C541U_99 Datasheet, PDF (131/192 Pages) Siemens Semiconductor Group – 8-BIT CMOS MICROCONTROLLER

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 kΩ 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 µs (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 µs (30 full speed data bit times).
D+
FS / LS USB
Transceiver ^D-
Host or
R₁
R₁
Hub Port
Twisted Pair Shielded
5 meters max.
R₂
D+
FS / LS USB
D- _Transceiver
Full Speed Device
D+
FS / LS USB
Transceiver ^D-
Host or
R₁
R₁
Hub Port
Unwisted, Unshielded
3 meters max.
R₂
D+
FS / LS USB
D- _Transceiver
Low Speed Device
MCS03416
Figure 6-38
Low Speed / High Speed Device Cable and Resisitor Connection
Resistor R₂ 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 R₂ can be made
switchable with a port a transistor.
Semiconductor Group
6-86
1999-04-01