Serial Communications Interface (SCI) Michael LennardZachary PetersBao Nguyen

Overview  Types of Data TransmissionMichael Lennard Parallel Serial  Serial Communication Synchronous Asynchronous  Baud and Bit Rates  Asynchronous Serial TransmissionZachary Peters  Serial Communication With the HCS12Bao Nguyen  Examples of data words are transmitted

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 Presenter: Michael Lennard Transmitter Receiver One ‘Word’

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 Presenter: Michael Lennard Transmitter Receiver One Word

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

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 Presenter: Michael Lennard Transmitter Receiver Data Clock Ticks 123

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 Presenter: Michael Lennard Transmitter Receiver Data Word ParityStop Start

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

Baud Rate vs. Characters Per Second Presenter: Michael Lennard

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

Overview  Types of Data TransmissionMichael Lennard Parallel Serial  Serial Communication Synchronous Asynchronous  Baud and Bit Rates  Asynchronous Serial TransmissionZachary Peters  Serial Communication With the HCS12Bao Nguyen  Examples of data words are transmitted

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

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

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

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

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

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

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

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

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

Overview  Types of Data TransmissionMichael Lennard Parallel Serial  Serial Communication Synchronous Asynchronous  Baud and Bit Rates  Asynchronous Serial TransmissionZachary Peters  Serial Communication With the HCS12Bao Nguyen  Examples of data words are transmitted

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

SCI Control Register 1 SCICR1 - $00CA Presenter: Bao Nguyen  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

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 Presenter: Bao Nguyen

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 Presenter: Bao Nguyen

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 Presenter: Bao Nguyen

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 Presenter: Bao Nguyen

Asynchronous Data Transmission  Example 1:  Hex# 4A 16 is to be sent with one start bit, even parity, 8-bit data length and two stop bits  4A 16 =  Note: Little endian communication used (LSB sent first) Start BitData Bit 0Data Bit 1 Data Bit 2Data Bit 3Data Bit 4Data Bit 5Data Bit 6Data Bit 7 Parity BitStop Bit Presenter: Bao Nguyen

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

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

Thank You! Any Questions?