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

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


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


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


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


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


SymbolBits

sBitBitRatesSymbolBd

/

)/()/(

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


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.





Baud and Bit Rates

Asynchronous Serial TransmissionZachary Peters



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


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


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)


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


Data Bits Example:

8-bit mode:1100010101111 ASCII Code in Data “T”

9-bit mode: 11011010100011 Hex equivalent in Data $2B


DataStart Bit

Stop Bits

Parity Bit

DataStart Bit

Stop Bits

Parity Bit

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)


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


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


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





Baud and Bit Rates

Asynchronous Serial Transmission Zachary Peters



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 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

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


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


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


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


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 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 Stop Bit Stop Bit

0 0 1 0 1 0 0 1 0 1 1 1



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


0 0 0 1 0 1 1 0 1 0 1 1



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


0 0 0 1 0 1 1 0 1 1 1 1


Thank You!

Any Questions?

Serial Communications Interface (SCI)

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