USB System Architecture

Figure 11-11: Example Eye Diagram for Receiver Sensitivity Test

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High-Speed Start of Packet & Synchronization Sequence

A clock is transmitted at the beginning of each packet so that the input receiver can synchronize to the incoming packet. This synchronization sequence consists of a series of K to J transitions. The duration of this sequence is 32 bit times when originated by the host or responding device. See Figure 11-12 on page 235.

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Figure 11-12: High-Speed Synchronization Sequence and SOP

The synchronization sequence may be shorter than 32-bits when finally received by the device or host. This can occur as packets traverse a hub. Figure 11-13 illustrates the delay associated with detecting the beginning of a packet via the transmission envelope detector and the receiver being enabled. This can cause hub to drop up to 4 bits from the synchronization pattern when repeating packets. The topology restricts the number of hubs that can be connected in-line between the host and a device downstream. Thus, a maximum of 5 hub crossings may result in a sync pattern containing only 12 bits.

Figure 11-13: Squelch Detection Can Cause Hubs to Drop up to Four Bits from Synchronization Sequence

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High-Speed End of Packet (EOP)

Every packet ends with an EOP sequence as illustrated in Figure 11-14. The EOP sequence is 8 bits long except after the Start of Frame packet. In this case only, the EOP is extended to 40 bits. The 8-bit EOP consists of an NRZI encoded bit pattern of 01111111 without bit stuffing. Receivers detect the end of packet via an intentional bit stuffing error. High-speed receivers always detect EOP when a bit stuffing error occurs, even when the bit stuffing error is not during a normal EOP.

Figure 11-14: High-Speed EOP Detection

Detection of High-Speed Device Removal

The high-speed termination resistors keep both D+ and D- pulled down when no differential signaling is occurring. When a device is disconnected, no significant difference in the line states will be detected, because the 45Ω termination will still be present at the hub interface, thereby keeping the lines in the idle state.

The method used to detect that the high-speed device has been removed takes advantage of the missing terminations at the opposite end of the D+ and D-

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lines. When the device is removed, the high-speed packets continue to be transmitted from the port where the device was previously attached. When the incident wave reaches the end of the trace with no termination, a large reflection will travel back toward the hub interface, causing nearly a doubling of the original signal amplitude. This has no effect of the hub interface, because its transceiver is in the transmit mode when this amplitude doubling occurs. The hub uses this phenomenon to detect that the device is no longer attached to the port.

Hubs check for high-speed device removal at the end of each microSOF packet as illustrated in Figure 11-15.

Figure 11-15: Device Removal is Checked at End of MicroSOF Packet

The downstream facing hub ports include a disconnect receiver that watches for a differential signal significantly larger than the normal 400mv. Figure 11-16 on page 238 depicts the hub interface and the disconnect envelope detector. The receiver produces an output when the differential voltage reaches 625mv. This should occur only if the termination resistors are missing at one end of the cable due to a device having been removed.

The first bit of a high-speed EOP always forces a transition in the bit stream. No additional transitions occur until the next packet starts. EOP is a convenient mechanism to detect device removal because of predictable transition at the

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