (SCI)

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  1. Serial Communications Interface (SCI) Michael Lennard Zachary Peters Bao Nguyen

  2. Overview • Types of Data Transmission Michael Lennard • Parallel • Serial • Serial Communication • Synchronous • Asynchronous • Baud and Bit Rates • Asynchronous Serial Transmission Zachary Peters • Serial Communication With the HCS12 Bao Nguyen • Examples of data words are transmitted

  3. Parallel Data Transmission • Simultaneous transmission • Requires separate data lines • Uses a clock to keep bits synchronized • Fast but Expensive • Requires short cables to prevent Cross-Talk/Skewing • Example: Printer Cable Receiver One ‘Word’ Transmitter Presenter: Michael Lennard

  4. Serial Data Transmission • One bit sent at a time • Slow compared to Parallel • Requires only a single transmission line & port • Cheap! • Can often be clocked faster than parallel data • Example: USB, Firewire, Ethernet Receiver One Word Transmitter Presenter: Michael Lennard

  5. Serial Data Transmission • Two Basic Types of Serial Data Transmission • Synchronous • Asynchronous • Two solutions to same problem - Receiver needs to know • When data starts • When data stops • If data was processed correctly Presenter: Michael Lennard

  6. Synchronous Serial Communication • Transmitter and Receiver have synchronized clocks • Continuous data must be sent to maintain synchronization • Any data not on a clock cycle is considered noise • Establish transmission parameters before sending data • 30% faster than asynchronous transmission for large continuous blocks of data Data Receiver Transmitter 3 2 1 Clock Ticks Presenter: Michael Lennard

  7. Asynchronous Serial Communication • Transmitter & Receiver are independent • Transmitter sends ‘Start’, ‘Stop’ and ‘Parity’ bits with each word of data • Simpler to implement and less expensive than synchronous • Data received between a Stop bits and the next Start bit is ignored Data Word Receiver Transmitter Start Stop Parity Presenter: Michael Lennard

  8. Baud Rate vs. Bit Rate • Baud Rate (Bd) is the rate at which Symbols (Signaling Events) are transferred • Number of bits per Symbol is Hardware Specific • Our hardware uses just 1’s and 0’s and thus just 1 bit/Symbol • Bit rate is the absolute rate at which bits are transmitted • Can be changed for each port Presenter: Michael Lennard

  9. Baud Rate vs. Characters Per Second • Not all bits sent are data • Asynchronous Serial Communication has Start/Stop/Parity bits • Characters per Second (cps) is a measure of data throughput • Throughput = Rate of actual data sent • Standard character = 1 bit • Characters = Total bits – Overhead bits Presenter: Michael Lennard

  10. Example Problem • You have an asynchronous serial connection with 2 bits/Symbol and a 9600 bd line speed. You want to send data in an 8 bit data format with 1 parity, 1 start bit and 1 stop bit. Calculate the throughput in cps. Presenter: Michael Lennard

  11. Overview • Types of Data Transmission Michael Lennard • Parallel • Serial • Serial Communication • Synchronous • Asynchronous • Baud and Bit Rates • Asynchronous Serial Transmission Zachary Peters • Serial Communication With the HCS12 Bao Nguyen • Examples of data words are transmitted

  12. Asynchronous Serial Communication • Transmitter and Receiver Operate independently • Transmitter sends data at any time • Receiver is always ready to accept data • No need for clock signals • However… format and transfer rate must match during transmission Presenter: Zachary Peters

  13. Asynchronous Transmission • Data word contains information before and after that specifies the beginning and end of word • This synchronizes transmitter and receiver during transmission • Bit transfer rate is determined by programmer, but limited by interfaces Presenter: Zachary Peters

  14. Data Format • Start bit – indicates the beginning of word • Data bit – data user is transmitting • Parity bit – checks integrity of data • Stop bit – indicates the end of the word Presenter: Zachary Peters

  15. Start Bit • Opposite polarity from idle bit state • Idle state for HCS12 = all 1’s so start bit = 0 • Alerts receiver that the data transmission is about to begin • Accuracy verification methods to reduce noise (discussed later) Presenter: Zachary Peters

  16. Data Bits • Actual Data being sent or received plus parity bit • Most common mode: 8-bit transmission • Used for ASCII character transmission (ASCII code is 7-bit + 1 parity bit = 8-bit) • Less common mode: 9-bit transmission • Can be used to send a full byte of data + parity bit • Example: sending an address • LSB transmitted first Presenter: Zachary Peters

  17. Data Bits • Example: • 8-bit mode: 1100010101111 ASCII Code in Data  “T” • 9-bit mode: 11011010100011 Hex equivalent in Data $2B Stop Bits Stop Bits Data Data Start Bit Start Bit Parity Bit Parity Bit Presenter: Zachary Peters

  18. Parity Bit • 1 Bit • Located at end of data bits • Used as a method of ensuring proper data transmission • Even Parity • Parity bit = 1, if # of ones in the set is odd (makes the total # of ones even) • Odd Parity • Parity bit = 1, if # of ones in the set is even (makes the total # of ones odd) Presenter: Zachary Peters

  19. Stop Bit • 1 or 2 bits • Only used due to asynchronous nature (Synchronous transmitters/receivers don’t need start/stop bits) • Occurs directly after the parity bit • Bit is the same as the polarity of the data-line’s idle state • Idle state for HCS12 = all 1’s so stop bits = 1 Presenter: Zachary Peters

  20. Noise Detection • Problem: • A premature bit (1 or 0) can cause the receiver to think it is receiving data before it should be, or receive incorrect data • Solution: Sample at higher freq than baud rate and take “average” of samples • RT Clock = Baud rate * 16 • 16 samples of each bit • RT3, RT5, RT7 are recorded Presenter: Zachary Peters

  21. Noise Detection for Start Bit • Samples taken after the signal falls to 0 to verify if it is indeed a start bit • If two ‘1’s in sample then not a start bit • If one ‘1’ in sample then noise bit flagged Presenter: Zachary Peters

  22. Overview • Types of Data Transmission Michael Lennard • Parallel • Serial • Serial Communication • Synchronous • Asynchronous • Baud and Bit Rates • Asynchronous Serial Transmission Zachary Peters • Serial Communication With the HCS12 Bao Nguyen • Examples of data words are transmitted

  23. SCI Baud Rate Registers SCIBDH & SCHBDL - $00C8-$00C9 • 13-Bit 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 Presenter: Bao Nguyen

  24. SCI Control Register 1SCICR1 - $00CA • LOOPS (loop operation enable) – 0: Normal, 1: Loop Operation • SCISWAI (SCI wait mode enable) – 0: Off 1: On • 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 Presenter: Bao Nguyen

  25. SCI Control Register 2SCICR2 - $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 Presenter: Bao Nguyen

  26. SCI Status Register 1SCISR1 - $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 Presenter: Bao Nguyen

  27. SCI Status Register 2SCISR2 - $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 Presenter: Bao Nguyen

  28. SCI Data RegistersSCIDRH &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 Presenter: Bao Nguyen

  29. Start Bit Data Bit 0 Data Bit 1 Parity Bit Stop Bit Stop Bit Data Bit 2 Data Bit 3 Data Bit 4 Data Bit 5 Data Bit 6 Data Bit 7 0 1 1 1 0 0 1 0 1 0 0 1 Asynchronous Data Transmission • Example 1: • Hex# 4A16 is to be sent with one start bit, even parity, 8-bit data length and two stop bits • 4A16 = 0100 10102 • Note: Little endian communication used (LSB sent first) Presenter: Bao Nguyen

  30. Start Bit Data Bit 0 Data Bit 1 Parity Bit Stop Bit Stop Bit Data Bit 2 Data Bit 3 Data Bit 4 Data Bit 5 Data Bit 6 Data Bit 7 1 0 1 1 0 0 0 1 0 1 1 0 Asynchronous Data Transmission • Example 2: • Hex# B416 is to be sent with one start bit, even parity, 8-bit data length and two stop bits • B416 = 1011 01002 Presenter: Bao Nguyen

  31. Start Bit Data Bit 0 Data Bit 1 Parity Bit Stop Bit Stop Bit Data Bit 2 Data Bit 3 Data Bit 4 Data Bit 5 Data Bit 6 Data Bit 7 1 1 1 1 0 0 0 1 0 1 1 0 Asynchronous Data Transmission • Example 3: • Hex# B416 is to be sent with one start bit, odd parity, 8-bit data length and two stop bits • B416 = 1011 01002 Presenter: Bao Nguyen

  32. Thank You! Any Questions?

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