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Bidirectional Control Endpoint FIFO0 Operation
FIFO0 should be used for the bidirectional control endpoint
0. It can be configured to receive data sent to the default ad-
dress with the DEF bit in the EPC0 register. Isochronous
transfers are not supported for the control endpoint.
The Endpoint 0 FIFO can hold a single receive or transmit
packet with up to 8 bytes of data. Figure 30 shows the basic
operation in both receive and transmit direction.
Note: The actual current operating state is not directly vis-
ible to software.
FLUSH Bit, TXC0 Register
Write to TXD0
TXFILL
TX_EN Bit,
TXC0
Register
TX_EN Bit,
TXC0 Register
(Zero-Length
Packet)
TXWAIT
IN Token
TX
Figure 30. Endpoint 0 Operation
A packet written to the FIFO is transmitted if an IN token for
the respective endpoint is received. If an error condition is
detected, the packet data remains in the FIFO and transmis-
sion is retried with the next IN token.
The FIFO contents can be flushed to allow response to an
OUT token or to write new data into the FIFO for the next IN
token.
If an OUT token is received for the FIFO, software is in-
formed that the FIFO has received data only if there was no
error condition (CRC or STUFF error). Erroneous recep-
tions are automatically discarded.
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FLUSH Bit, RXC0 Register
IDLE
RX_EN Bit,
RXC0 Register
RXWAIT
SETUP
Token
OUT or
SETUP
Token
Transmission
Done
FIFO0 Empty
(All Data Read)
RX
DS050
Transmit Endpoint FIFO Operation (TXFIFO1, TXFIFO2,
TXFIFO3)
The Transmit FIFOs for endpoints 1, 3, and 5 support bulk,
interrupt, and isochronous USB packet transfers larger than
the actual FIFO size. Therefore, software must update the
FIFO contents while the USB packet is transmitted on the
bus. Figure 31 illustrates the operation of the transmit
FIFOs.
FLUSH (Resets TXRP and TXWP)
-
TFnS
1
TX FIFO n
-
TCOUNT = TXRP
TXWP (= TFnS
Figure 31. Transmit FIFO Operation
TFnS
The Transmit FIFO n Size is the total number
of bytes available within the FIFO.
TXRP
The Transmit Read Pointer is incremented ev-
ery time the Endpoint Controller reads from
the transmit FIFO. This pointer wraps around
to zero if TFnS is reached. TXRP is never in-
cremented beyond the value of the write
pointer TXWP. An underrun condition occurs if
TXRP equals TXWP and an attempt is made
to transmit more bytes when the LAST bit in
the TXCMDx register is not set.
TXWP
The Transmit Write Pointer is incremented ev-
ery time software writes to the transmit FIFO.
This pointer wraps around to zero if TFnS is
reached. If an attempt is made to write more
bytes to the FIFO than actual space available
(FIFO overrun), the write to the FIFO is ig-
nored. If so, TCOUNT is checked for an indi-
cation of the number of empty bytes
remaining.
TXFL
The Transmit FIFO Level indicates how many
bytes are currently in the FIFO. A FIFO warn-
ing is issued if TXFL decreases to a specific
value. The respective WARNn bit in the FWR
register is set if TXFL is equal to or less than
the number specified by the TFWL bit in the
TXCn register.
TCOUNT
The Transmit FIFO Count indicates how many
empty bytes can be filled within the transmit
FIFO. This value is accessible by software in
the TXSn register.
92
TXRP
0X0
+
TXFL = TXWP
+
TXWP
+
-
TXFL)
-
TXRP
DS051
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