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CP3BT26
18.2 ENDPOINT OPERATION
18.2.1 Address Detection
Packets are broadcast from the host controller to all nodes
on the USB network. Address detection is implemented in
hardware to allow selective reception of packets and to per-
mit optimal use of CPU bandwidth. One function address
with seven different endpoint combinations is decoded in
parallel. If a match is found, then that particular packet is re-
ceived into the FIFO; otherwise it is ignored.
The incoming USB Packet Address field and Endpoint field
are extracted from the incoming bit stream. Then the ad-
dress field is compared to the Function Address register
(FADR). If a match is detected, the Endpoint field is com-
pared to all of the Endpoint Control registers (EPCn) in par-
allel. A match then causes the payload data to be received
or transmitted using the respective endpoint FIFO.
Figure 29. USB Function Address/Endpoint Decoding
18.2.2 Transmit and Receive Endpoint FIFOs
The CR16 USB node uses a total of seven transmit and re-
ceive FIFOs: one bidirectional transmit and receive FIFO for
the mandatory control endpoint, three transmit FIFOs, and
three receive FIFOs. As shown in Table 39, the bidirectional
FIFO for the control endpoint is 8 bytes deep. The additional
unidirectional FIFOs are 64 bytes each for both transmit and
receive. Each FIFO can be programmed for one exclusive
USB endpoint, used together with one globally decoded
USB function address. Software must not enable both trans-
mit and receive FIFOs for endpoint zero at any given time.
Table 39 Endpoint FIFO Sizes
If two endpoints in the same direction are programmed with
the same endpoint number and both are enabled, data is re-
ceived or transmitted to/from the endpoint with the lower
number, until that endpoint is disabled for bulk or interrupt
transfers, or becomes full or empty for ISO transfers. For ex-
ample, if receive EP2 and receive EP4 both use endpoint 5
and are both isochronous, the first OUT packet is received
into EP2 and the second OUT packet into EP4, assuming
no software interaction in between. For ISO endpoints, this
allows implementing a ping-pong buffer scheme together
with the frame number match logic.
Endpoints in different directions programmed with the same
endpoint number operate independently.
Receive/
Transmit FIFO0
USB Packet
EPC0 Register
FADR Register
ADDR Field Endpoint Field
Match
Match
Transmit FIFO1
EPC1 Register
Receive FIFO1
EPC2 Register
Transmit FIFO2
EPC3 Register
Receive FIFO2
EPC4Register
Transmit FIFO3
EPC5 Register
Receive FIFO3
EPC6 Register
DS049
Endpoint
Number
TX FIFO RX FIFO
Size
(Bytes)
Name
Size
(Bytes)
Name
0 FIFO0 (bidirectional, 8 bytes)
1 64 TXFIFO1 - -
2 - - 64 RXFIFO1
3 64 TXFIFO2 - -
4 - - 64 RXFIFO2
5 64 TXFIFO3 - -
6 - - 64 RXFIFO3