USB System Architecture

Full-Speed Device Connect

Figure 5-4 on page 98 illustrates a FS device connected to a hub port. When a FS device is attached to the port, current flows across the voltage divider created by the hub’s pull-down resistor and the device’s pull-up resistor on D+. Since the pull-down resistor value is 15KΩ and the device’s pull-up resistor is a value of 1.5KΩ , D+ will raise to approximately 90% of Vcc. When the hub detects that D+ approaches Vcc while the other remains near ground, it knows that a fullspeed device has been attached.

Figure 5-4: Full-Speed Device Detection

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Figure 5-5 on page 99 illustrates signal behavior when a FS device is attached. When D+ raises above VIH (max), for 2.5µs or longer but no more than 2ms

(TDCNN), the hub recognizes device attachment. The hub sets the appropriate status bits in its port status register after detecting device attachment.

The hub also sets a bit to indicate that the attached device operates at full-speed. Since the hub detects the D+ pull-up, it knows that the device operates at fullspeed. The hub must configure the port interface for full-speed device signaling. Table 5-1 illustrates the port status register. Bit 9, when cleared to zero, indicates that a full-speed device is attached. Note that the register definition shown is based on the 1.x version of the specification. Additional bits support detection of high-speed devices in the 2.0 version of this register.

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Chapter 5: LS/FS Signaling Environment

Table 5-1: Format of Port Status Fields Returned During the Get Port Status Request

Host software polls each hub periodically to check for port events including device connection. Software then resets the device and performs device configuration as discussed in Chapter 19, entitled "USB Device Configuration," on page 347.

Figure 5-5: Signal States During FS Device Attachment

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99

USB System Architecture

Low-Speed Device Connect

Designers of low-speed devices place a pull-up on the D- line versus the D+ line used for full-speed devices. Figure 5-6 on page 100 illustrates a low-speed device connected to a hub port. Like the full-speed device, the signal states and detection timing is based on the TDCNN timing parameter as illustrated in Figure 5-7 on page 101. When the hub detects low-speed device attachment, it sets both the connect and LS bits in the port status register and configures the interface for low-speed operation. Table 5-1 on page 99 depicts the port status register in which bit 9 is set to indicate that a LS device is attached to the port.

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Chapter 5: LS/FS Signaling Environment

Detecting Device Disconnect

The hub also monitors each port that is currently connected for the possibility of the device being removed. The transition that will be seen by the hub when a device is removed is illustrated in Figure 5-8. A hub detects the disconnect when it observes a single-ended zero (when D+ and D- fall below VIL) for the duration of the TDDIS timing parameter. Upon detecting a disconnect event, the hub will set status bits indicating the event.

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