ADLINK Technology NuDAQ ACL-8454, Руководство пользователя

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8254 Timer Chip Operation
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53
Control Byte
Before loading or reading any of these individual counters, the
control byte (BASE+3) must be loaded first. The format of the control
byte is :
Bit 7 6 5 4 3 2 1 0
SC1 SC0 RL1 RL0 M2 M1 M0 BCD
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SC1 & SC0 - Select Counter ( Bit 7 & Bit 6)
SC1 SC0 COUNTER
0 0 Select Counter 0
0 1 Select Counter 1
1 0 Select Counter 2
1 1 ILLEGAL
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RL1 & RL0 - Select Read/Load operation ( Bit 5 & Bit 4)
RL1 RL0 OPERATION
0 0 COUNTER LATCH FOR STABLE READ
0 1 READ/LOAD LSB ONLY
1 0 READ/LOAD MSB ONLY
1 1 READ/LOAD LSB FIRST, THEN MSB
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M2, M1 & M0 - Select Operating Mode ( Bit 3, Bit 2, & Bit 1)
M2 M1 M0 MODE
0 0 0 0
0 0 1 1
x 1 0 2
x 1 1 3
1 0 0 4
1 0 1 5
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BCD - Select Binary/BCD Counting ( Bit 0)
0 16-BITS BINARY COUNTER
1 BINARY CODED DECIMAL (BCD) COUNTER (4
DIGITAL)
Note The count of the binary counter is from 0 up to
65,535 and the count of the BCD counter is from
0 up to 9,999