USB System Architecture

be dropped from the start of packet.

2.When a packet is received by a 2.0 hub, it re-clocks the data when sending it to the next device.

3.Re-clocking the data requires buffering some of the packet.

High-Speed Hub Repeater

Figure 16-1 provides a conceptual view of the receiver, repeater, and transmitter path. Each of these blocks are discussed below. Note that the maximum propagation delay through the buffer is specified as 36 bit times.

Figure 16-1: Repeater Function Within Hub

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Chapter 16: 2.0 Hubs During HS Transactions

Receiver Squelch

As described in Chapter 11, high-speed receive circuitry is squelched when the bus is idle and enabled when a differential voltage of 150mv is detected by the packet envelope detector. The hub specification allows up to 4 bit times between detecting the packet and enabling the receiver. Thus, the first 4 bits of each packet can be dropped by a hub. This results in the synchronization clock being reduced by as many as 4 bits. Since the maximum number of hubs between any USB device and the root hub is 5, the maximum number of synchronization bits dropped is 20 out of the 32 produced at the originator.

The same 4 bit times affect the end of packet as well. When the receiving port returns to the idle state, the repeater is disabled; however, it may take up to 4 bit times to actually disable the repeater after EOP. This results in up to 4 random bits added to the end of the packet. These bits are termed dribble bits, but have no adverse effect on the detection of the packet at the receiver. They will accumulate with each hub crossing so that the fifth hub may send a packet with a maximum of 20 dribble bits. When the receiver of this packet detects EOP (via a bit stuffing error), it will check CRC and detect a valid value and the following dribble bits are simply discarded.

Re-clocking the Packet

Re-clocking packets is performed to reduce the jitter seen at a receiver so that jitter remains within the limits defined by the specification. Re-clocking involves extracting the data from the received NRZI stream and re-transmitting the stream using the hub’s local clock.

Port Selector State Machine

The block represents a hub state machine whose job it is to verify that the incoming packet is valid. The state machine detects removal of squelch (which could be caused by noise) and awaits the priming of the elasticity buffer (12 bit times). The state machine then checks for the repeating pattern of “JK” or “KJ” within the elasticity buffer, which indicates the synchronization pattern and a valid packet. If no repeating pattern is detected, then transmission of the packet to the downstream ports is not enabled.

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Elasticity Buffer

The elasticity buffer handles the frequency differences between the receive clock derived from the receive packet and transmit clock generated locally within the hub. The specification allows clock tolerance of 500ppm, resulting in the maximum difference between the receive and transmit clocks of 1000ppm. The elasticity buffer must handle the case where the receive clock is faster than the transmit clock and vice versa. To handle both conditions, the buffer must be filled half-way (primed) before data is clocked out of the buffer. In this way, if data is taken out of the buffer more quickly than it is being filled, no underrun will occur, and buffer space is also available to prevent overflow when data is stored faster than it is taken out.

The half-depth of the buffer must be equal to the maximum difference in clock rate over the length of a maximum-sized packet. Calculation of the half-depth of the buffer is as follows:

•given that the maximum clock difference is 1000ppm, and

•the maximum packet length is: 1024 byte data payload + total overhead including 20 dribble bits = 9644 bits;

•then the maximum overrun or underrun is approximately 10 bits (1000ppm

*9644 = 9.644 bits).

The specification requires a buffer half-depth of 12 bits to provide 2 bits of additional margin.

The Repeater State Machine

The hub repeater states are the same for low-, full-, and high-speed hub operation (See Figure 16-2 on page 287). However, when a hub is operating at highspeed, it repeats only high-speed packets. The operation of the high-speed state machine includes the following characteristics:

•Connectivity is established upon detecting a Start of High-Speed Packet (SOHP). Transitions caused by SOHP occur when the port selector state machine has ensured that a valid packet has been detected. See Figure 16-1 on page 284.

•Connectivity is torn down upon detecting a High-Speed End Of Packet (HEOP). The state transitions take place after the hub has repeated the last bit in the elasticity buffer.

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