Semiconductor Group
6-86
1999-04-01
On-Chip Peripheral Components
C541U
6.4.10 Detection of Connected Devices
Full speed and low speed USB devices are differentiated by the position of the pullup resistor on the
downstream end of the cable. Full speed devices are terminated with the pull-up on the D+ line and
low speed devices are terminated with the pull-up in the D- line (see figure 6-). The pull-up
terminator is a 1.5 k
Ω
resistor tied to a voltage source between 3.0 and 3.6 V referenced to the local
ground.
When a device is attached to the host or hub, the data line with the pull-up is above 2.8 V and the
other data line is near ground (idle state). A connect condition will be detected when one of the data
lines is pulled above the single-ended high threshold level for more than 2.5
µ
s (30 full speed data
bit times).
When a device is detached from the USB, the pull-down resistors will cause both D+ and D- lines
to be pulled below the single-ended low threshold of the host or hub port. This creates a state called
a single-ended zero (SE0) on the downstream port. A disconnect condition is indicated if an SE0
persists on a downstream port for more than 2.5
µ
s (30 full speed data bit times).
Figure 6-38
Low Speed / High Speed Device Cable and Resisitor Connection
Resistor R
2
has to be external. In some cases it might be necessary to hide the USB device from
the host even when it is plugged in. To accomplish this, the pull-up resistor R
2
can be made
switchable with a port a transistor.
FS / LS USB
Transceiver
Host or
Hub Port
D+
D-
Transceiver
FS / LS USB
Hub Port
Host or
D-
D+
Transceiver
FS / LS USB
FS / LS USB
Transceiver
D+
D-
D+
D-
R
1
R
1
R
1
R
1
R
2
2
R
Low Speed Device
Full Speed Device
Twisted Pair Shielded
5 meters max.
Unwisted, Unshielded
3 meters max.
MCS03416
Содержание C541U
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