Serial Communications Interface (SCI) Michael Lennard Zachary Peters Bao Nguyen Overview Types • • of Data Transmission Parallel Serial Michael Lennard Serial Communication • Synchronous • Asynchronous Baud and Bit Rates Asynchronous Serial Serial Transmission Communication With the HCS12 Examples of data words are transmitted Zachary Peters Bao Nguyen Parallel Data Transmission Receiver Simultaneous transmission Requires separate data lines Uses a clock to keep bits synchronized One ‘Word’ Fast but Expensive Requires short cables to prevent Cross-Talk/Skewing Transmitter Presenter: Michael Lennard Example: Printer Cable Serial Data Transmission Receiver One Word Transmitter Presenter: Michael Lennard 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 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 Data Transmitter Receiver 3 2 1 Clock Ticks Presenter: Michael Lennard 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 Stop Presenter: Michael Lennard Parity 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 BitRate( Bit / s) Bd ( Sym bol/ s) Bits / Sym bol Presenter: Michael Lennard Baud Rate vs. Characters Per Second Not all bits sent are data Characters per Second (cps) is a measure of data throughput Asynchronous Serial Communication has Start/Stop/Parity bits Throughput = Rate of actual data sent Standard character = 1 bit Characters = Total bits – Overhead bits ℎ = ∗ 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. = 9600 ∗2 = 19200 8 ℎ ℎ = 19200 ∗ = 10,155 11 Presenter: Michael Lennard Overview Types • • of Data Transmission Parallel Serial Michael Lennard Serial Communication • Synchronous • Asynchronous Baud and Bit Rates Asynchronous Serial Serial Transmission Communication With the HCS12 Examples of data words are transmitted Zachary Peters Bao Nguyen 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 Less common mode: 9-bit transmission Used for ASCII character transmission (ASCII code is 7-bit + 1 parity bit = 8-bit) 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: 1100010101111 ASCII Code in Data “T” Data Stop Bits Start Bit Parity Bit 9-bit mode: 11011010100011 Hex equivalent in Data $2B Data Stop Bits Start Bit Parity Bit Presenter: Zachary Peters Parity Bit 1 Bit Even Parity Located at end of data bits Used as a method of ensuring proper data transmission 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: Solution: Sample at higher freq than baud rate and take “average” of samples RT Clock = Baud rate * 16 A premature bit (1 or 0) can cause the receiver to think it is receiving data before it should be, or receive incorrect data 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 Transmission Parallel Serial Michael Lennard Serial Communication • Synchronous • Asynchronous Baud and Bit Rates Asynchronous Serial Serial Transmission Zachary Peters Communication With the HCS12 Bao 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 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 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# 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) Start Bit 0 Data Bit 0 Data Bit 1 Data Bit 2 Data Bit 3 Data Bit 4 Data Bit 5 Data Bit 6 Data Bit 7 0 1 Presenter: Bao Nguyen 0 1 0 0 1 0 Parity Bit 1 Stop Bit Stop Bit 1 1 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 Start Bit 0 Data Bit 0 Data Bit 1 Data Bit 2 Data Bit 3 Data Bit 4 Data Bit 5 Data Bit 6 Data Bit 7 Parity Bit 0 0 Presenter: Bao Nguyen 1 0 1 1 0 1 0 Stop Bit 1 Stop Bit 1 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 Start Bit 0 Data Bit 0 Data Bit 1 Data Bit 2 Data Bit 3 Data Bit 4 Data Bit 5 Data Bit 6 Data Bit 7 Parity Bit 0 0 Presenter: Bao Nguyen 1 0 1 1 0 1 1 Stop Bit 1 Stop Bit 1 Thank You! Any Questions?