User manual

PCI-DA12-8/16 Manual
Page 6-2
Manual MPCI-DA12-16.Dh
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
On these cards, the 8254 counters occupy the following addresses (hex):
Base Address + 24: Read/Write Counter #0
Base Address + 25: Read/Write Counter #1
Base Address + 26: Read/Write Counter #2
Base Address + 27: Write to Counter Control register
The counters are programmed by writing a control byte into a counter control register at Base Address
+ 27. 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.