29
Appendix A
A.3 Counter Operating Modes
A.3.1 MODE 0 – Stop on Terminal Count
The output will be initially low after you set this mode of operation. After
you load the count into the selected count register, the output will remain
low and the counter will count. When the counter reaches the terminal
count, its output will go high and remain high until you reload it with the
mode or a new count value. The counter continues to decrement after it
reaches the terminal count. Rewriting a counter register during counting
has the following results:
1.
Writing to the first byte stops the current counting.
2.
Writing to the second byte starts the new count.
A.3.2 MODE 1 – Programmable One-shot
The output is initially high. The output will go low on the count following
the rising edge of the gate input. It will then go high on the terminal
count. If you load a new count value while the output is low, the new
value will not affect the duration of the one-shot pulse until the succeed-
ing trigger. You can read the current count at any time without affecting
the one-shot pulse. The one-shot is retriggerable, thus the output will
remain low for the full count after any rising edge at the gate input.
A.3.3 MODE 2 – Rate Generator
The output will be low for one period of the input clock. The period from
one output pulse to the next equals the number of input counts in the
counter register. If you reload the counter register between output pulses,
the present period will not be affected, but the subsequent period will
reflect the value.
The gate input, when low, will force the output high. When the gate input
goes high, the counter will start from the initial count. You can thus use
the gate input to synchronize the counter.
With this mode the output will remain high until you load the count regis-
ter is loaded. You can also synchronize the output by software.
Summary of Contents for PCI-1751U
Page 1: ...PCI 1751U 48 bit Digital Input Output Card with Universal PCI Bus User Manual ...
Page 5: ...2 CHAPTER 1 General Information ...
Page 9: ...2 CHAPTER 2 Installation ...
Page 15: ...11 Chapter2 2 5 PCI 1751U Block Diagram Figure 2 2 PCI 1751U Block Diagram ...
Page 18: ...PCI 1751U User Manual 14 ...
Page 19: ...2 CHAPTER 3 Operation ...
Page 28: ...PCI 1751U User Manual 24 ...
Page 29: ...2 APPENDIX A Function of 8254 Counter Chip ...