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COSMOS Summer 2008
Peripheral Interfaces: I2C & USB
Rajesh K. Gupta
Computer Science and Engineering
University of California, San Diego.
©2008 R. Gupta, UCSD
Roadmap
• Topic:
– Computer interfaces: mechanicals, electricals, signaling,
protocols
• This lecture
– Concepts covered: Input/Output, Addressing, Synchronous,
Asynchronous, Access Arbitration, Protocols, Packets, Messaging
– Concepts covered: Interface standardization, character coding,
serial data transfers, Star network, Host controller
• Next lecture
– Cap Sensing
• Reference
– None
©2008 R. Gupta, UCSD
Keywords:
Bidirectional
Address space
Address decoding
Multi-master
Wired AND
I2C, Master/Slave, RS232c,
Dsub connector, UART, USB,
USB Hub, USB host
What is this connector?
9-pin RS232C: DE-9
A “Serial” Port
Then What Are These?
From RS232c to USB
• “D-Sub” connectors
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A: 15 pin
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B: 25 pin
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C: 37 pin
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D: 50 pin
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E: 9 pin
• RS232 was DB25
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RS232c was DE9
• Specified by EIA in 1969
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Electricals (voltage levels)
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Signaling rate, timing, slew-rate
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Mechanicals
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But not: character encoding,
character framing, protocols
Question 1: How many keys on your keyboard?
Question 2: How many bits it will take to encode these?
What is ASCII?
• American Standard Code for Information Interchange
– Published in 1963, revised 1967, 1986
• 128 characters, incl. 33 non-printing or control
– 94 printable characters: 26 + 10 + 11-25 symbols
– Is SPACE printable?
• 8-bit extension by MAC OS Roman
• Unicode and Universal Character Set (UCS)
– UTF-8, UTF-16, UTF-32
Interface Basics
1. Who/Where to send/receive information?
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Ports: mechanicals, electrical
2. What information to send?
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Signals and Packets: Electrical signaling, logical encoding
3. How to send the information?
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Protocols: synchronous, asynchronous
How do we measure goodness of an interface?
1. Ports
• Which door to knock at or open?
• All processors already have one door: memory
– Memory-mapped IO
• They may have additional I/O ports
– How are these ports identified?
– How are devices connecting to these ports identified?
• Mechanically, Electrically, or at a ‘higher level’
• Memory-mapped versus dedicated IO
– What happens to CPU when I/O operation is in progress?
2. Signaling
• How many wires? What do they carry?
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Serial signaling: Send one bit at a time
• Direction of signaling: Half and Full Duplex
–
Synchronous versus Asynchronous
• Asynchronous serial communication
–
–
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Send a START signal prior to each byte
And a STOP signal after each byte
Generally use more than 8-bit to transmit a byte (10 to 12)
• UART: Universal Asynchronous Receiver Transmitter
–
–
Again, no shared clock. The RX must lock onto data and detect individual
bits
TX is a Parallel-to-Series converter
• RX is a Series-to-Parallel converter
3. Protocols
• Request/Acknowledge Handshakes
– RTS = Request to Send: Transmitter (TX) asserts RTS
– CTS = Clear to Send: Receiver (RX) asserts CTS
• This gives you flow control
– i.e., data transfer can proceed at a rate that is acceptable
• Let us examine two protocols
– I2C and USB
I2C: inter-integrated circuit
• Two-wire
– A microcontroller can control a network of devices with just two
general-purpose IO pins and software. (upto a few meters)
• Connects multiple devices on a multi-drop bus
• Devices can be attached or detached without
affecting other devices
– 7-bit address space, 16 reserved, 112 nodes maximum
• 10 kbps (low), 100 kbps, 400 kbps (fast), FM+ 1
©2008 R. Gupta, UCSD
Mbps, HS 3.4 Mbps
I2C Wires
• Two bidirectional wires
– SDA: Serial Data
– SCL: Serial Clock
• ‘Open drain’: normally high when not in use
– MASTER node issues the SCL and addresses SLAVES
– SLAVE node receives the SCL and the address
– “Wired AND” logical function.
©2008 R. Gupta, UCSD
So, how do we write or read?
• Normally, both SDA and SCL are ‘high’
– “sense” before you drive a line
• A device that wants to write pulls SDA low
– Followed by SCL going low
• So, everyone else knows that a transmission is
starting
©2008 R. Gupta, UCSD
START, Data, Data,…,Data, STOP!
Putting it together
• Start Condition: With SCL low, SDA goes HL
• Bits are ‘sampled’ on the rising edge of SCL
• Stop Condition: With SCL high, SDA goes LH
©2008 R. Gupta, UCSD
QUIZ
• Question 1: What is on SDA when it makes a
transition with SCL low?
• Question 2: What is on SDA when it makes a
transition with SCL high?
• Question 3: What happens if Slave is not ready
to receive next BIT?
I2C Protocol
• Any number of bytes in an I2C packet
– MSB first, each bytes transmitted must be acknowledged by
the receiver
– After each 8th bit, MASTER releases SDA and then generates
an additional clock pulse on SCL
• Receiver can then acknowledge by pulling SDA low
• Receiver can always abort the transmission by
holding SCL low
– Can not go up by the MASTER, thus no bit sampling edge
©2008 R. Gupta, UCSD
Bi-directional Data Transfers
I2C Protocol Actions
• Master sends START
– followed by 7-bit address of the Slave
• followed by single bit representing write to (0) or read
from (1) the slave.
• Slave responds with ACK bit for that address
– Master then continues in either TX or RX mode
• Communication transfer follows.
• All other MASTERS monitor START and STOP bits.
©2008 R. Gupta, UCSD
Note the direction of signaling
Exercise
• Is I2C communication synchronous or
asynchronous?
• Is I2C duplex? Fully duplex?
• Draw SDA and SCL waveforms in case of
transmission of START followed by $A2 = (1010
0010)
©2008 R. Gupta, UCSD
Stepping Back in Time:
RS232 to USB
• RS232C originally designed for connecting a teletype
(DTE) to a modem (DCE)
–
The modem connected over the phone line to other modems
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Question: Why ‘modem’? Hint: interface to the telephone line.
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In truth, DTE, DCE is a bit arbitrary
• RS232C is ‘unbalanced’
–
Logical High = typically -12V (-5 to -15 V)
–
Logical Low = typically +12V (+5 to +15 V)
–
This means that one typically needs interface circuits to the
microprocessor (level shifters)
• 7-bit data frames
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(instead of more common 8-bit)
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You may receive email that specifies 7-bit character set just in case it is
routed via a serial connection that supports only 7-bit transmission
Standardizing the Standard: USB
• RS232C was not standard enough
– Too much flexibility (on data rate, parity, flow control)
• USB: standardized the door and the lane
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–
–
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Software takes care of the data, information side
The ‘OS’ is aware of the device interface
Up to 127 devices. One standard cable.
Devices identify themselves. Not the interface.
• USB 1.1: 12 Mbps (Normal), 1.5 Mbps (Low)
• USB 2.0: 480 Mbps (High)
• USB 3.0: 4.8 Gbps (Super)
USB
• Shielded 4-wire cable
1
Vbus
USB device power (+5 V)
Red
3
D+
Differential data line
Green
2
D-
Differential data line
White
4
GND
Power and signal ground
Black
• One host in a network: host controller
–
Upstream versus downstream connection and connectors (A versus B)
• Host controller either directly connects to device (star) or through
a hub (tiered star)
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Because of the connectors, no device-to-device connections
• When a device is attached to the network
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Based on its identification, the host OS determines the software driver to
be used,
device is assigned a unique address and
host requests its internal configuration.
Classes of Devices
• Host controller know about the following class of devices
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Audio
HID
Hub
IrDA
Mass storage: HD, CDROM, DVD
Monitor
Communications
Physical interface device
Power
Printer
Imaging
Common class…
USB Packets
• All data transfers under the control of the host controller
• Four type of data transfers
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Control transfer: configure the bus and devices on the bus
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Bulk transfer: move data asynchronously
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Isochronous transfer: move time-critical data (e.g, audio)
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Interrupt transfer: retrieve data at regular intervals
• A packet contains
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SYNC byte, Packet ID, Content, CRC
• Four type of packets
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Token, data, handshake, preamble
• Defined processes for how a device is recognized, joins the
network and how it communicates with the host controller.
Embedding Clock
• Recover clock from data
• Manchester Encoding
– Each data bit (1 or a 0) has at least one transition
– So, we can ‘recover’ clock from data
Coding Schemes
• Note that each data transition requires two transitions of
clock
– Higher frequency, signal bandwidth, electrical effects
• A coding scheme can reduce the number of transitions
– to be equal to that of the data rate
• NRZ (duration of each bit = 1/f)
• NRZI (signal is toggled only when a logic 1 is to be
transmitted)
• 4B/5B encoding translated 4 bits of data into a 5-bit
symbol
• for the 16 possible permutations there are 32 symbols
• carefully choose symbols to guarantee that at least one signal
transition within any 3 bit period.
4b/5b code
Code
11110
01001
10100
10101
01010
01011
01110
01111
10010
10011
10110
11010
Data
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1100
Recap
• Computers interfaces with devices via memory or via direct
ports
• I2C defines a 2-pin interface that
– Performs Master-Slave communications
• The interface defines electrical behavior (signaling)
– And the data transfer protocols (messaging)
– What about the mechanicals?
• Serial interfaces are simpler and cost effective in
embedded applications
• Standardization helps in proliferating their use
• The data interface can also be used to supply power
– Typically use ‘unused’ pins in the standard
• E.g., A RTS ‘powers up’ a device
– PoE, PoweredUSB, …
©2008 R. Gupta, UCSD