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USB Pull-up/Pull-down Resistors
USB uses pull-up or pull-down resistors to determine when an attach or detach event occurs on
the bus. Host mode complicates the resistors, since it requires devices to operate as either a
USB target device or a USB host. Figure 4-5 shows the two resistor combinations required for
USB targets and hosts.
Normally, the USB operates in normal mode with HOST_MODE_EN=0. This mode enables
resistor R1 and disables the R2 resistors. When the device connects to a PC host, the host
recognizes that DPLUS is pulled up, indicating that a full-speed device is attached.
When the device is in host mode (HOST_MODE_EN=1), the R2 resistors are enabled and the
R1 resistor is disabled. When a USB target connects to the USB, the R1 in the target causes
the DPLUS signal (or DMINUS for a low-speed device) to go HIGH, activating the ATTACH
interrupt.
Figure 4-5. Pull-up/Pull-down USB