Manual Number: 00650-014-4
Page 8-2
AD12-8 Manual
Mode 2: Rate Generator
This mode provides a divide-by-N capability where N is the count loaded into the counter. When
triggered, the counter output goes low for one clock period after N counts, reloads the initial count,
and the cycle starts over. This mode is periodic, the same sequence is repeated indefinitely until
the gate input is brought low. This mode is used on the AD12-8 card in counters 1 and 2 to generate
periodic A/D start commands. This mode also works well as an alternative to mode 0 for event
counting.
Mode 3: Square Wave Generator
This mode operates periodically like mode 2. The output is high for half of the count and low for
the other half. If the count is even, then the output is a symmetrical square wave. If the count is
odd, then the output is high for (N+1)/2 counts and low for (N-1)/2 counts. Periodic triggering or
frequency synthesis are two possible applications for this mode. Note that in this mode, to achieve
the square wave, the counter decrements by two for the total loaded count, then reloads and decre-
ments by two for the second part of the wave form.
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. This mode can be used to
provide a delayed software trigger for initiating A/D conversions.
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
On the AD12-8, the 8253 counters occupy the following addresses:
BASE A 4:
Read/Write Counter #0
BASE A 5:
Read/Write Counter #1
BASE A 6:
Read/Write Counter #2
BASE A 7:
Write to Counter Control register
The counters are programmed by writing a control byte into a counter control register at BASE
A 7. 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:
%
%
%
%
%
%
%
%
6&
6&
5: 5:
0
0
0
%&'
