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
u
sed by a task. Doing so may ca
u
se the device to go into an
u
nknown
state. Make s
u
re that all tasks
u
sing a reference clock are stopped before disconnecting it.
Enabling or disabling the PLL thro
u
gh the
u
se of a reference clock affects the clock
distrib
u
tion to all s
u
bsystems. For this reason, the PLL can only be enabled or disabled
when no other tasks are r
u
nning in any of the device s
u
bsystems.
10 MHz Reference Clock
The 10 MHz reference clock can be
u
sed to synchronize other devices to
yo
u
r X Series device. The 10 MHz reference clock can be ro
u
ted to the
RTSI <0..7> or PFI <0..15> terminals. Other devices connected to the
RTSI b
u
s can
u
se this signal as a clock inp
u
t.
The 10 MHz reference clock is generated by dividing down the onboard
oscillator.
Synchronizing Multiple Devices
Refer to the following sections for information abo
u
t synchronizing
m
u
ltiple X Series devices.
PXI Express Devices
On PXI Express systems, yo
u
can synchronize devices to PXIe_CLK100.
In this application the PXI Express chassis acts as the initiator. Each
PXI Express mod
u
le ro
u
tes PXIe_CLK100 to its external reference clock.
Another option in PXI Express systems is to
u
se PXI_STAR. The Star
Trigger controller device acts as the initiator and drives PXI_STAR with a
clock signal. Each target device ro
u
tes PXI_STAR to its external reference
clock.
PCI Express Devices
With the RTSI and PFI b
u
ses and the ro
u
ting capabilities of X Series
PCI Express devices, there are several ways to synchronize m
u
ltiple
devices depending on yo
u
r application.
To synchronize m
u
ltiple devices to a common timebase, choose
one device—the initiator—to generate the timebase. The initiator device
ro
u
tes its 10 MHz reference clock to one of the RTSI <0..7> or PFI <0..15>
signals.
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