Serial Communication Interface (SCI) 1 Ellenor Brown Howard Liles Algan Samur.
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Transcript of Serial Communication Interface (SCI) 1 Ellenor Brown Howard Liles Algan Samur.
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Serial Communication Interface (SCI)
Ellenor BrownHoward LilesAlgan Samur
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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
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Data Transmission
Electronic devices communicating with each other
Desktop computer
Turbine generators
Hard drive
Printer
Lights
Power supply circuit
Types of Data Transmission
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• Serial Data TransmissionOne bit at a time
• Parallel Data TransmissionOne word (N bits) at a time
Transmitter (microprocessor)
Receiver (printer)
Word
Receiver (Monitor)
Transmitter (CPU)
= bits
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Parallel vs. Serial
• 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
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Receiver
ReceiverTransmitter
Transmitter
Serial CommunicationSynchronous 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 CommunicationTransmitter and receiver do not have synchronized clocks and act independentlySimpler and less expensive than synchronous Start, Stop and Parity “caution” bits are sent with each word of data
Word
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Comparisons
Andrew Byrley
SYNCHRONOUS ASYNCHRONOUS
How It Works Clock Start and Stop bits
Advantages Lower overheadGreater throughputFaster
SimpleCheap hardware
Disadvantages More complex hardwareMore expensive
Large overheadSlower
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Baud and Bit Rates
• 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
Symbol
Bits
ond
Symbols
ond
Bits*
secsec
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Baud and Bit Rates Cont.’d
• 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 sent1 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!
bitsTotal
bitsCharacter
ond
Bits
ond
Characters
_
_*
secsec
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Baud and Bit Rates Example
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
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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
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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
Parity Start
Transmitter Receiver
DataStop
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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
Parity Start
Transmitter Receiver
DataStop
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Start Bit
• One bit• Indicates the beginning of word• Opposite polarity from idle bit state
– Idle state for HCS12 is 1’s– Start bit = 0
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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
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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
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Parity Bit
• 2 types of Parity functionality• Even Parity
– Parity bit is set to 1 if there is an odd number of 1’s in data bits # of 1’s becomes even
• Odd Parity– Parity bit is set to 1 if there is an even number of
1s in data bits # of 1’s becomes odd)
• Even/Odd Parity is set by user on HCS12
Evan Johnson
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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-line’s idle state– Idle state for HCS12 = all 1’s– Stop bits = 1
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Asynchronous Data Transmission
• Example 1:– Hex# 4A16 is to be sent with one start bit, even parity,
8-bit data length and one stop bit– 4A16 = 0100 10102
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
0 0 1 0 1 0 0 1 0 1 1
LSB
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Asynchronous Data Transmission
• Example 2:– Hex# B416 is to be sent with one start bit, even parity,
8-bit data length and one stop bit– B416 = 1011 01002
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
0 0 0 1 0 1 1 0 1 0 1
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Asynchronous Data Transmission
• Example 3:– Hex# B416 is to be sent with one start bit, odd parity, 8-
bit data length and one stop bit– B416 = 1011 01002
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
0 0 0 1 0 1 1 0 1 1 1
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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
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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
1 2 3 4 5 6 7
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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
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Noise Detection
• SCI detects noise on the receiver input
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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
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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
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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
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SCI Control Register 1SCICR1 - $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
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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
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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
• 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 1SCISR1 - $00CC
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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
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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
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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
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 52.0833– Actual baud rate is 9615.3 (0.160% error)
0]:SCIBR[1216
clockmoduleSCIratebaudSCI
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Example
Write SCIBR ($0034) to SCIBDH/SCIBDLFor 8-bit, no parity, no interrupts,
default values will workSimply enable transmit and receive in
SCICR2Read from SCIDRL to receive 8-bit data Write data to SCIDRL to send 8-bit data
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Code Example
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Code Example
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#include <hidef.h> /* common defines and macros */#include <mc9s12c32.h> /* 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 */}
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Thank You!
Any Questions?