Real Time Devices AD1200 User Manual page 94

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82C54
OPERATIONAL DESCRIPTION
General
Atter power-up, the state of the 82C54 is undefined.
The Mode, count value, and output of all Counters
are undefined.
How each Counter operates is determined when it is
programmed. Each Counter must be programmed
before it can be used. Unused counters need not be
programmed.
Programming the 82C54
Counters are programmed by writing a ControlWord
and then an initial count. The control word format is
shown in Figure 7.
All Control Words are written into the Oontrol Word
Register, which is selected when 41, Ao : 11. The
ControlWord itself specifies which Counter is being
programmed.
By contrast, initial counts are written into the Coun-
ters, not the Control Word Register. The 41, A9 in-
puts are used to select the Counter to be written
into. The format of the initial count is determined by
the ControlWord used.
ControlWord Format
A 1 , A 9 : 1 1 G : O f f i : 1
W H : O
D7 D5 D5 Da
D3 D2 D1 Ds
SC - Select Counter:
scl
sco
M - MODE:
n2 Ml MO
RW - Read/Wrlte:
RWl RWo
NOTE: Don't care bits (X) should be 0 to insure
compatibility with future Intel products.
scl sc0
RW1 RWO M2 M 1 MO
BCD
0 0 Select Counter 0
0 1 Select Counter 1
1
0
Select Counter 2
1
1
Read-Back Command
(See Read Operations)
0 0 0
Mode 0
0 0
1
Mode 1
X
1
0
Mode 2
x
1
1
Mode 3
1
0
0
Mode 4
1 0 1
Mode 5
0
0 Counter Latch Command (see Read
Operations)
0
1 Read/Write least significant byte only.
1 0 Read/Write most signilicant byte only.
1 1
Read/Write least significant byte first,
then most significant byte.
BCD:
0 Binary Counter 16-bits
1
Binary Coded Decimal (BCD) Counter
(4 Decades)
Figure 7. GontrolWord Format
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