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PROGRAMMING INFORMATION
PROGRAMMING FORMAT
ALTERNATE PROGRAMMING FORMAT
Step
Step
1
Mode Control Word
Counter n
1
Mode Control Word
Counter 0
2
LSB
Count Register Byte
Counter n
2
Mode Control Word
Counter 1
3
MSB
Count Register Byte
Counter n
3
Mode Control Word
Counter 2
4
LSB
Counter Register Byte
Counter 1
5
MSB
Count Register Byte
Counter 1
6
LSB
Count Register Byte
Counter 2
7
MSB
Count Register Byte
Counter 2
8
LSB
Count Register Byte
Counter 0
9
MSB
Count Register Byte
Counter 0
Figure 3-2.
PIT Programming Sequence Examples
When a selected count register is to be loaded, it must be loaded with
the number of bytes programmed in the mode control word.
One or two
bytes can be loaded, depending on the appropriate count.
These two bytes
can be programmed at any time following the mode control word, as long as
the correct number of bytes is loaded in order.
The count mode selected in the control word controls the counter output.
As shown in Figure 3-1, the PIT chip can operate in any of six modes:
a.
Mode 0:
Interrupt on terminal count.
In this mode, the counters can
be used for auxiliary functions, such as generating real-time
interrupt intervals.
After the count value is loaded into the count
register, the count output goes low and remains low until the
terminal count is reached.
The output then goes high until the count
register is reloaded or the mode word is rewritten.
b.
Mode 1:
Programmable one-shot.
In this mode
~he
output of the
counters will go low on the count following the rising edge off the
gate input.
The output will go high on the terninal count.
If a new
count value is loaded while the output is low, it will not affect the
duration of the one-shot pulse until the succeeding trigger.
The
current count can be read at any time without affecting the one-shot
pulse.
The one-shot is retriggerable, hence the output will remain
low for the full count after any rising edge of the gate input.
3-10
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