Chapter 9 Digital Routing and Clock Generation
© National Instruments 9-3 X Series User Manual
Caution Do not disconnect an external reference clock once the devices have been
synchronized or are used by a task. Doing so may cause the device to go into an unknown
state. Make sure that all tasks using a reference clock are stopped before disconnecting it.
Enabling or disabling the PLL through the use of a reference clock affects the clock
distribution to all subsystems. For this reason, the PLL can only be enabled or disabled
when no other tasks are running in any of the device subsystems.
10 MHz Reference Clock
The 10 MHz reference clock can be used to synchronize other devices to
your X Series device. The 10 MHz reference clock can be routed to the
RTSI <0..7> or PFI <0..15> terminals. Other devices connected to the
RTSI bus can use this signal as a clock input.
The 10 MHz reference clock is generated by dividing down the onboard
oscillator.
Synchronizing Multiple Devices
Refer to the following sections for information about synchronizing
multiple X Series devices.
PXI Express Devices
On PXI Express systems, you can synchronize devices to PXIe_CLK100.
In this application the PXI Express chassis acts as the initiator. Each
PXI Express module routes PXIe_CLK100 to its external reference clock.
Another option in PXI Express systems is to use PXI_STAR. The Star
Trigger controller device acts as the initiator and drives PXI_STAR with a
clock signal. Each target device routes PXI_STAR to its external reference
clock.
PCI Express Devices
With the RTSI and PFI buses and the routing capabilities of X Series
PCI Express devices, there are several ways to synchronize multiple
devices depending on your application.
To synchronize multiple devices to a common timebase, choose
one device—the initiator—to generate the timebase. The initiator device
routes its 10 MHz reference clock to one of the RTSI <0..7> or PFI <0..15>
signals.
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