Specification
NET2272 USB Peripheral Controller
______________________________________________________________________________
NetChip Technology, Inc., 2003
Patent Pending
335 Pioneer Way, Mountain View, California 94041
TEL (650) 526-1490
FAX (650) 526-1494
http://www.netchip.com
Rev 1.2, October 15, 2003
39
5.6.3.3 Isochronous In Transactions
Isochronous IN endpoints are used to transfer data from the NET2272 local bus to a USB host. An isochronous IN
transaction consists of the following:
Stage
Packet Contents
Number of bytes
Source
IN Token
IN PID, address, endpoint, and CRC5
3
Host
Data
DATA0 PID, N data bytes, and CRC16
N+3
NET2272
The USB host initiates an Isochronous IN transaction by sending an IN token to an Isochronous IN endpoint. The
Data IN Token Interrupt bit is set when the IN token is recognized. If there is data in the endpoint’s buffer, it is
returned to the host. If the endpoint has no data to return, a zero length packet is returned to the host. The NET2272
responds to the IN token according to the following table.
Packet Validated
Amount of Data in Buffer
Action
0
< Max Packet Size
Zero length packet to host; USB
IN NAK Sent status bit set
X
>= Max Packet Size
Return data to host.
1
empty
Zero length packet to host
1
>0
Return data to host
After the packet has been sent to the host, the Data Packet Transmitted Interrupt bit is set. If an IN token arrives
and there is no valid packet in the endpoint buffer, the NET2272 returns a zero-length packet, and the FIFO
Underflow status bit is set. No handshake packets are returned to the host, but the USB IN ACK Sent, and Timeout
status bits are still set to indicate the status of the transaction. After every data packet is transmitted, the local CPU
should sample these status bits to determine if the packet was successfully transmitted to the host.
By definition, isochronous endpoints do not utilize handshaking with the host. Since there is no way to return a stall
handshake from an isochronous endpoint to the host, data that is requested from a stalled isochronous endpoint will
be transmitted normally.
5.6.3.4 High Bandwidth Isochronous IN Transactions
A USB device is required to send ISO PID sequences for each microframe according to the Additional Transaction
Opportunities field in the Endpoint Descriptor and the EP_n_HS_MAXPKT register as follows:
Additional
Opportunities
PID Sequence
0
DATA0 (normal ISO)
1
DATA1, DATA0 (one extra transaction)
2
DATA2, DATA1, DATA0 (two extra transactions)
When the first IN token of a microframe arrives, the NET2272 copies the Additional Opportunities field from the
EP_n_HS_MAXPKT register to determine the initial PID. On each succeeding IN token of the microframe, the
PID advances to the next token.
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