Measurement Computing CIO-PDMAxx - Manual (Page 15)

5.1.4 8254 COUNTER LOAD & READ REGISTERS
COUNTER 0
BASE ADDRESS + 4
304h, 772
10
D0 D1 D2 D3 D4 D5 D6 D7
0 1 2 3 4 5 6 7
COUNTER 1
BASE ADDRESS + 5
305h, 773
10
D0 D1 D2 D3 D4 D5 D6 D7
0 1 2 3 4 5 6 7
COUNTER 2
BASE ADDRESS + 6
306h, 774
10
D0 D1 D2 D3 D4 D5 D6 D7
0 1 2 3 4 5 6 7
The data in the counter read register, and the action taken on the data in a counter load
register, is wholly dependent upon the control code written to the control register.
The counters have 16-bits, each with an 8-bit window, the read / load register. Data is
shifted in and out of the 16-bit counters through these 8-bit windows according to the
control byte.
You will need an 8254 data sheet if you want to program the 8254 directly in
assembly language or in BASIC or some other high level language.
5.1.5 COUNTER CONTROL REGISTER
BASE ADDRESS + 7 307h, 775
10
BCD M0 M1 M2 RL0 RL1 SC0 SC1
0 1 2 3 4 5 6 7
WRITE ONLY
SC1 to SC0 are the counter select bits. They are binary coded between 0 and 2.
SC1 SC0 Counter
0 0 0
0 1 1
1 0 2
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