Introduction to High Speed USB

As mentioned before, the high speed additions to the specification were introduced in USB 2.0 as a response to the higher speed of Firewire.

As High Speed was added as an afterthought, and had to maintain compatibilty without compromising performance, we have left the description of High Speed until we had covered the basics of the original specification.

Subjects covered in this part...

• Data Transmission
• Packet Sync
• End of Packet
• Compatibility
• Negotiating High Speed
• Frames and Microframes

• New Packet Identifiers
• High Speed Hubs with Full and Low Speed Devices
• Reset
• Suspend
• Resume
• Detecting a Device Unplug

Data Transmission

The data rate achieved by High Speed is 480 Mb/s. This needs to be transmitted down cables which were originally specified for a 12 Mb/s transmission rate,

To achieve this, when the link is conveying high speed data, each end of D+ and each end of D- is terminated with a 45 Ohm resistance to ground.

Data is sent by steering a current of 17.78 mA (derived from the positive supply) into either the D+ or the D- line. This results in a voltage of 400mV on the line being fed with current. The differential state of the line is detected at the receiving end by a differential receiver. This arrangement is able to reliably receive data sent at 480 Mb/s.

In fact the 45 Ohm resistors are provided by the Full Speed / Low Speed driver, at each end of the link, applying a Single Ended Zero. The FS/LS driver is designed to provide as accurate a termination resistance as possible. By switching off the high speed transceiver current source, the line conditions are as defined for full speed / low speed.

Basics of High Speed Transmission

In addition to the differential receiver, there is also a 'transmission envelope detector' and a 'differential envelope detector'.

The transmission envelope detector produces a 'squelch' signal if there is less than 100uV between the data lines, which means that there is no data being received.

The differential envelope detector detects if the far end has been unplugged, as the differential voltage will double to about 800 mV if the far end terminating resistors are not present.

(Further down the page you will see how this is used by the host to detect the unplugging of a high speed device.)

Packet Sync

Just prior to the packet sync, both data lines are low. The sync is sent using the NRZI sequence KJKJKJKJKJKJKJKJ KJKJKJKJKJKJKJKK.

A hub may drop up to 4 bits from the sync pattern. After 5 hubs the sync field of a packet may be only 12 bits long.

High Speed Synchronisation Pattern

End of Packet

On a low or full speed link, a brief Single Ended Zero (SE0) state is used to indicate End Of Packet (EOP), and idle is indicated by a J condition.

On a high speed link, the idle state is effectively a SE0, so that state is not available to indicate EOP, and a different method for indicating the end of packet is used. During normal data transmission there can not be a run of more than 6 1's in a row, because a 0 is automatically inserted (and will be removed on reception). This guarantees that there will be sufficient transitions in the NRZI encoded data stream to allow clock recovery.

At high speed, the EOP is indicated, by deliberately sending a byte which contains a bit-stuffing error; '01111111'. This applies at the end of all packets except SOF.

Each high speed SOF packet is terminated with 5 NRZI bytes containing bit-stuffing errors: 01111111 - 11111111 -11111111 -11111111 -11111111. This pattern allows the 'disconnection envelope detector' to detect a rise in data amplitude above 625 mV, in the event that the device, along with its termination resistors, has been unplugged.

Compatibility

Care has been taken to provide as much compatibity between high speed and full / low speed host, and high speed and full / low speed devices. USB is a plug-and-play system and must not confuse the user.

So a low or full speed device will always work with a high speed capable host.

Negotiating High Speed

To maintain the required compatibility, a high speed device will always present itself initially as a Full Speed device (by a 1.5K pullup resistor on D+).

The negotiation for High Speed takes place during the Reset, which is, as we remember, the first thing a host must do to a device before attempting data communication.

The high speed detection handshake is initiated by the device.

The hub will respond to it, if it is high speed capable.

What the device does

The device leaves its D+ 1.5K pullup resistor connected, and does not terminate the lines with 45 Ohm resistors as it would for high speed. But it drives high speed current (17.78mA) into the D- line for at least a millisecond. Now, remember that the hub is applying a reset condition to the lines, so effectively is already terminated as for high speed data. As only one end of the link is terminated, the hub will see about 800 mV on D-. This condition is called a K-chirp.

A full / low speed hub will pay no attention to this condition, but a high speed hub will detect it using its differential receiver and the absence of a squelch signal.

If the hub does not respond, then the rest of the reset, and subsequent data transmissions will take place as is normal for a full speed device.

Hub Response

If the hub is high speed capable then it will monitor the K-chirp from the device until it sees it completing. It must, within 100us, send a series of K-J chirp pairs to the device. This means that it will inject 17.78 mA alternately into the D- and the D+ lines. Each of these chirps lasts around 50us, and there are no gaps between them. The device has to see at least 3 chirp pairs before assuming that the hub is high speed capable.

Switching to High Speed

At this point the device disconnects its 1.5K pullup resistor, applies the 45 Ohm high speed terminations (using its full speed data driver in SE0 mode), and is thus in a state to perform high speed data transmission and reception. The hub will continue to send chirp pairs up until 100 - 500 us before the end of reset, and the device will monitor these chirps. At the point in time when the device termination is applied, the amplitude of the chirp signals, viewed on an oscilloscope would be seen to halve in amplitude from 800mV to 400mV.

Frames and Microframes

The 1 ms frame rate in full speed / low speed USB, is used for a number of purposes, such as scheduling access to the bus, and as a timing reference for interrupt and isochronous transfers.

For high speed, a higher frame rate was deemed appropriate, while still maintaining a relationship with the existing 1 kHz rate.

To this end, high speed uses the 'Microframe' which is 125us long (8 Microframes per millisecond). The correspondence with the 1ms frame numbering is maintained in the high speed SOF packets by repeating each frame number in 8 successive Microframes.

Packet Length

The maximum length of packets was increased for high speed, see table to right.

Packets per (Micro)frame

At high speed it is possible to specify up to 3 isochronous or interrupt transfers per microframe, rather than the 1 transfer per frame of full speed; giving a maximum possible isochronous or interrupt transfer rate of 192 Mb/s.

New Packet Identifiers

Some new PIDs were added for high speed, partly to overcome some inefficiencies which were recognised in the full speed protocol, and partly to support new isochronous transfer features, and the new requirement for 'split transactions' (more about this below).

The identifiers which are designed to overcome some inefficiencies and improve bandwidth usage at high speed are:

• NYET
• PING

Identifiers which allow for control over multiple isochronous packets per microframe are:

• DATA2
• MDATA

The identifiers added to assist with split transfers are:

• SPLIT
• ERR

* Bits are transmitted lsb first

Reset

The host will maintain its SE0, but not send any data, when it wants to Reset the device. The device will initially see a SE0 (with no data activity) and will not be able to distinguish this condition from a Suspend. After, at the latest, 3.125 ms of this condition the device must revert to full speed termination itself, and then test whether it sees SE0 or Idle. If it sees SE0 then it knows it is being reset, and will procede with the chirp handshake described above.

Suspend

A high speed host suspends a device by reverting to a Full Speed idle state. Again the device will initially see a SE0 (with no data activity) and will not be able to distinguish this condition from a Reset.

After, at the latest, 3.125 ms of this condition the device must revert to Full Speed termination itself, and then test whether it sees SE0 or Idle. If it detects idle it must assume that it it being suspended, and must go to its lower power suspended mode.

Note that both ends of the link must remember that they were in high speed mode, so that when Resume takes place, no high speed handshake is required.

Resume

As for full / low speed, the Resume is signalled by a K state for 20 ms. When the link was previously in high speed mode, the resume is completed by a transition back to SE0 at the end of the resume, and both host and device must be in high speed terminated mode within 2 low speed bit times.

Detecting a Device Unplug

See the description of the EOP signal above.

Summary

We have looked at the nuts and bolts of High Speed USB to see how a 480 Mb/s bit rate can be achieved in the same system designed originally to support Full Speed and Low Speed USB.

Coming up...

Next we will examine the contents of the new packets and how they are used in the various data transfer types. We will also see how the bus can support three such different data rates without compromising bus performance at High Speed.

Copyright © 2006-2008 MQP Electronics Ltd