Transcript Slide 1

Lecture 20: Communications
Lecturers:
Professor John Devlin
Mr Robert Ross
Overview
• Communications overview
• Overview: UART (RS-232)
• Brief overview:
– I2C
– SPI
– 1-Wire
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Digital Communications
• Different types of communication:
– Serial – data only transmitted one bit at a time on one
lane (per direction) (differential pairs of signals are
referred to as a lane)
– Parallel – data transmitted typically words at a time
(on different lines) with additional lines for parity and
control (Typically doesn’t use lanes)
• Different types of signals:
– Data – The message which is to be sent
– Control – Signals used to ensure to control the flow of
data, eg. Clock, start of frame, end of frame
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Universal Asynchronous
Receiver/Transmitter UART
• Common standard: RS-232
• Three wires used:
– Transmit
– Receive
– Ground
• Data encapsulated as 7 or 8 bit chunks
• Optional Parity – monitor data integrity
• Allow serial communication between
devices
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Asynchronous
• Signals are self clocking – no separate
clock is transmitted with data
• Receiver must lock onto data and decode
individual bits without using a specific
receive clock
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Operation
• UARTs have two sectors:
– Receive (RX): read the incoming serial bitstream, converts to parallel data (Serial to
Parallel Conversion)
– Transmit (TX): converts parallel data from the
microprocessor registers into a serial bitstream to transmit (Parallel to Serial
Conversion)
• Receiver has to detect phase of incoming
data and phase lock it’s local clock to the
data pattern
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Parity Bit
• Parity is a simple check to verify data
integrity
• An extra bit is added to each byte (or a
defined chunk of data) called a parity bit
• 4 different cases:
– Even Parity: Even number of bits in total
– Odd Parity: Odd number of bits in total
– Mark Parity: Parity bit is always ‘1’
– Space Parity: Parity bit is always ‘0’
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Parity Bit
• For each byte the number of ‘1’s is summed
• In even parity this sum should be even (parity bit
is set to make it even)
• In odd parity this sum should be odd (parity bit is
set to make it odd)
• Example:
– 10010111 (5 ‘1’s present)
• If even parity: parity bit should be ‘1’
• If odd parity: parity bit should be ‘0’
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Parity Bit
• Receiver performs same parity count – if it
doesn’t match the receiver knows that the
data wasn’t received correctly
• What is a problem with Parity?
– Only detects odd numbers of bit errors
effectively
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UART interfacing
UART device
Microprocessor
TX
RX
Examples:
GPS Receiver
Bluetooth Transceiver
RX
TX
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UART RS-232 interfacing
• RS-232 is the serial standard used on PC serial
ports
• The voltage levels are inverted and have a
larger swing than most microprocessors provide
• Use a specialised IC to convert: eg. MAX232
PC
Microprocessor
TX
RX
MAX232
RX
TX
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I2C
•
•
•
•
Inter-Integrated Circuit
Developed by Philips
Multi-master serial computer bus
Two wires – configured as open drain
lines, pulled up with resistors
– SDA: Serial Data
– SCL: Serial Clock
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I2C
• Master sends Start bit, followed by address of desired
slave followed by read/write bit
• Slave sends acknowledgement
• Master continues in requested mode (read or write),
slave continues in complementary mode (write or read)
until stop bit is sent
• Communications are in Big Endian bit format – most
significant bit first
• Start bit: high->low transition of SDA with SCL high
• Stop bit: low->high transition of SDA with SCL high
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SPI
• Serial Peripheral Interface Bus
• Developed by Motorola
• 4 signals used:
– SCLK: Serial Clock (from master)
– MOSI: (Master Output, Slave Input)
– MISO: (Master Input, Slave Output)
– SS: (Slave Select) (one pin for each slave
device)
– http://en.wikipedia.org/wiki/Serial_Peripheral_I
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nterface_Bus
SPI
• Allows full duplex (bi-directional at the same
time) communication
• Higher throughput than I2C
• No flow control
• Short distance communication
• Applications:
–
–
–
–
SD Memory cards
Sensors
Memory
IC communications
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Summary
• Digital communications is the process
where digital devices exchange data
encoded in a bit format
• Communications can occur in two forms
(with various adaptations of each) – serial
or parallel
• UART, I2C and SPI are common methods
used for serial communications
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