
Programmable Four-Channel Communications Controller
—
CD2481
Datasheet
67
5.6
Hardware Configurations
To demultiplex the A/D[0
–
15] bus into separate address and data buses, external buffers and
latches are required. To reduce external circuitry, these external devices can be shared in multi
–
CD2481 applications. The common control lines (ADLD*, AEN*, DATDIR*, DATEN*) to the
external devices are wire-OR
’
ed together. These pins are tristate, not open collector, but an external
pull-up resistor (2.2K
–
5.0K) must be connected to each line to ensure a logic
‘
1
’
when no CD2481
is a bus master.
When no higher-priority alternate bus masters are present, a daisy-chain priority scheme can be
implemented by wire OR
’
ing the BR* and BGACK* and connecting directly to the 680X0. The
680X0 BG* signal is then connected to the first device in the chain and daisy-chained to the
remaining devices. A lower priority bus master can then be connected at the end of the chain.
If a higher priority bus master is present, the BG* signal must be qualified before being passed into
the highest priority CD2481. If a priority encoded scheme is required, the BR* signals must be
prioritized externally and BG* signals routed to individual devices.
Table 9. Data Clock Selection Using External Clock
Bit Rate
External Clock Frequency
Divisor (hex)
Clock = 35 MHz
50
9.765 kHz
C2
110
9.765 kHz
57
150
9.765 kHz
40
300
39.062 kHz
81
600
39.062 kHz
40
1200
156.250 kHz
81
2481
156.250 kHz
40
3600
625.00 kHz
EF
4800
625.00 kHz
81
7200
1.250 MHz
AC
9600
1.250 MHz
81
19200
1.250 MHz
40
38400
1.250 MHz
1F
56000
1.250 MHz
15
64000
1.250 MHz
12
76800
1.250 MHz
0F
115200
2.00 MHz
10
128000
2.00 MHz
0F