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Inside USB

Halt feature is set. High-speed bulk OUT endpoints can also return a NYET

handshake code, which means that the endpoint accepted the data in the cur-

rent transaction but isn’t yet ready for more data.

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The data toggle is a data-sequencing value that guards against lost or duplicated

data. If you’re debugging a device where it appears that the proper data is trans-

mitting on the bus but the receiver is discarding the data, chances are good that

the device isn’t sending or expecting the correct data toggle.

Each endpoint address maintains its own data-toggle value, which alternates

between DATA0 and DATA1. Devices typically store the value in a register bit.

When the host configures a device on power up or attachment, the host and

device each set their data toggles to DATA0. On detecting an incoming data

packet, the host or device compares the state of its data toggle with the data tog-

gle in the received data packet. If the values match, the data packet’s receiver

toggles its value for the next transaction and returns an ACK. On receiving the

ACK, the data packet’s sender toggles its value for the next transaction.

The next received packet should contain a data toggle of DATA1, and again the

receiver toggles its bit and returns an ACK. In additional transactions, the data

toggle continues to alternate between DATA0 and DATA1. An exception is

control transfers, where the Status stage always uses DATA1.

If the receiver is busy and returns a NAK, or if the receiver detects corrupted

data and returns no response, the sender doesn’t toggle its bit and tries again

with the same data and data toggle.

Table 14-2: Transactions that carry control-transfer requests and other data contain
token, data, and handshake packets.

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Token Device address, endpoint

number and direction,

transaction type

Host

Data Transaction-specific data.

Not used in some

transactions.

Host or device

Handshake Status code In transactions with a data packet, the receiver of

the data packet.

In transactions with no data packet, the device.

Not used in isochronous transactions.