107 www.national.com
CP3BT26
TFWL The Transmit FIFO Warning Limit bits specify
how many more bytes can be transmitted from
the respective FIFO before an underrun con-
dition occurs. If the number of bytes remaining
in the FIFO is equal to or less than the select-
ed warning limit, the TXWARN bit in the
FWEV register is set. To avoid interrupts
caused by setting this bit while the FIFO is be-
ing filled before a transmission begins, TX-
WARN is only set when transmission from the
endpoint is enabled (TX_ENn in the TXCn
register is set). See Table43.
IGN_ISOMSK
The Ignore ISO Mask bit has an effect only if
the endpoint is set to be isochronous. If set,
this bit disables locking of specific frame num-
bers with the alternate function of the TOG-
GLE bit. Therefore, data is transmitted upon
reception of the next IN token. If clear, data is
only transmitted when FNL0 matches TOG-
GLE. This bit is cleared after reset.
0 – Data transmitted only when FNL0 match-
es TOGGLE.
1 – Locking of frame numbers disabled.
18.3.34 Transmit Data Register n (TXDn)
Each transmit FIFO has one TXDn register. Data written to
the TXDn register is loaded into the transmit FIFO n at the
current location of the transmit write pointer. The TXDn reg-
isters provide write-only access from the CPU bus.
TXFD The Transmit FIFO Data Byte is used to load
the transmit FIFO. Software is expected to
write only the packet payload data. The PID
and CRC16 are inserted automatically in the
transmit data stream.
18.3.35 Receive Status Register n (RXSn)
Each receive endpoint pipe (2, 4, and 6) has one RXSn reg-
ister with the bits defined below. To allow a SETUP packet
to be received after a zero length OUT packet is received,
hardware contains two copies of this register. One holds the
receive status of a zero length packet, and another holds the
status of the next SETUP packet with data. If a zero length
packet is followed by a SETUP packet, the first read of this
register indicates the zero-length packet status, and the
second read, the SETUP packet status. This register pro-
vides read-only access from the CPU bus. After reset it is
clear.
RCOUNT The Receive Counter holds the number of
bytes presently in the endpoint receive FIFO.
If this number is greater than 15, a value of 15
is actually reported.
RX_LAST The Receive Last Bytes bit indicates that an
ACK was sent on completion of a successful
receive operation. This bit is cleared when this
register is read.
0 – No ACK was sent.
1 – An ACK was sent.
TOGGLE The function of the Toggle bit differs depend-
ing on whether ISO or non-ISO operation is
used (as controlled by the ISO bit in the EPCn
register).
Non-Isochronous mode—A value of 0 in-
dicates that the last successfully received
packet had a DATA0 PID, while a value of
1 indicates that this packet had a DATA1
PID.
Non-Isochronous mode—This bit reflects
the LSB of the frame number (FNL0) after
a packet was successfully received for this
endpoint.
This bit is cleared by reading the RXSn regis-
ter.
SETUP The Setup bit indicates that the setup packet
has been received. This bit is cleared when
this register is read.
0 – Setup packet has not been received.
1 – Setup packet has been received.
RX_ERR The Receive Error indicates a media error,
such as bit-stuffing or CRC. If this bit is set,
software must flush the respective FIFO.
0 – No receive error occurred.
1 – Receive error occurred.
Table 43 Transmit FIFO Warning Limit
TFWL Bytes Remaining in FIFO
00 TFWL disabled
01 ≤ 4
10 ≤ 8
11 ≤ 16
7 0
TXFD
7 6 5 4 3 0
RX_ERR SETUP TOGGLE RX_LAST RCOUNT