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SA-1100 Developer’s Manual 11-57
Peripheral Control Module
11.8.1.1 Signalling Levels
USB uses differential signalling to encode data and to communicate various bus conditions. The
USB specification refers to the J and K data states to differentiate between high- and low-speed
transmission. Because the UDC supports only 12-Mbps transmission, references are made only to
actual data state 0 and actual data state 1.
Four distinct states are represented using differential data by decoding the polarity of the UDC+
and UDC- pins. Two of the four states are used to represent data. A one is represented when UDC+
is high and UDC- is low; a zero is represented when UDC+ is low and UDC- is high. The
remaining two states and pairings of the four encodings are further decoded to represent the current
state of the USB bus. Table 11-10 shows how seven different bus states are represented using
differential signalling.
Hosts and hubs have pull-down resistors on both the D+ and D- lines. When a device is not attached
to the cable, the pull-down resistors cause D+ and D- to be pulled down below the single-ended low
threshold of the host or hub. This creates a state called single-ended zero (SE0). A disconnect is
detected by the host when an SE0 persists for more than 2.5 µs (30-bit times). When the UDC is
connected to the USB cable, the pull-up resistor on the UDC+ pin causes D+ to be pulled above the
single-ended high threshold level. After 2.5 µs elapse, the host detects a connect.
After this point, the bus is in the idle state because UDC+ is high and UDC- is low. A start of
packet is signalled by transitioning the bus from the idle to the resume state (a 1 to 0 transition).
The beginning of each USB packet begins with a sync field, which starts with the 1-to-0 transition
(see the Section 11.8.1.1, “Signalling Levels” on page 11-57). After the packet data has been
transferred, an end of packet is signalled by pulling both UDC+ and UDC- low for 2-bit times,
followed by an idle for 1-bit time. If the idle persists for more than 3 ms, the UDC enters suspend
mode and it is placed in low-power mode. The UDC can be awakened from the suspend state by
the host by switching the bus to the resume state via normal bus activity, or by signalling a reset.
Under normal operating conditions, the host ensures that devices do not enter the suspend state by
periodically signalling an end of packet (EOP).
Table 11-10. USB Bus States
Bus State UDC+/UDC- Pin Levels
Idle
UDC+ high, UDC- low (same as a 1).
Resume
UDC+ low, UDC- high (same as a 0).
Start of Packet
Transition from idle to resume.
End of Packet
UDC+ AND UDC- low for 2-bit times followed by an idle for 1-bit time.
Disconnect
UDC+ AND UDC- below single-ended low threshold for more than 2.5 µs.
(Disconnect is the static bus condition that results when no device is plugged into a hub
port.)
Connect
UDC+ OR UDC- high for more than 2.5 µs.
Reset
UDC+ AND UDC- low for more than 2.5 µs. (Reset is driven by the host controller and
sensed by a device controller.)