Harrison Jones
Alexis Noel
William Allen

 Alexis Noel
 Types of data transmission
 Parallel & Serial
 Serial Communication
 Synchronous & Asynchronous
 Harrison Jones
 Baud & Bit Rates
 Asynchronous Serial Transmission
 Start, Data, Stop, Parity Bits
 Noise
 William Allen
 Registers
 Examples of data transmission



b
0 b
3
Byte b b
1
4 b
6 b
7 b
2 b
5
b
0 b
1 b
2 b
3 b
4 b
5 b
6 b
7 b
0 b
3
Byte b b
1
4 b
6 b
7 b
2 b
5


b
0 b
1
Byte b
0 b
1 b
2 b
3 b
4 b
5 b
6 b
7 b
6 b
7 b
2 b
3 b
4 b
5
Byte b
0 b
1 b
2 b
3 b
4 b
5 b
6 b
7

NOT
Ethernet
USB Fiber Optic Cable
Serial Port Parallel Port HDMI

Issues with parallel communication:
• Inter-symbol interference (ISI) and noise cause corruption over long distances
• Wires have small amounts of capacitance and mutual inductance
• Bandwidth of parallel wires is much lower than bandwidth of serial wires
Parallel communication is faster than serial for short distances

Why serial connection is better for long distances:
• Differential signals are used to increase power
• Double the signal to noise ratio (SNR)
• Reach higher bitrate without noise
• USB 2.0 is capable of
480Mbits/sec!
(At this rate, it would take only
46.5 seconds to transfer a
2.19GB BluRay movie over from a hard drive) Differential Signal
Fun Fact: Longest ever deep sea Fiber-Optic cable will run through thawing artic between UK and Japan (that’s 9,693 miles of cable!)

Synchronous Serial Transmission
• Stream of data is encoded in chunks
• Various bytes at the beginning of the data provide an embedded clock
• The data stream can also be synchronized by an external clock
Asynchronous Serial Transmission
• Data transmitted one character at a time
• Each character contains its own clock
• Start bits and stop bits
• Resynchronizes with each character

Synchronous Serial Transmission
Advantages
• Amount of transmission information restricted to few characters for each block
• Not prone to distortion
• Can be used at higher speeds
Disadvantages
• If error were to occur, whole block of data is lost
(100+characters)
• User cannot transmit characters instantaneously
• Requires storage
Synchronous used for high-speed communication between computers

Advantages
Asynchronous Serial Transmission
Disadvantages
• Each character is its own complete timer system
• Dependence on recognition of start bits
• Corruption will not spread
• Many bits are used only for control purpose and carry no
• Good for irregular interval character generation
• Keyboards useful information
• Limits transmission speed
Used for speeds up to 3000 bits/second with only simple single-character error detection

Asynchronous Serial Transmission
Start Bit
• Signals start of transmission of data bits
• Transition from logic 1 to logic 0
Data Bits
• Typically 8 data bits (not including parity bit)
• Least significant bit is transmitted and received first
Stop Bit
• Signals end of data word

 The rate at which symbols are sent
 Measured in symbols per second (Bd)
 Also known as baud or modulation rate
 Often incorrectly referred to as bits per second
 Important Baud Variables
 Bd – Baud rate
 M – Number of symbols used (voltages, tones, etc)
 Number of symbols used (M) = 2 N where N = bits / symbol
 N – Bits per symbol (binary = 1)

 The rate at which bits are transmitted
 Baud * Bits / Symbol
 Measured in bits per second (bps) NOT bytes per second (Bps)
 Often incorrectly referred to as data rate
 Gross Bit Rate – total number of bits transmitted per second
 Includes protocol overhead bits and data bits
 R b
= 1 / T b where T b is the bit transmission time
 Symbol Rate ≤ Gross Bit Rate
 Only equal when 2 bits per symbol (binary)
 Information Rate – rate at which useful data is transmitted
 Information rate ≤ Gross Bit Rate
 I
R
 Unless there is no protocol (risky!)
= R b
* Data Bit Number / Total Bit Number

 Symbol Number (M)
 Analog modem capable 64 different voltage levels (symbols) sends out how many bits per symbol?
 M = 65 = 2 N . N = 6 bits per symbol
 Baud Rate
 A baud rate of 100 Baud = 100 symbols / second
 Bit Rate
 At 9,600 Baud with 3 voltage levels what is the bits per second?
 Bits
 BPS = 9,600 * 1.5850 = 15,216 bps
 Information Rate
 Given a protocol with 3 bits of protocol, 8 bits of data, 9600 baud, and 1 bit per symbol (binary) what is the IR?
 I
R
= 9600 * log2(2) * 8/11 = 6981 data bits per second

 Overhead Bits
 Start bit – Start of “frame” bit
 Parity Bit – Error check bit
 Stop Bits – End of “frame” bit
 Data Bit – the actual data

 One bit
 The first bit of a serial data word
 Signals the start of data transmission
 Detected as a transition from the idle state to the active state
 Referred to as a “mark-to-space” transition
 Idle state for HCS12 is high (“1”)
 Active state for HCS12 is low (“0”)

 One bit
 Used as a crude form of error detection
 Even / Odd / No Parity
 Even – Start Bit + Data Bits + Stop Bits + Parity = 0
 Odd – Start Bit + Data Bits + Stop Bits + Parity = 1
 None – No parity bit included
 IMPORTANT: for this class, simply count Data Bits + Parity Bit
 Calculated by transmitter, checked by receiver
 User specified on the HCS12

 One+ bits
 Indicate the end of transmission
 Usually 1 or 2 idle state bits
 One stop bit on the HCS12

 Number of data bits established by protocol
 Common Transmission Mode
 7 bits of data + 1 parity bit = 8 total data bits
 Other Mode
 8 bits of data (full byte) + 1 parity bit = 9 total data bits
 Bit order is hardware dependent
 HCS12 sends LSB first

 Sending #$AB with one start bit, one stop bit, even parity , and
8 bit data
 Binary is #%1010 1011
 Parity Bit is 0 : 0+1+0+1+0+1+0+1+1+P+1 = EVEN
Parity Bit
0 1 0 1 0 1 0 1 1 0 1
Start Bit Data Bits Stop Bit
Direction of Transmission

 Noise – Noisy signals cause bits to “flip”
 Overrun – Slow receivers cause loss of data
 Framing Error – Timing errors cause issues

 Noise can cause “1” to appear to be “0” and vice verse
 Effect lessened by sampling

 Each bit after start bit is sampled X pules after center of start bit. X depends on the mode (1,16, and 64)

 Very simple concept
 Receiver doesn’t read data in SCI data register fast enough
 New data lost

 The result of reading incoming bits using the wrong starting point
 Can be detected by using the parity bit
 Often the result of mismatched baud rates

B I L LY A L L E N
 The SCI Module communicates over PS1 ( Tx) and PS0 (Rx).
 Writing to the SCI Data Registers transmits data over the Tx line.
 Incoming Rx data is automatically stored in the SCI Data
Register.
 Flags in the SCI Status Registers can be used to trigger interrupts.

 $00CE
 $00CF
 Reading from this register accesses the data received by the
SCI receiver.
 Writing to this register transmits the data through the SCI.
 T8 and R8 are the 9 th data bit when the SCI is in 9-bit mode.
 R0-R7 is the data received by the SCI.

 Writing to SCBIR to set the Baud Rate:
 $00C8
 $00C9
 Baud rate = SCI module clock / (16 * SCIBR[12:0])

 Example: To achieve a baud rate of 9600 with an 8MHz module clock:
 LDX
 STX
#0052
$00C8
(loads 52 into X)
(stores X into the SCIBR Registers
$00C8 and $00C9)
 Note that due to the way the baud rate is calculated, there will be some error in the actual rate that the 12C32 uses.
 This example actually yields a baud rate of 9615, but that error (0.156%) is small enough to be safely ignored.

 $00CA
 LOOPS: If set to 1, connects the output of TX to the input of RX.
 SCISWAI: If set to 1, disabled the SCI while in wait mode.
 RSRC: If LOOPS = 1, RSRC selects the source of the incoming TX signal.
 If RSRC = 0, the receiver is connected to the transmitter internally.
 If RSRC = 1, the receiver is connected to an external TX signal.
 M: Controls the number of data bits.
 If M = 0, the SCI uses one start bit, eight data bits, one stop bit.
 If M = 1, the SCI uses one start bit, nine data bits, one stop bit.

 $00CA
 WAKE: Defines the input condition that wakes up the receiver.
 If WAKE = 0, an idle input (0) will wake up the receiver.
 If WAKE = 1, a high input (1) wakes up the receiver.
 ILT: Defines when the SCI starts counting to wait for idle bits.
 If ILT = 0, the counter begins when the start bit is sensed.
 If ILT = 1, the counter begins when the stop bit is sensed.
 PE: If PE = 1, a parity bit is added to the data being transferred.
 PT: Defines how the parity bit it set.
 If PE = 0, parity bit it set for ODD parity.
 If PE = 1, parity bit is set for EVEN parity.

 $00CB
 TIE: If TIE = 1, the Transmit Data Register Empty flag, TDRE, can generate interrupt requests.
 TCIE: If TCIE = 1, the Transmission Complete flag, TC, can generate interrupt requests.
 RIE: If RIE = 1, the Receive Data Register Full flag, RDRF, or the OverRun flag, OR, can generate interrupt requests.
 ILIE: If ILIE = 1, the idle line flag, IDLE, can generate interrupt requests.

 $00CB
 TE: Transmitter Enable. If TE = 1, the transmitter is enabled.
 RE: Receiver Enable. If RE = 1, the receiver is enabled.
 RWU: Receiver Wakeup Bit. If RWU = 1, the SCI goes into a standby state and will not trigger interrupt requests until it wakes up from an external signal.
 SBK: Sent Break Bit. If SBK = 1, sends a pattern of break characters on the TX line.

 $00CC
 TDRE: Transmit Data Register Empty Flag
 This is set to 1 if the transmit data register becomes empty.
 TC: Transmit Complete Flag
 This is set to 1 if the transmission is done and nothing new is being sent.
 RDRF: Receive Data Register Full Flag
 Set to 1 if there is received data available in the SCI data register.
 IDLE: Idle Line Flag
 Set to 1 if 10 or 11 consecutive logic 1 signals appear on the receiver input line.

 $00CC
 OR: OverRun Flag
 Is set to 1 if the software fails to read the contents of the data register before the shift register receives more data.
 NF: Noise Flag
 Is set to 1 if the SCI detects noise on the receiver input.
 FE: Framing Error Flag
 Is set to 1 If the SCI detects that a framing error has occurred.
 PF: Parity Error Flag
 Is set to 1 if the parity bit is incorrect.

 $00CD
 BK13: Break Transmit Character Length
 Defines the length of the break character used.
 If BK13 = 0, break character is 10 or 11 bits long.
 If BK13 = 1, break character is 13 or 14 bits long.
 TXDIR: Transmitter Pin Data Direction in Single -wire mode
 If single wire mode is active, this controls the data direction for TXD.
 If TXDIR = 0, TXD is an input.
 If TXDIR = 1, TXD is an output.
 RAF: Receiver Active Flag
 This is set to 1 if the receiver is currently receiving data.

 After a short delay, send the value $AB over the Tx line with an ODD parity bit .

 Wait until data is received on Rx, then load it into Accumulator
X.