Protocol Layer
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Bottom-up view of the USB protocol
– Byte/Bit Ordering
– SYNC Field
– Packet Field Formats
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PID Field
Address Field
Frame Number Field
Data Field
CRC
– Packet Formats
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Token Packets
Split Transaction Special Token Packets
Start-of-Frame Packets
Data Packets
Handshake Packets
Byte/Bit Ordering
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Bits are sent onto the bus
– LSB first, MSB last
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Multiple bytes are interpreted
– Little endian order, i.e., LSB to MSB
Endian Order
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Little Endian
– The low-order byte of the number is stored in memory at the lowest
address, and the high-order byte at the highest address (The little end
comes first)
– Example, a 4 byte Long Int
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Byte3 Byte2 Byte1 Byte0
– Will be arranged in memory as follows
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Base
Base
Base
Base
Big Endian
Address+0 Byte0
Address+1 Byte1
Address+2 Byte2
Address+3 Byte3
– Will be arrange in memory as follows
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Base
Base
Base
Base
Address+0 Byte3
Address+1 Byte2
Address+2 Byte1
Address+3 Byte0
SYNC Field
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Synchronization field
All packets begin with SYNC field
Align incoming data with the local clock
Length
– 8 bits for full/low speed
– 32 bits for high speed
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Serves only synchronization mechanism
The last two bits are a marker of the end of
the SYNC field
Packet Field Formats
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Field formats for
– Token packets
– Data packets
– Handshake packets
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SOP / EOP
– All packets have distinct Start-Of-Packet
(SOP) and End-Of-Packet (EOP) delimiters
PID (Packet IDentifier Field)
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Immediately follows the SYNC field of every USB
packets
Indicates the type of packet
A PID consists of
– 4 bits packet type field
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Packet format, error type
– 4 bits check field
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Complements of the their respective packet identifier bits
PID (Packet IDentifier Field)
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The host and all functions must perform a
complete decoding of all received PID fields
Any PID received with a failed check field or
which decodes to a non-defined value is
assumed to be corrupted  ignored by the
packet receiver
Valid PID for a transaction but does not
support, the function must not respond
– IN-only endpoint must ignore an OUT token
PID Types
Address Fields
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Function endpoints are addressed
using two fields
– The function ADDRess field (ADDR)
– The ENDPoint field (ENDP)
Address Field (ADDR)
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Specifies the function via its address
Either the source or destination of a data
packet, depending on the value of the token
PID
128 addresses are specified as ADDR<6:0>
Function address zero is reserved as the
default address
Endpoint Field (ENDP)
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More flexible addressing of functions in which more
than one endpoint is required
Is defined for IN, SETUP, OUTPUT and PING token
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Device Endpoint:
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– A uniquely addressable portion of a USB device that is the
source or sink of information in a communication flow
between the host and device
Frame Number Field
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11-bit field that is incremented by the
host on a per-frame basis
Maximum value of 7FFh and rolls over
after reaching its maximum value
Data Field
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Data packet size varies with the
transfer type
May range from zero to 1,024 bytes
Data bits within each bytes are shifted
out LSB first
CRC-Cyclic Redundancy Checks
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CRCs are used to protect all non-PID
fields in token and data packets
Token and data packet CRCs provide
100% coverage for all single and
double bit errors
Packet Formats
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Token packets
Data packets
Handshake packets
Token Packets
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PID
– IN, OUT, or SETUP
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ADDR / ENDP
– For OUT, SETUP
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Identify the endpoint that will receive the subsequent Data packet
– For IN
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Identify which endpoint transmit a Data packet
– For PING
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Identify which endpoint will respond with a handshake packet
A five bit CRC that covers that address and endpoint fields
Split Transactions
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A high speed split transaction is used
only between the host controller and a
hub when the hub has full/low speed
devices attached to it
Only defined to be used between the
host controller and a hub
No other high speed or full/low speed
devices ever use split transactions
Split Transaction Special Token
Packets
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SPLIT
– A special token for split transaction
– 4 byte token packets
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Other normal 3 byte token packets
– Provides additional transaction types with its specific
information
– Is used to support split transactions between the host
controller communicating with a hub operating at high
speed with full/low speed devices to some of its
downstream facing ports
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SSPLIT / CSPLIT
– Start SPLIT transaction
– Complete SPLIT transaction
Packets in a split Transaction
Relationship of Interrupt IN
Transaction to High-speed Split
Transaction
Relationship of Interrupt OUT
Transaction to High-speed Split
OUT Transaction
Start-Split (SSPLIT)
Transaction Token
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SC (Start/Complete)
– Zero indicates that this is a start-split transaction (SSPLIT)
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Port
– The port number of the target hub for which this full/low
speed transaction is destined
Start-Split (SSPLIT)
Transaction Token
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For full-speed isochronous OUT start-splits,
– S and E fields specify how the high speed data payload
corresponds to data for a full-speed data packet as show
in this table
– Allow the hub to detect various error cases due to lack of
receiving start-split transactions
Start-Split (SSPLIT)
Transaction Token
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ET (Endpoint Type)
– Tells the hub which split transaction state
machine to use for this full/low speed
transaction
Complete-Split Transaction
Token
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A SPLIT special token packet with the
SC field set to one
U: reserved/unused and must be reset
to zero
Start-of-Frame Packets
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Start-of-Frame (SOF) packets are
issued by the host at a nominal rate of
once every 1 ms for full-speed bus and
125 μs for a high-speed bus
USB Frames and Microframes
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A full speed 1 ms frame time
A high speed microframe with a 125 μs frame time
High speed devices see an SOF packet with the same
frame number each times during each 1 ms period
Data Packets
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Four types of data packets
– DATA0, DATA1, DATA2, MDATA
Handshake Packets
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Used to report the status of a data transaction
ACK
– The data packet was received without bit stuff or CRC errors over
the data field and that the data PID was received correctly
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NAK
– Unable to accept data from the host (OUT) or has no data to
transmit to the host (IN)
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STALL
– Unable to transmit or receive data or a control pipe request is
not supported
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NYET and ERR
– High speed only handshake