Chapter 2
NI 5450 Overview
NI PXIe-5450 User Manual
2-40
ni.com
External Sample Clock Considerations
The NI 5450 incorporates high-speed digital clocking technology and
requires a stable, free-running Sample clock to operate properly. When the
signal generator is committed—either explicitly by calling the niFgen
Commit VI or the
niFgen_Commit
function or implicitly by writing
waveforms or sequences or initiating a generation—the external Sample
clock must be available to the device. If the external clock becomes
unstable due to glitching, changing frequency, or is removed entirely,
NI-FGEN returns a hardware clocking error.
If necessary, you can change the rate or the source of the external Sample
clock between subsequent generations by first calling the niFgen Abort
Generation VI or the
niFgen_AbortGeneration
function, changing the
rate or source, and then calling the niFgen Commit VI or the
niFgen_Commit
function. NI-FGEN reprograms the NI 5450 for the new
settings, and you can call the niFgen Initiate Generation VI or the
niFgen_InitiateGeneration
function to start the next generation.
If you must remove the external Sample clock between generations (after
calling the niFgen Abort Generation VI or
niFgen_AbortGeneration
function, but before calling the niFgen Initiate VI or the
niFgen_Init
function), but are not changing the frequency or source of the external
clock, you can choose one of the following options:
•
Call the niFgen Initiate VI or the
niFgen_Init
function, which
returns a hardware clocking error because the external Sample clock is
gone, then clear the error and call the niFgen Initiate VI or the
niFgen_Init
function again. NI-FGEN then reprograms the
hardware to use the external clock again.
•
Force the device to be recommitted by changing a property or attribute
to another value and then back to its original value. This action causes
NI-FGEN to re-commit the settings to hardware, which does not
happen otherwise because NI-FGEN does not know that the external
Sample clock is gone.
Caution
When configuring an external Sample clock, you can set the sample rate to the
exact frequency you are using to avoid data errors by calling the niFgen Set Sample Rate
VI or the
niFgen_ConfigureSampleRate
function.
