Counter/Timers (8254)
If a counter is programmed to read or write two-byte counts, the
following precaution applies: a program must not transfer control
between writing the first and second byte to another routine that also
writes into the same counter. If this happens, the counter will be
loaded with an incorrect count.
D7
D6
D5
D4
D3
D2
D1
D0
SC1
SC0
RW1
RW0
M2
M1
M0
BCD
I/O Address = 43h
RD = 1
WR = 0
1
0
1
0
0
1
1
SC --- Select Counter:
SC1
0
0
1
1
SC0
0
1
0
1
Select Counter 0
Select Counter 1
Select Counter 2
Read-Back Command
(See Read Operations)
RW --- Read/Write:
RW1
RW0
0
Counter Latch Command (See Read )t
(Operations)
Read/Write least significant byte only
Read/Write most significant byte only
Read/Write least significant byte first,
then most significant byte
Note: Don’t care bits (X) should be 0 to ensure compatibility with future products
M --- MODE:
M2
M1
M0
0
0
0
Mode 0
0
1
0
Mode 1
X
0
1
Mode 2
X
1
1
Mode 3
1
0
0
Mode 4
1
1
0
Mode 5
BCD:
0
1
Binary Counter 16-bits
Binary Coded Decimal (BCD) Counter
(4 Decades)
Figure 11–3. Control Word Format.
11-7
