CertusPro-NX SerDes/PCS Usage Guide
Preliminary
Technical Note
108
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FPGA-TN-02245-0.81
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Port Name
Descriptions
1’b1 – DIFFIOCLK1
1’b0 – DIFFIOCLK0
CLKSEL
Dynamic clock source selection.
2’b11 – CLK
2’b10 – DIFFIOCLK0 or DIFFIOCLK1
2’b01 – PLLCLK1
2’b00 – PLLCLK0
CLKOUT
Reference clock output.
Spread Spectrum Clocking Support
Spread Spectrum Clocking (SSC) is a technique used in electronics design to intentionally modulate the ideal position of
the clock edge such that the resulting signal spectrum is spread around the ideal frequency of the clock. In timing
circuits, this technique has the advantage of reducing Electromagnetic Interference (EMI) associated with the
fundamental frequency of the signal. The amount of EMI a system is allowed to generate is set by various regulatory
bodies to ensure systems not interfacing with one another. System spectrum clocking is often used to help meet the
regulated EMI requirements. A spread spectrum signal has the disadvantage of having much higher jitter than the
un-modulated signal.
Spread spectrum clocking is commonly used for microprocessor clocks, USB and PCI Express reference clocks to reduce
EMI. CertusPro-NX SerDes/PCS supports the following optional SSC features defined by PCI Express Base Specification:
The reference clock can be modulated by +0% to -0.5% from nominal (5000 ppm), referred to as down spreading.
The modulation rate must be between 30 KHz and 33 KHz.
In PCI Express applications, the Root Complex (RC) is responsible for spreading the reference clock. The Endpoint uses
the same clock to pass back the spectrum through the Transmitter. What should be noted is that the reference clock
from RC needs to be used in PCI Express Endpoint applications based on CertusPro-NX SerDes/PCS, when the SSC
feature has been enabled. Otherwise, reference clock source from local OSC can be used for CertusPro-NX SerDes/PCS.
However, using a local OSC as reference clock is not recommended on account of the system compatibility.
The Tx PLL and CDR PLL inside PMA have no capability to generate SSC clock, but can be compatible with SSC clock.
Although CertusPro-NX SerDes allows the mixing of a PCI Express channel and other protocol channels within the same
quad, using the PCI Express reference clock with SSC can cause a violation of the Gigabit Ethernet, Serial Rapid IO
(SRIO), SGMII and QSGMII transmit jitter specifications.
Unused Quad/Channel and Power Supply
Some power supply pins can be left floating for power saving purpose, when specific quads or channels are unused.
Follow guidelines below for unused quads or channels power pins connections:
For unused quads:
Connect all VSSDQ pins to board ground.
Leave corresponding VCCAUXSDQx [x = 0, 1] pins floating (not connected).
Leave corresponding VCCSDx [x = 0 ~ 7] and VCCPLLSDx [x = 0 ~ 7] pins floating (not connected).
For unused channels:
Connect all VSSDQ pins to board ground.
Connect corresponding VCCAUXSDQx [x = 0, 1] to a clean and stable power rail.
Connect corresponding VCCDQx [x = 1, 5] and VCCPLLSDx [x = 1, 5] pins to a clean and stable power rail.
Connect used VCCSDx [x = 0 ~ 7] and VCCPLLSDx [x = 0 ~ 7] pins to a clean and stable power rail.
Leaves other unused VCCSDx [x = 0 ~ 7] and VCCPLLSDx [x = 0 ~ 7] pins floating (not connected).
Note: Channel 1 of the used quad must be powered even though channel 1 is unused in this quad.
shows the numbering for CertusPro-NX SerDes power pins.