53
Chapter 7
7.2 Counter Read/write and Control Register
The 8254 programmable interval timer uses four registers at addresses
BASE+12, BASE+13, BASE+14 and BASE+15. Register functions are
listed below:
Since the 8254 counter uses a 16-bit structure, each section of read/write
data is split into a least significant byte (LSB) and most significant byte
(MSB). To avoid errors it is important that you make read/write opera-
tions in pairs and keep track of the byte order.
The data format for the control register appears below:
SC1 & SC0
Select counter
Register
Function
BASE+12
Counter 0 read/write
BASE+13
Counter 1 read/write
BASE+14
Counter 2 read/write
BASE+15
Counter control word
BASE+15 - 8254 control, standard mode
Bit
D7
D6
D5
D4
D3
D2
D1
D0
Value
SC1
SC0
RW1
RW0
M2
M1
M0
BCD
Counter
SC1
SC0
0
0
0
1
0
1
2
1
0
Read-back command
1
1
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