Freescale Semiconductor PowerQUICC MPC885 Reference Manual page 858

Universal Serial Bus (USB)
Token
OUT Transmission begins when the USB host controller fetches a TxBD containing OUT token and a data
TxBD and loads them to the host FIFO. The token and data are transmitted and a handshake is expected.
If a handshake is not received within the expected time interval, the USB controller clears TxBD[E] of data
BD, sets the TxBD[TO] indication and generates an interrupt if TxBD[I] = 1.
When stall or nak is received within the expected time interval, the USB controller clears TxBD[E] of data
BD, sets the TxBD[stall] or TXBD[nak] indication and generates an interrupt if TxBD[I] = 1.When ack is
received within the expected time interval, the USB controller clears TxBD[E] of data BD, generates an
interrupt if TxBD[I] = 1. No indication is set.
The token TxBD[R] is cleared right after the OUT token transmission.
IN Transmission begins when the USB host controller fetches a TxBD containing an IN token and loads the
token to FIFO. After the IN token is transmitted the USB host controller waits for reception of data within
expected time interval. On reception of a correct DATA PID an RxBD is fetched. The received data and
DATA PID are stored in the receive FIFO. If RxBD[E] is set PID, data will be moved to the buffer. While
receiving the data, the USB host controller calculates CRC16, performs bit un-stuffing. At the end of
reception, the calculated CRC is compared to received and octet alignment is checked, RxBD[E] is
cleared, RxBD[PID] is set according to received DATA PID and error indications are set if required:
RxBD[CR] for failed CRC check, RxBD[NO] for non octet sized data and RxBD[AB] if bit stuffing error
occurred.
If no correct DATA PID or no data at all received during the expected time interval a TO indication in the
token TxBD is set.
SETUP The format of setup transactions is similar to OUT but uses a SETUP rather than an OUT PID. A SETUP
token is recognized only by a control endpoint and cannot be answered with nak or stall, there for host
expects either ack or no handshake at all.
31-10
Table 31-3. USB Tokens (Host Mode)
Token Data
OUT Sent by host
Data Transmitted
Token
by Function
IN Received correctly
Received
corrupted
None
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MPC885 PowerQUICC Family Reference Manual, Rev. 2
Description
USB Out transaction
Handshake Generated
Indication on TxBD
by Function
None (Data Discarded)
ACK
NAK
STALL
USB In transaction
Handshake Generated
Indication on RxBD
by Host
ACK RxBD[E] is cleared
None RxBD[CR] or RxBD[AB]
or RxBD[NO]
None TxBD[TO]
TO
None
nak
stall
Freescale Semiconductor