7.1.5 Device Speed Identification The following sections specify the speed identification mechanisms for low-speed, full-speed, and high-speed.
7.1.5.1 Low-/Full-speed Device Speed Identification The USB is terminated at the hub and function ends as shown in Figure 7-20 and Figure 7-21. Full-speed and low-speed devices are differentiated by the position of the pull-up resistor on the downstream end of the cable: - Full-speed devices are terminated as shown in Figure 7-20 with the pull-up resistor on the D+ line. - Low-speed devices are terminated as shown in Figure 7-21 with the pull-up resistor on the D- line. - The pull-down terminators on downstream facing ports are resistors of 15 kΩ ±5% connected to ground.
The design of the pull-up resistor must ensure that the signal levels satisfy the requirements specified in Table 7-2. In order to facilitate bus state evaluation that may be performed at the end of a reset, the design must be able to pull-up D+ or D- from 0 V to VIH (min) within the minimum reset relaxation time of 2.5 μs. A device that has a detachable cable must use a 1.5 kΩ ±5% resistor tied to a voltage source between 3.0 V and 3.6 V (VTERM) to satisfy these requirements. Devices with captive cables may use alternative termination means. However, the Thevenin resistance of any termination must be no less than 900 Ω.
Note: Thevenin resistance of termination does not include the 15 kΩ ±5% resistor on host/hub.
The voltage source on the pull-up resistor must be derived from or controlled by the power supplied on the USB cable such that when VBUS is removed, the pull-up resistor does not supply current on the data line to which it is attached.
楼主
标题置顶
标题高亮
