Transcript Serial Communication Interface (SCI)

SERIAL
COMMUNICATION
INTERFACE
(SCI)
Harrison Jones
Alexis Noel
William Allen
INTRODUCTION & OUTLINE
 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
SERIAL COMMUNICATION
 Serial = one after the other
 Serial Communication = sending one bit at a time
over a channel
Serial Communication
Byte
b0 b1 b2
b3 b4 b5
b6 b7
b0
b1
b2
b3
b4
b5
b6
b7
Byte
b0 b1 b2
b3 b4 b5
b 6 b7
PARALLEL COMMUNICATION
 Parallel Communication = several bits sent at a
time on several parallel channels
Parallel Communication
b0
Byte
b0
b1
b2
b3
b4
b5
b6
b7
b1
Byte
b2
b0
b1
b2
b3
b4
b5
b6
b7
b3
b4
b5
b6
b7
WHICH OF THESE DOES NOT SEND DATA IN A
SERIAL STREAM?
Ethernet
Serial Port
USB
Parallel Port
Fiber Optic Cable
HDMI
WHICH T YPE SHOULD I USE?
PARALLEL COMMUNICATION ISSUES
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
SERIAL COMMUNICATION FOR LONG
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 VS. ASYNCHRONOUS
COMMUNICATION
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 COMMUNICATION
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
ASYNCHRONOUS COMMUNICATION
Asynchronous Serial Transmission
Advantages
• Each character is its own
complete timer system
• Corruption will not
spread
• Good for irregular interval
character generation
• Keyboards
Disadvantages
• Dependence on recognition
of start bits
• Many bits are used only for
control purpose and carry no
useful information
• Limits transmission
speed
Used for speeds up to 3000 bits/second with only simple
single-character error detection
DATA WORD AND CONTROL BITS
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
BAUD RATES
 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)
BIT RATES
 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
 Unless there is no protocol (risky!)
 I R = R b * Data Bit Number / Total Bit Number
BAUD RATES EXAMPLES
 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
ASYNCHRONOUS SERIAL TRANSMISSION
 Overhead Bits
 Start bit – Start of “frame” bit
 Parity Bit – Error check bit
 Stop Bits – End of “frame” bit
 Data Bit – the actual data
START BIT
 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”)
PARIT Y BIT
 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
STOP BITS
 One+ bits
 Indicate the end of transmission
 Usually 1 or 2 idle state bits
 One stop bit on the HCS12
DATA BITS
 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
EXAMPLE
 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
Start Bit
1
0
1
0
1
0
1
Data Bits
Direction of Transmission
1
0
1
Stop Bit
TRANSMISSION ERRORS
 Noise – Noisy signals cause bits to “flip”
 Overrun – Slow receivers cause loss of data
 Framing Error – Timing errors cause issues
NOISE
 Noise can cause “1” to appear to be “0” and vice verse
 Effect lessened by sampling
NOISE
 Each bit after start bit is sampled X pules after center of start
bit. X depends on the mode (1 ,16, and 64)
OVERRUN
 Very simple concept
 Receiver doesn’t read data in SCI data register fast enough
 New data lost
FRAMING ERROR
 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
SCI BLOCK DIAGRAM
SCI MODULE ON THE HCS12
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.
OVERVIEW OF SCI REGISTERS
SCI DATA REGISTERS
 $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.
SETTING THE BAUD RATE
 Writing to SCBIR to set the Baud Rate:
 $00C8
 $00C9
 Baud rate = SCI module clock / (16 * SCIBR[12:0])
SETTING THE BAUD RATE
 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.
SCI CONTROL REGISTER 1
 $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.
SCI CONTROL REGISTER 1
 $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.
SCI CONTROL REGISTER 2
 $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.
SCI CONTROL REGISTER 2
 $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.
SCI STATUS REGISTER 1
 $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.
SCI STATUS REGISTER 1
 $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.
SCI STATUS REGISTER 2
 $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.
DATA TRANSMIT EXAMPLE
 After a short delay, send the value $AB over the Tx line with
an ODD parity bit .
DATA RECEIVE EXAMPLE
 Wait until data is received on Rx, then load it into Accumulator
X.
QUESTIONS?