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Appendix D Register Map and Descriptions
© National Instruments Corporation D-23 Lab-PC+ User Manual
8253 Counter/Timer Register Groups A and B
The nine registers making up the two Counter/Timer Register Groups access the two onboard
8253 Counter/Timers. Each 8253 has three counters. For convenience, the two Counter/Timer
Groups and their respective 8253 integrated circuits have been designated A and B. The three
counters of Group A control onboard data acquisition timing and waveform generation. The
three counters of Group B are available for general-purpose timing functions.
Each 8253 has three independent 16-bit counters and one 8-bit Mode Register. The Mode
Register is used to set the mode of operation for each of the three counters. Writing to the Timer
Interrupt Clear Register clears the interrupt request asserted when a low pulse is detected on the
output of Counter A2 or on the EXTUPDATE* line.
Bit descriptions for the registers in the Counter/Timer Register Groups are given in the following
pages.