Chapter 4
General-Purpose Counter/Timer
DAQ-STC Technical Reference Manual
4-62
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National Instruments Corporation
to the load register, so that on the next reload the counter will load the incremented value.
Refer to Gi_Autoincrement for more information.
Each counter also has two 24-bit save registers, designated the hardware save register
(HW_Save_Register) and the software save register (SW_Save_Register). The G_GATE
signal latches the counter contents into the HW_Save_Register. Thus, the hardware
determines the time that the counter contents are saved, hence the name HW_Save_Register.
The HW_Save_Register makes possible buffer-oriented, interrupt-driven period and
pulsewidth measurements. The DAQ-STC provides error-detection mechanisms for cases
where gap-free readings are not possible due to the interrupt latency of the system. The
software save register provides the ability to peek at the counter contents any time without
disturbing any buffer-oriented measurements. The software save registers enable the software
to monitor the counter contents for status reporting.
4.8.1 Internal Signals and Operation
Table 4-4 lists internal signals used in the GPCT hardware description and their relationship
to the external signals.
Table 4-4.
Internal Signal Description
Signal
Description
G_CONTROL
Counter Control—The hardware generates G_CONTROL by passing
G_GATE through the G_CONTROL conditioning circuit.
G_GATE
Counter Gate—G_GATE can come from two sources. If Gi_OR_Gate
is set to 0, the hardware generates G_GATE by passing the output of the
Gi_Gate selector through polarity selection. If Gi_OR_Gate is set to 1,
the hardware generates G_GATE by passing the output of the Gi_Gate
selector through polarity selection and OR-ing the result with the output
of the other general-purpose counter.
G_IN_TIMEBASE1
Internal Timebase—G_IN_TIMEBASE1 is the internal timebase for
the general-purpose counter/timer module. G_IN_TIMEBASE1 can be
the same as IN_TIMEBASE, or it can be IN_TIMEBASE divided by
two. Related bitfields: G_Source_Divide_By_2.
G_SOURCE
Counter Source—The hardware generates G_SOURCE by passing the
output of the Gi_Source selector through polarity selection. Related
bitfields: Gi_Source_Select.
G_TC
Counter TC—G_TC is the actual general-purpose counter TC signal.
G_OUT
Counter Output—G_OUT is the G_TC signal after output and polarity
selection. Related bitfields: Gi_Output_Mode, Gi_Output_Polarity.