Cp3bt26 – National CP3BT26 User Manual

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CP3BT26

20.7.8

Audio Transmit Status and Control Register
(ATSCR)

The ASCR register controls the basic operation of the inter-
face. It also holds bits which report the current status of the
audio communication. The CPU bus master has read/write
access to the ASCR register. At reset, this register is loaded
with F003h.

TXAE

The Transmit FIFO Almost Empty bit is set
when the number of data bytes/words in
transmit buffer is equal to the specified warn-
ing limit.
0 – Transmit FIFO above warning limit.
1 – Transmit FIFO at or below warning limit.

TXE

The Transmit FIFO Empty bit is set when the
transmit buffer is empty. The TXE bit is set to
one every time the TRP is equal to the TWP
and the last access to the FIFO was read op-
eration (into ATSR).
0 – Transmit FIFO not empty.
1 – Transmit FIFO empty.

TXF

The Transmit FIFO Full bit is set when the
TWP is equal to the TRP and the last access
to the FIFO was write operation (write to AT-
DR).
0 – Transmit FIFO not full.
1 – Transmit FIFO full.

TXU

The Transmit Underflow bit indicates that the
transmit shift register (ATSR) has underrun.
This occurs when the transmit FIFO was al-
ready empty and a complete data word has
been transferred. In this case, the TRP will be
decremented by 1 and the previous data will
be retransmitted. No transmit interrupt and no
DMA request will be generated (even if en-
abled).
0 – Transmit underrun occurred.
1 – Transmit underrun did not occur.

TXSA

The Transmit Slot Assignment field specifies
during which slots the transmitter is active and
drives data through the STD pin. The STD pin
is in high impedance state during all other
slots. If the frame consists of less than 4 slots,
the TXSA bits for unused slots are ignored.
For example, if a frame only consists of 2
slots, TXSA bits 2 and 3 are ignored. The fol-
lowing table shows the slot assignment
scheme.

After reset, the TXSA field is clear, so soft-
ware must load the correct slot assignment.

TXDSA

The Transmit DMA Slot Assignment field
specifies which slots (audio channels) are
supported by DMA. If the TXDSA bit is set for
an assigned slot n (TXSAn = 1), the data to be
transmitted within this slot will not be read
from the transmit FIFO, but will instead be
read from the corresponding Transmit DMA
data register (ATDRn). A DMA request n is as-
serted when the ATDRn is empty. If the TSA
bit for a slot is clear, the TXDSA bit is ignored.
The following table shows the DMA slot as-
signment scheme.

TFWL

The Transmit FIFO Warning Level field speci-
fies when a transmit interrupt is asserted. A
transmit interrupt is asserted when the num-
ber of bytes or words in the transmit FIFO is
equal or less than the warning level value. A
TXFWL value of Fh means that a transmit in-
terrupt is asserted if one or more bytes or
words are available in the transmit FIFO. At
reset, the TXFWL field is loaded with Fh.

TXSA

TXU

TXF

TXE

TXAE

TXFWL

TXDSA

TXSA Bit

Slots Enabled

TXSA0

TXSA1

TXSA2

TXSA3

TXDSA Bit

Slots Enabled

for DMA

TXDSA0

TXDSA1

TXDSA2

TXDSA3