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
45
5.7.4 Deadlock Considerations:
Usually, the USB host-controller retries NAKed BULK transactions in a "round-robin" priority, so deadlocks will
not normally occur. However, it may be possible that some host drivers may be susceptible to deadlocks. Specific
device and host driver implementations should be evaluated specifically for deadlock exposure.
For example, step 2a of re-assigning an IN endpoint (above) requires firmware to wait until the physical endpoint is
empty. If the host is not requesting data on the (old) logical endpoint, but is instead waiting for a transaction on the
new logical endpoint (the logical endpoint firmware is trying to switch to, but is prevented because the physical
endpoint is not empty), deadlock occurs.
This particular deadlock can be broken by flushing the IN endpoint if the device is prepared to reload the packet(s),
or if the data can be discarded. Alternatively, the deadlock can be avoided entirely by not loading any data into the
IN logical endpoint until the host has sent an IN token request, or by using method 2b-4b instead.
Another potential source of deadlock is the USB-inherent problem discussed in section 8.5.3.3 of the USB 2.0
Specification. In this situation, the endpoint is not able to flush the final packet of a transfer because it does not
know that the host has received it correctly, and the host may not send another IN on the same endpoint for an
indeterminate time.
5.7.5 Buffer Control
The Net2272 buffers can be read and written from the local bus. This feature can be used for both chip diagnostics
(power-on tests) and during Virtual Endpoint context switching.
When the Endpoint Direction bit of an endpoint (bit 4 of EP_CFG) is set, the endpoint in an IN endpoint and
EP_DATA is a write-only FIFO-style register. After data is loaded into the endpoint, the packet can be validated by
writing 0 to EP_TRANSFER (as a convenience, if the upper two bytes of EP_TRANSFER are already 0, writing 0
to EP_TRANSFER0 validates the packet with a single register write).
After a packet has been validated (or both if the endpoint is configured to be double-buffered), the Endpoint
Direction bit can be switched to OUT (by clearing bit 4 of EP_CFG) and the buffer data can be read out.
EP_AVAIL indicates the number of bytes available in the buffer.
Zero-length packets behave the same as OUT zero-length packets sent by the host, and they are similarly flushed by
a dummy read from EP_DATA. The presence of a zero-length packet is indicated by Local OUT ZLP (bit 6 of
EP_STAT1) or by the buffer states in EP_BUFF_STATES.
Note that data is only available for reading on the OUT EP_DATA register after the packet has been validated while
the endpoint is configured for IN. Data written but not validated is lost when the direction is switched to OUT.
Note also that the endpoint should be disabled before switching directions, and that the disable operation should be
checked (by reading back bit 7 of EP_CFG: Endpoint Enable). This ensures that the USB host does not read the IN
packet before local firmware reads it out.
EP_DATA buffers can be read or written while the endpoint is disabled.
5.7.6 Summary
NetChip’s Virtual Endpoint hardware support allows a USB device to utilize the full potential of USB endpoints by
providing the capability to expose any number of endpoints to the USB host. Firmware can track and assign the
available physical endpoints to the dynamically required logical endpoints with full flexibility. Any number of
endpoints required by any host driver (including USB Class Drivers) can be supported.
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