
Programmable Four-Channel Communications Controller
—
CD2481
Datasheet
39
5.1.2
Byte and Word Transfers
Data can be moved to and from the CD2481 in either byte or word transfers. Many registers are 16-
bits wide and, while these can be accessed as individual bytes, some, such as timers, should be
accessed only with word transfers. To accommodate various families of host processors, the
BYTESWAP input pin is set to indicate the system byte-ordering scheme. The size pins (SIZ[1,0])
are used to indicate whether the transfer is 1 or 2 bytes wide.
In systems where the even addresses represent the high-order byte, the BYTESWAP input pin
should be tied low, and byte transfers occur on the A/D[15:8] pins for even addresses and on the A/
D[7:0] pins for odd addresses. In systems where the high-order byte is on the odd address, the
situation is reversed, and BYTESWAP should be tied high. Byte transfers to even addresses occur
on the A/D[7:0] pins, and to odd addresses on the A/D[15:8] pins.
5.2
Interrupts
The CD2481 uses interrupt requests to alert the host that certain events requiring its attention have
occurred. Interrupt operations on the CD2481 are tightly coupled with several registers described
below. The concept of context affects the accessibility of these and other registers.
Figure 3. Host Write Cycle
CS*
DS*
R/W*
A/D[0
–
15]
A[0
–
7], SIZ[0
–
1]
DTACK*
DATEN*
DATDIR*
DIN