Upload
kelli
View
72
Download
0
Tags:
Embed Size (px)
DESCRIPTION
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 - PowerPoint PPT Presentation
Citation preview
Harrison JonesAlexis NoelWilliam Allen
SERIAL COMMUNICATION
INTERFACE(SCI)
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
INTRODUCTION & OUTLINE
SERIAL COMMUNICATION
Serial = one after the otherSerial Communication = sending one bit at a
time over a channel
Byteb0 b1 b2b3 b4 b5
b6 b7
Byteb0 b1 b2b3 b4 b5
b6 b7
b0 b1 b2 b3 b4 b5 b6 b7
Serial Communication
PARALLEL COMMUNICATION
Parallel Communication = several bits sent at a time on several parallel channels
b0
Parallel Communication
b1
b2
b3
b4
b5
b6
b7
b0b1b2b3b4b5b6b7
Byteb0b1b2b3b4b5b6b7
Byte
WHICH OF THESE DOES NOT SEND DATA IN A SERIAL STREAM?
Ethernet USB
Serial Port
Fiber Optic Cable
HDMIParallel Port
WHICH TYPE 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
Asynchronous 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
• 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 TransmissionAdvantages
• 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
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
speedUsed for speeds up to 3000 bits/second with only simple single-character error detection
Asynchronous Serial Transmission
DATA WORD AND CONTROL BITS
Asynchronous Serial TransmissionStart Bit
• Signals start of transmission of data bits• Transition from logic 1 to logic 0Data Bits• Typically 8 data bits (not including parity bit)• Least significant bit is transmitted and
received firstStop Bit• Signals end of data word
The rate at which symbols are sentMeasured 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) = 2N where N = bits / symbol N – Bits per symbol (binary = 1)
BAUD 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 Rb = 1 / Tb where Tb 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!) IR = Rb * Data Bit Number / Total Bit Number
BIT RATES
Symbol Number (M) Analog modem capable 64 different voltage levels (symbols)
sends out how many bits per symbol? M = 65 = 2N. 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? IR = 9600 * log2(2) * 8/11 = 6981 data bits per second
BAUD RATES EXAMPLES
Overhead Bits Start bit – Start of “frame” bit Parity Bit – Error check bit Stop Bits – End of “frame” bit
Data Bit – the actual data
ASYNCHRONOUS SERIAL TRANSMISSION
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”)
START 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 BitCalculated by transmitter, checked by receiver
User specified on the HCS12
PARITY BIT
One+ bits
Indicate the end of transmission
Usually 1 or 2 idle state bits
One stop bit on the HCS12
STOP 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
DATA BITS
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
EXAMPLE
0 1 0 1 0 1 0 1 1 0 1
Start Bit Stop Bit
Parity Bit
Data Bits
Direction of Transmission
Noise – Noisy signals cause bits to “flip”
Overrun – Slow receivers cause loss of data
Framing Error – Timing errors cause issues
TRANSMISSION ERRORS
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)
NOISE
Very simple concept
Receiver doesn’t read data in SCI data register fast enough
New data lost
OVERRUN
The result of reading incoming bits using the wrong starting point
Can be detected by using the parity bitOften the result of mismatched baud rates
FRAMING ERROR
SCI BLOCK DIAGRAM
SCI MODULE ON THE HCS12B 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
$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.
SCI DATA REGISTERS
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 #0052 (loads 52 into X) STX $00C8 (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.
SETTING THE BAUD RATE
$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 1
$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 CONTROL REGISTER 2
$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 1
$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.
SCI STATUS REGISTER 2
After a short delay, send the value $AB over the Tx line with an ODD parity bit .
DATA TRANSMIT EXAMPLE
Wait until data is received on Rx, then load it into Accumulator X.
DATA RECEIVE EXAMPLE
QUESTIONS?