
Programmable Four-Channel Communications Controller
—
CD2481
Datasheet
183
global register used by all channels. It is not automatically reloaded by the CD2481. If the host
CPU wishes to avoid bus error interrupts, it should periodically reload BERCNT to prevent it from
reaching a zero value.
9.6.3
DMA Buffer Status (DMABSTS)
When CD2481 requires an external buffer for DMA transfer, it checks Ntbuf/Nrbuf bits to decide
which buffer to use. Note that it only checks the status of the buffer indicated by Ntbuf and Nrbuf;
if Ntbuf/Nrbuf point to buffer A, but buffer A is not available (as indicated by 2481Own), it will
not check to see if buffer B is available, even if 2481Own is set for buffer B. Once CD2481 starts
using the buffer, it toggles Ntbuf/Nrbuf bits, and sets Tbusy/Rbusy bits. Ntbuf and Nrbuf bits are
set to Buffer A at system initialization.
Bit 7
This status bit is used internally to manage data alignment in the transmit FIFO.
Bit 6
Reset Append mode is set after the terminate append buffer command in STCR has
been recognized, and is cleared after the remaining data has been flushed from the
buffer.
Bit 5
Current transmit buffer is used internally to mark the actual buffer in use.
Bit 4
Append (only Buffer A can be used as an append buffer)
Transmit append buffer usage indicator
0 = Append buffer is not in use.
1 = Append buffer is in use.
Bit 3
Ntbuf
Next transmit buffer
0 = Buffer A is the next transmit buffer.
1 = Buffer B is the next transmit buffer.
This bit is toggled when transmission starts from a buffer, that is, when data is first
read from Buffer A, the bit is set to indicate that Buffer B is next.
Bit 2
Tbusy
Current transmit buffer is in use
0 = No buffer is in use.
1 = Current transmit buffer is in use.
Bit 1
Nrbuf
Next receive buffer
0 = Buffer A is the next receive buffer.
1 = Buffer B is the next receive buffer.
Register Name: DMABSTS
Register Description: DMA Buffer Status Register
Default Value: x
’
00
Access: Byte Read Only
Intel Hex Address: x
’
1A
Motorola Hex Address: x
’
19
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
TDAlign
RstApd
CrtBuf
Append
Ntbuf
Tbusy
Nrbuf
Rbusy