Mode 4: Software Triggered Strobe
This mode sets the output high and, when the count is loaded, the counter begins to count down. When
the counter reaches zero, the output will go low for one input period. The counter must be reloaded to
repeat the cycle. A low gate input will inhibit the counter.
Mode 5: Hardware Triggered Strobe
In this mode, the counter will start counting after the rising edge of the trigger input and will go low for
one clock period when the terminal count is reached. The counter is retriggerable. The output will not
go low until the full count after the rising edge of the trigger.
Programming the 8254
The counters are programmed by writing a control byte into the counter control register. The control
byte specifies the counter to be programmed, the counter mode, the type of read/write operation, and
the modulus. The control byte format is as follows:
B7 B6 B5 B4 B3
B2
B1
B0
SC1 SC0 RW1 RW0 M2 M1
M0
BCD
SC0-SC1: These bits select the counter that the control byte is destined for.
SC1
SC0
Function
0
0
Program Counter #0
0
1
Program Counter #1
1
0
Program Counter #2
1 1
Read/Write
Cmd.*
* See section on Reading and Loading the Counters.
RW0-RW1:
These
bits
select the read/write mode of the selected counter.
RW1
RW0
Counter Read/Write Function
0
0
Counter Latch Command
0
1
Read/Write LS Byte
1
0
Read/Write MS Byte
1
1
Read/Write LS Byte, then MS Byte
Manual 104-DIO-48E, 24E
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