1. Serial Communication Interface (SCI)
Ellenor Brown
Howard Liles
Algan Samur

2. Presentation Outline Types of data transmission Serial Communication
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

3. Electronic devices communicating with each other
Data Transmission
Power supply circuit
Electronic devices communicating with each other
Desktop computer
Lights
Turbine generators
Printer
Hard drive

4. Types of Data Transmission
Serial Data Transmission
One bit at a time
Parallel Data Transmission
One word (N bits) at a time
Receiver (Monitor)
Receiver (printer)
= bits
Word
Transmitter (CPU)
Transmitter (microprocessor)

5. 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

6. Serial Communication Synchronous 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
Transmitter
Receiver
Asynchronous Communication
Transmitter and receiver do not have synchronized clocks and act independently
Simpler and less expensive than synchronous
Start, Stop and Parity “caution” bits are sent with each word of data
Transmitter
Receiver
Word

7. Comparisons SYNCHRONOUS ASYNCHRONOUS How It Works Clock
SYNCHRONOUS
ASYNCHRONOUS
How It Works
Clock
Start and Stop bits
Advantages
Lower overhead Greater throughput
Faster
Simple Cheap hardware
Disadvantages
More complex hardware More expensive
Large overhead
Slower
Andrew Byrley

8. 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

9. 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 sent
1 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!

10. Baud and Bit Rates Example
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

11. Presentation Outline Types of data transmission Serial Communication
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

12. 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
Data
Stop
Transmitter and Receiver operate independently
Transmitter sends data at any time
Receiver is always ready to accept data
The baud rate clock is synchronized with the bus clock and drives the receiver. The baud rate clock divided by 16 drives the
transmitter. The receiver has an acquisition rate of 16 samples per bit time.
No synchronized clock signals… but format and transfer rate must match during transmission
The SCI allows asynchronous serial communications with peripheral devices and other CPUs.

13. 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
Data
Stop

14. 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
Alerts receiver that the data transmission is about to begin

15. 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
128 characters
9-bit example: sending an address

16. 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

17. Parity Bit 2 types of Parity functionality Even Parity Odd 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

18. 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
(Synchronous transmitters/receivers don’t need start/stop bits)

19. 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 =
LSB
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 1 1 1 1 1

20. 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 =
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 1

21. 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 =
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 1

22. Error sources Frame shift Overrun Noise
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

23. Occurs when stop bit is not where receiver expects it to be
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

24. 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
SOFTWARE
REGISTER
TRANSMITTER

25. Noise Detection
SCI detects noise on the receiver input

26. Presentation Outline Types of data transmission Serial Communication
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

27. 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

28. 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

29. SCI Control Register 1 SCICR1 - $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

30. 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

31. 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

32. SCI Status Register 1 SCISR1 - $00CC
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

33. 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, : TXD pin used as output. (used only in single wire mode)
RAF (receiver active flag) – 0: no reception in progress, 1: reception in progress

34. 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

35. 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

36. SCI module runs at bus speed Desired value for SCIBR is 52
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
Actual baud rate is (0.160% error)

37. Example Write SCIBR ($0034) to SCIBDH/SCIBDL
For 8-bit, no parity, no interrupts, default values will work
Simply enable transmit and receive in SCICR2
Read from SCIDRL to receive 8-bit data
Write data to SCIDRL to send 8-bit data

38. Code Example

39. Code Example

40. #include <hidef.h> /* common defines and macros */
MISC = 0x03;
#include <mc9s12c32.h> /* derivative information */
PEAR = 0x0C;
#pragma LINK_INFO DERIVATIVE "mc9s12c32"
MODE = 0xE2;
void SCI_init(void){
//Call function to setup SCI
int BR = 0x0034;
SCI_init();
SCIBDH = BR>>8; //stores high Byte
//Main loop
SCIBDL = BR; //stores low Byte
while(1) {
SCICR2 = 0x0C; //sets TE and RE to 1 }
char data = SCI_getByte();
char SCI_getByte(void){
SCI_sendByte(data);
while (!(SCISR1_RDRF))
} /* loop forever */
;//waits FOREVER until receive register is full
/* please make sure that you never leave this function */
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

41. Thank You!
Any Questions?