Protocol analysis
R&S
®
RTO6
815
User Manual 1801.6687.02 ─ 05
each data packet, but disregard it unless it carries the correct address. In reply, only
the addressed device (one at a time) can send data upstream, to the host.
The USB architecture supports four different kinds of data transfer: control, bulk, inter-
rupt or isochronous transfer. For example, a "bulk OUT" transfer (from host to device)
would look like this:
Figure 13-37: Example of a bulk OUT transfer. Blue: host speaks, white: device speaks
Packet structure
All packets must start with a
SYNC
field, also called
SOP
(start of packet), which indi-
cates data transmission. It consists of "KJ" pairs, followed by one "KK". At low speed
and full speed (USB
1.x), it is 8 bits long, encoded as "KJKJKJKK". At high speed
(USB
2.0 and HSIC), it is up to 32 bits long, encoded as "KJKJKJKJKJKJ...KK".
The SYNC field is used to synchronize the clock of the receiver with that of the trans-
mitter. The final 2 bits ("KK") indicate where the PID fields starts.
Figure 13-38: Start of a high-speed packet: after an SE0, the packet starts with a SYNC field (or SOP),
followed by the data packet
SE0 = Single-ended zero, both D+ and D- wire are on low level
1
= High-speed SYNC field (or SOP), length up to 32 bits
2
= Receiver-squelched differential envelope, below 100 mV
3
= Differential envelope exceeds 150 mV, reception enabled with 4 symbol times
4
= Clock recovery must lock in time to detect end of SYNC
5
= End of SYNC detected, receiver begins data recovery
After the SYNC (or SOP), USB data packets consist of (in this order):
●
PID
(packet identifier): specifies the type of packet. Examples are:
–
Token packets (IN, OUT, SETUP) are always the first packet in a USB transac-
tion, containing the address of the device and its endpoint.
–
Token SOF packets (start of frame) are there to schedule a data transfer.
–
Handshake packets provide the status of the transaction.
–
Data packets are dedicated for the data transfer payload.
–
Various special packets provide special characteristics: for instance, SPLIT
packets are used to communicate with low or full speed devices on a high-
speed link.
USB (option R&S
RTO6-K570)
