CP3UB26 datasheet(98/268 Pages) NSC

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ACK_STAT

The Acknowledge Status bit is valid when the

TX_DONE bit is set. The meaning of the

ACK_STAT bit differs depending on whether

ISO or non-ISO operation is used (as selected

by the ISO bit in the EPCn register).

Non-Isochronous mode

—This bit indi-

cates the acknowledge status (from the

host) about the ACK for the previously

sent packet. This bit itself is set when an

ACK is received; otherwise, it is clear.

Isochronous mode

—This bit is set if a

frame number LSB match occurs (see

Section 17.3.33), and data was sent in re-

sponse to an IN token. Otherwise, this bit

is cleared, the FIFO is flushed, and

TX_DONE is set.

The ACK_STAT bit is cleared when this regis-

ter is read.

TX_URUN

The Transmit FIFO Underrun indicates wheth-

er the transmit FIFO became empty during a

transmission, and no new data was written to

the FIFO. If so, the Media Access Controller

(MAC) forces a bit stuff error followed by an

EOP. This bit is cleared when this register is

read.

0 – No transmit FIFO underrun event oc-

curred.

1 – Transmit FIFO underrun event occurred.

17.3.33 Transmit Command Register n (TXCn)

Each of the transmit endpoints (1, 3, and 5) has a Transmit

Command Register, TXCn. These registers provide read/

write access from the CPU bus. After reset the registers are

clear.

TX_EN

The Transmission Enable bit enables data

transmission from the FIFO. It is cleared by

hardware after transmitting a single packet or

after a STALL handshake in response to an IN

token. It must be set by software to start pack-

et transmission.

0 – Transmission disabled.

1 – Transmission enabled.

LAST

The Last Byte bit indicates whether the entire

packet has been written into the FIFO. This is

used especially for streaming data to the FIFO

while the actual transmission occurs. If the

LAST bit is not set and the transmit FIFO be-

comes empty during a transmission, a stuff er-

ror followed by an EOP is forced on the bus.

Zero length packets are indicated by setting

this bit without writing any data to the FIFO.

The transmit state machine transmits the pay-

load data, CRC16, and the EOP signal before

clearing this bit.

0 – Last byte of the packet has not been writ-

ten to the FIFO.

1 – Last byte of the packet has been written to

the FIFO.

TOGGLE

The function of the Toggle bit differs depend-

ing on whether ISO or non-ISO operation is

used (as selected by the ISO bit in the EPCn

Non-Isochronous mode

—The TOGGLE

bit specifies the PID used when transmit-

ting the packet. A value of 0 causes a

DATA0 PID to be generated, while a value

of 1 causes a DATA1 PID to be generated.

Isochronous mode

—The TOGGLE bit

and the LSB of the frame counter (FNL0)

act as a mask for the TX_EN bit to allow

pre-queueing of packets to specific frame

numbers. (I.e. transmission is enabled

only if bit 0 in the FNL register is set to

TOGGLE.) If an IN token is not received

while this condition is true, the contents of

the FIFO are flushed with the next SOF. If

the endpoint is set to ISO, data is always

transferred with a DATA0 PID.

This bit is not altered by hardware.

FLUSH

Writing 1 to the Flush bit flushes all data from

the corresponding transmit FIFO, resets the

endpoint to Idle state, and clears both the

FIFO read and write pointers. If the MAC is

currently using the FIFO to transmit, data is

flushed after the transmission is complete. Af-

ter data flushing, this bit is cleared by hard-

ware.

0 – Writing 0 has no effect.

1 – Writing 1 flushes the FIFO.

RFF

The Refill FIFO bit is used to repeat a trans-

mission for which no ACK was received. Set-

ting the LAST bit to 1 automatically saves the

Transmit Read Pointer (TXRP) to a buffer.

When the RFF bit is set, the buffered TXRP is

reloaded into the TXRP. This allows software

to repeat the last transaction if no ACK was re-

ceived from the host. If the MAC is currently

using the FIFO to transmit, TXRP is reloaded

only after the transmission is complete. After

reload, this bit is cleared by hardware.

0 – No action.

1 – Reload the saved TXRP.

IGN_ISOMSK TFWL RFF FLUSH TOGGLE LAST TX_EN

CP3UB26 Datasheet(PDF) 98 Page - National Semiconductor (TI)

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