Universal Serial
Bus
Grant Heileman
The History of USB
In 1994 a collaborative effort to design a standard for
peripheral devices was made between Compaq, DEC, IBM,
Intel, Microsoft, NEC, and Nortel.
USB 1.0
USB 1.0 was released January 1996. It has a data transfer
rate of 12 Mbit/s. However USB did not become popular
until its first revision, in 1998. This revision featured the
ability to use either 12 Mbit/s(FS) or 1.5 Mbit/s(LS)
depending on the device being used.
USB 2.0
April 2000, the specifications for USB 2.0 are released.
With a data transfer rate of 480 Mbit/s(HS) this revision
was much more superior than its predecessors. However,
just because this speed is manageable does not mean that
it is often met.
USB cables
Signals are sent across USB cables by a twisted set of
differential pair wires D+ and D- ( labeled W and G in the
picture). USB uses Non Return to Zero Invert (NRZI) data
encoding. This means that a 1 will be produced when
there is no change in the level of the signal and a 0 will be
produced whenever the signal changes. Power is supplied
to the devices by the Vbus (+5v) and GND (-5v) wires.
Terms you should know
 Slave - A peripheral USB device, could be anything from an Ipod to a
vacuum cleaner. A compound device is a slave that contains several subdevices, referred to as device functions, incased within it.
 Hub - A hub is a device that contains multiple ports. Their will always be a
root hub. This is the hub that contains the ports connected to the host.
After this condition has been meet you may have a device with the sole
purpose of giving access to more ports.
 Host - Initiates all transactions and bandwidth usage. A host is usually
thought of as a computer; however a host can be any device that manages
data transactions to and from slaves. The host controls most of the
protocol complexity allowing for cheaper slave devices to be produced.
Host Controller
A host controller is an additional piece of hardware that is
used to guarantee that the data transmitted over the bus
is within the USB specifications and no errors have
occurred. Along with data processing and error
management the HC is used to generate Start of Frame
(SOF) packets every 1msec.
Serial Interface Engine
The SIE is used in both the host and slave. The SIE is used
in the serialization of USB transmissions. USB implements
bit stuffing. This means that after six consecutive ones a
zero is inserted. The SIE is responsible for bit stuff
encoding/decoding the NRZI stream. It also generates the
CRCs and confirms any incoming CRC, as well as detects
PID, SOP, EOP, reset signals, and resume signals.
Tiered star topology
Host
Root hub
Hub
Hub
Hub
Peripheral Device
Peripheral Device
Peripheral Device
Peripheral Device
Enumeration
Once you plug your device in a process called enumeration
starts. The host initiates enumeration by…
- Sending a reset signal to the device, as well as determine
it’s speed
- reading the device’s information
- assigning a unique 7 bit address to the device
- If the device is compatible with the host the device’s
drivers are loaded and it is set to a configurable state.
Endpoints
An interface is a group of endpoint. A endpoint is the
physical structure, located on a USB device, that is the
destination of the Packets. Managing interfaces allows
the use to send information from the hosts buffers to
specific endpoints on the interface.
Endpoints can be Out (host to device)
or In (device to host)
Endpoint types
 Control transfers: used for short, simple commands to the device, and
status response to the host. Used by the bus control pipe number 0.
 Isochronous transfers: guarantees a set data rate with possible data
loss. It is usually the fastest data rate possible. (e.g. realtime audio or
video).
 Interrupt transfers : devices that need guaranteed quick responses
(bounded latency) (e.g. pointing devices and keyboards).
 Bulk transfers: large sporadic transfers using all remaining available
bandwidth, but with no guarantees on bandwidth or latency (e.g. file
transfers).
Pipes
A pipe is a logical connection between the appropriate
software in the host and a particular endpoint on the
device. There are two types of pipes, stream and message.
A stream pipe is uni-directional and utilizes the
isochronous, interrupt, and bulk data transfers. A
message pipe is bi-directional and is exclusively used for
control data transfers. Endpoint Zero is usually reserved
for message pipes.
Packet Fields
USB data is transmitted LSB first and contains packages
consisting of these fields.
Sync
 This is a mandatory field that synchronizes the slave’s
clock with the host’s clock. This field is 8 bits long in low
and full speed devices and 32 bits long in high speed
devices. The last two bits identify the start of the PID
field.
PID
 A “Packet ID” is a 8 bit field used to identify which type of
packet is being sent. Only 4 bits are needed for this but to
guarantee accuracy these 4 bits are complemented and
repeated.
ADDR
 This is a 7 bit field used to distinguish between slave
device. The all 0’s address is used for devices that have
not been assigned an address yet. This allows for a
maximum of 127 devices to be connected.
ENDP
 The endpoint field contains 4 bits, this allows for a total of
16 endpoints. However low speed devices may only have 4
endpoint, this includes the default pipes.
CRC
 The Cyclic redundancy check is a field used to check for
transmission errors in the token and data packets of any
non-PID transaction.
EOP
 The End of packet field consists of a Single Ended Zero
(SE0, D+ and D- are low) for two bit times followed by a J
(differential 1 for full-speed devices and 0 for low-speed
devices) for 1 bit time. This field is used so the slave can
identify when a packet ends and a new sync can start.
Packet Types
USB uses four different packet types. Token, Data,
Handshake, and start of frame.
Token Packets
Token packets are used to identify the type of transaction that will be
taking place.
•
In – Tells the slave that the host needs to read information.
•
Out – Tells the slave that the host needs to send information
•
SOF – Indicates the start of frame.
•
Setup – Tells the slave a control transaction is starting
Token packets must be formatted like this
Sync
PID
ADDR
ENDP
CRC5
EOP
Data Packets
Data packets contain the payload, low speed devices can
have a maximum size of 8 bytes while full and high speed
devices can have a maximum size of 1023 bytes and 1024
bytes respectively. The data must be sent in multiples of
bytes. Data0 and Data1 can transmit up to 1024 bytes of
data. High speed devices have an additional two packet
types, Data2 and Mdata.
Data packets must be formatted like this
Sync
PID
DATA
CRC16
EOP
Handshake packets
Handshake packets are used to determine whether or not
the the transmission of the data packet was successful.
The types of handshake packets consist of.
• ACK – Acknowledges the packet was received.
• NAK – Informs the host that the device momentarily can not
send or receive data.
• STALL – The device is in a state that requires intervention
form the host. Either the endpoint is frozen or the Setup is
not supported.
Handshake packets must be formatted like this.
Sync
PID
EOP
Start of frame Packets
 The SOF packet is sent by the host every 1msec (FS bus) or
125usec (HS bus). The frame number is 11 bytes long.
Sync
PID
Frame
Number
CRC5
EOP