Embedded Solutions
Page 7
Clocking
PCIe8LSwVPX3U VPX (PCIe downstream) port supports both PCI Express
Independent Non-Spread Spectrum Clocking (NSSC) and VITA Spread Spectrum
Clocking (SSC). The upstream/host port supports a PCIe REFCLK that is either a SSC
or a NSSC, or can be configured to use an on board NSSC if desired. Figure 1 shows
the clocking architecture of the PCIe8LSwVPX3U. The clocking mode is selected using
DIP Switches described in the DIP switch settings section.
PCIe8LSwVPX3U Clocking
EIA-899 M-LVDS
25MHz /-
with Modulation
+/- 100ppm max
50% duty cycle +/- 5%
PCI Express
Host
PCI
Express
Switch
VPX Add-In Card or
Carrier such as
Dynamic Engineering’s
VPX8LXMC3U
x8 PCIe link
REFCLKp/n
VPX/PCIe
Compliant
Downstream
Ports Clock
with Spread
Upstream
Port Clock with
Spread or
No Spread
X8 PCIe link
SSC Off
25MHz
Crystal
PCIe compliant
100MHz Clock
No Spread
Straps
Clock
Generator
SSC Off
Straps
25MHz in
100MHz out
with SSC
Generator
1.8V
M-LVDS
Transmitter
25MHz
100MHz
25MHz in
25MHz SSC out
Clock Generator
M-LVDS
Receiver
Note: VPX Spec allows up to 32
circuits to be connected to the
common transmission media
Backplane differential /- lines are differentially
terminated at each end with a resistor of 61.9 Ohms +/- 1%
PCIe
Compliant
Downstream
Ports Clock
No Spread
Downstream
Port
Clock Select
PCIe compliant
100MHz Clock
with Spread
VITA
25MHz in
100MHz out
with SSC
Generator
PCIe
Compliant
100MHz
NSSC
Generator
AND
OR
FIGURE 1
PCIE8LSWVPX3U CLOCKING
