Protocol analysis
R&S
®
RTE
818User Manual 1326.1032.02 ─ 20
●
Little Endian scheme: both multiple bits and multiple bytes are transmitted with the
least significant bit/byte (LSB) sent first, while the most significant bit/byte (MSB) is
sent last.
●
HSIC (High-Speed Inter-Chip): an industry standard for USB chip-to-chip intercon-
nection with a 2-signal (strobe, data) source synchronous serial interface, using
240 MHz DDR signaling to provide only high-speed (480 Mbps) data rate.
Table 13-21: Simplified symbolic representation from the USB standard
Bus State Protocol Levels
Differential "1"
D+ High, D- Low
Differential "0"
D+ Low, D- High
Data "J" State Low speed D+ Low, D- High (differential "0")
Full speed D+ High, D- Low (differential "1")
Data "K" State Low speed D+ High, D- Low (differential "1")
Full speed D+ Low, D- High (differential "0")
In the example in Table 13-21, High speed can be assumed to be like Full speed. For a
complete overview of bus states and for an electrical definition of High and Low speed,
refer to chapter 7 of the USB specification.
Data transfer
In contrast to a conventional bus, USB is more like a network protocol, using target
addresses and endpoints. However, USB features a bus master, called the host. It
transmits packets of data to all devices or hubs connected to the host (or connected to
a device or hub, which is in turn connected to the host). All connected devices receive
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 13-94: 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.
USB (option R&S
RTE-K60)
To Next Page
Loading...
