National CP3BT26 manual Bidirectional Control Endpoint FIFO0 Operation

CP3BT26

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 FLUSH Bit, RXC0 Register

	IDLE
Write to TXD0	RX_EN Bit,
	RXC0 Register
TXFILL	RXWAIT
TX_EN Bit,	SETUP
	Token
TXC0
Register	OUT or
TX_EN Bit,	SETUP
TXC0 Register	Token
(Zero-Length
Packet)	Transmission
TXWAIT
	Done
	FIFO0 Empty
IN Token	(All Data Read)
TX	RX
	DS050

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.

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)

		TXRP
TFnS - 1	0X0
		+	TXFL = TXWP - TXRP
		+
TX FIFO n			TXWP

+

TCOUNT = TXRP - TXWP (= TFnS - TXFL)		DS051
	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.

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