Additional Clocking Considerations for SSI Devices - 2020.2 English

UltraFast Design Methodology Guide for Xilinx FPGAs and SoCs

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2020.2 English

In general, all clocking considerations mentioned above also apply to SSI technology devices. However, there are additional considerations when targeting these devices due to their construction. When using a BUFMR, it cannot drive clocking resources across an SLR boundary. Accordingly, Xilinx recommends that you place the clocks driving BUFMRs into the bank or clocking region in the center clock region within an SLR. This gives access to all three clock regions on the left or right side of the SLR.

In terms of global clocking, for designs requiring sixteen or fewer global clocks (BUFGs), no additional considerations are necessary. The tools automatically assign BUFGs in a way to avoid any possible contention. When more than 16 (but fewer than 32) BUFGs are required, some consideration to pin selection and placement must be done to avoid any chance of contention of resources based on global clocking line contention and/or placement of clock loads.

As in all other Xilinx 7 series devices, Clock-Capable I/Os (CCIOs) and their associated Clock Management Tile (CMT) have restrictions on the BUFGs they can drive within the given SLR. CCIOs in the top or bottom half of the SLR can drive BUFGs only in the top or bottom half of the SLR (respectively). For this reason, pin and associated CMT selection should be done in a way in which no more that sixteen BUFGs are required in either the top or bottom half of all SLRs collectively. In doing so, the tools can automatically assign all BUFGs in a way to allow all clocks to be driven to all SLRs without contention.

For designs that require more than 32 global clocks, Xilinx recommends that you explore using BUFRs and BUFHs for smaller clock domains to reduce the number of needed global clock domains. BUFRs with the use of a BUFMR to drive resources within three clock regions that encompasses one-half of an SLR (approximately 250,000 logic cells in a Virtex-7 class SLR). Horizontally adjacent clock regions may have both left and right BUFH buffers driven in a low-skew manner enabling a clocking domain of one-third of an SLR (approximately 167,000 logic cells).

Using these resources when possible not only leads to fewer considerations for clocking resource contention, but many times improves overall placement, resulting in improved performance and power.

If more than 32 global clocks are needed that must drive more than half of an SLR or to multiple SLRs, it is possible to segment the BUFG global clocking spines. Isolation buffers exist on the vertical global clock lines at the periphery of the SLRs that allow use of two BUFGs in different SLRs that occupy the same vertical global clocking track without contention. To make use of this feature, more user control and intervention is required. In the figure below, BUFG0 through BUFG2 in the three SLRs have been isolated, and hence have independent clocks within their respective SLRs. On the other hand, the BUFG31 line has not been isolated. Hence, the same BUFG31 (located in SLR2 in the figure) drives the clock lines in all the three SLRs - and BUFG31 located in other SLRs should be disabled.

Careful selection and manual placement (LOCs) must be used for the BUFGs. Additionally, all loads for each clock domain must be manually grouped and placed in the appropriate SLR to avoid clocking contention. If all global clocks are placed and all loads managed in a way to not create any clocking contention and allow the clock to reach all loads, this can allow greater use of the global clocking resources beyond 32.

Figure 1. Optional Isolation on Clock Lines for SSI Devices