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AR# 37917

LogiCORE SPI-4.2 - Input Clocking requirement for source reference clock (SysClk)


The duty cycle distortion and jitter on the SPI-4.2 source core reference clock input (SysClk) appears on TDClk. Hence, there areseveral requirements and recommendations customers need to follow to ensure the quality of the TDClk.


Jitter requirements
It is required that the SysClk has jitter less than 50ps, since any jitter present on the SysClk appears on the TDClk.
Duty Cycle Distortion requirements
The duty cycle distortion on SysClk also appears on the TDClk clock.There are different duty cycle distortion requirements on SysClk depending on the clocking scheme used to generate the TDClk.
MMCM or DCM clocking scheme
For MMCM or DCM based designs, the input reference clock (SysClk) duty cycle requirement is 45/55 or tighter. The MMCM or DCM is capable of correcting the duty cycle of the input clock.
Regional clocking or PMCD scheme
For Regional Clock or PMCD based designs, the input reference clock (SysClk) duty cycle requirement is 48/52 or tighter. The duty cycle requirement for the input reference clock (SysClk) is tighter in a Regional Clock or PMCD based design because the duty cycle distortion for SysClk passes through the BUFIO or PMCD.
Placement of Clocking and Output I/O components
To further reduce the duty cycle distortion of TDClk, it is strongly recommended that customers place the clocking and I/O output components as close as possible to each other. This placement also reduces clock skew on TDClk which improves the available PCB and system noise budget. For more information, see the SPI-4.2 User Guide available in ISE Design Suite 12.4 and later.
Instantiating MMCM using the LogiCore Clocking Wizard (Virtex-6 FPGA only)
It is recommended that users utilize the LogiCORE Clocking Wizard IP to generate the optimum MMCM instantiation for their specific data rate; see (Xilinx Answer 39432) for step-by-step guidance on how to generate a MMCM instantiation for the SPI-4.2 core using the Clocking Wizard IP.The Clocking Wizard IP also provides jitter estimates in its summary page and this data can be used to calculate the timing budget of the SPI-4.2 source interface. For more information on calculating the timing budget for the SPI-4.2 source interface, see the SPI-4.2 User Guide available in ISE Design Suite 12.4 and later.

Linked Answer Records

Master Answer Records

Answer Number Answer Title Version Found Version Resolved
39376 SPI-4.2 v10.3 - Release Notes and Known Issues for ISE Design Suite 12.4 N/A N/A
AR# 37917
Date 12/15/2012
Status Active
Type General Article
  • SPI-4 Phase 2 Interface Solutions
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