net2890 ETC-unknow, net2890 Datasheet - Page 30

• Manufacturer Part Number: net2890
• Description: Interface Controller
• Manufacturer: ETC-unknow

Specification
4.8 Local Bus
Data passes between the local bus and the USB through the appropriate endpoint’s FIFO. The local bus is
an 8-bit non-multiplexed bus, and is controlled with a simple set of interface signals. The NET2890 acts
only as a slave on the local bus. When the NET2890 is acting as a local bus slave, a CPU on the local bus
can access all of the internal configuration registers, as well as the FIFOs.
4.8.1 Maximum Throughput
Note that maximum USB throughput is 1.5 Mbytes/sec, so the local bus utilization for accessing the
NET2890 is very low. This allows a fast local CPU to perform many other operations during USB data
transfers.
4.8.1.1 CPU writes to FIFO
4.8.1.2 CPU reads from FIFO
4.8.1.3 DMA writes to FIFO
4.8.1.4 DMA reads from FIFO
____________________________________________________________________________________
T1 (address setup) : 5
T3 (write strobe width) : 5
T7 (I/O recovery) : 64
Total = 74 ns
If the address setup can be overlapped with the I/O recovery, then
Total = 69 ns
T1 (address setup) : 5
T4 (read access time) : 28
Target setup time: (assume 5 ns)
T7 (I/O recovery) : 42
Total = 80 ns
If the address setup can be overlapped with the I/O recovery, then
Total = 75 ns
The maximum rate of DMA write transfers is determined by how quickly DACK# and IOW# are
asserted after DRQ. An internal state machine asserts DRQ on the rising edge of a 48MHz clock.
With a clock period of 20.8ns, a clock to output delay of 5ns on DRQ, and a 5ns setup time on
DACK# and IOW#, both DACK# and IOW# would have to be asserted within 10.8ns of DRQ in
order to be sampled on the next rising edge of the clock. Then DACK# and IOW# would both
need to be negated 25 nsec later to minimize the length of the cycle. If this could be accomplished,
the minimum cycle time would be 125ns yielding a burst rate of 8 Mbytes/sec.
The maximum rate of DMA read transfers is the same as the maximum rate of DMA write
transfers.
13.5 Mbytes/sec
14.5 Mbytes/sec
12.5 Mbytes/sec
13.3 Mbytes/sec
335 Pioneer Way, Mountain View, California 94041
TEL [phone] FAX [phone]
NetChip Technology, Inc., 1999
Rev 2.0, Draft 9, July 16, 1999
http://www.netchip.com
NET2890 USB Interface Controller
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