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
𝑐ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟 𝑏𝑖𝑡𝑠
𝑐𝑝𝑠 = 𝑏𝑖𝑡𝑟𝑎𝑡𝑒 ∗
𝑡𝑜𝑡𝑎𝑙 𝑏𝑖𝑡𝑠
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.
𝐵𝑖𝑡𝑟𝑎𝑡𝑒 = 9600
𝑆𝑦𝑚𝑏𝑜𝑙𝑠
𝑠
∗2
𝑏𝑖𝑡𝑠
𝑆𝑦𝑚𝑏𝑜𝑙
= 19200
𝑏𝑖𝑡𝑠
𝑠
𝑏𝑖𝑡𝑠 8 𝑐ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑠
𝑐ℎ𝑎𝑟𝑎𝑐𝑡𝑒𝑟𝑠
𝑐𝑝𝑠 = 19200
∗
= 10,155
𝑠
11 𝑏𝑖𝑡𝑠
𝑠
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?