UltraScale devices have a different clocking structure from previous device architectures, which blurs the line between global versus regional clocking. UltraScale devices do not have regional clock buffers like 7 series devices and instead use a common buffer and clock routing structure whether the loads are local/regional or global.
UltraScale devices feature smaller clock regions of a fixed size across devices, and the clock regions no longer span half of the device width in the horizontal direction. The number of clock regions per row varies per UltraScale device. Each clock region contains a clock network routing that is divided into 24 vertical and horizontal routing tracks and 24 vertical and horizontal distribution tracks. The following figure shows a device with 36 clock regions (6 columns x 6 rows). The equivalent 7 series device has 12 clock regions (2 columns x 6 rows).
The clocking architecture is designed so that only the clock resources necessary to connect clock buffers and loads for a given placement are used, and no resource is wasted in clock regions with no loads. The efficient clock resource utilization enables support for more design clocks in the architecture while improving clock characteristics for performance and power. Following are the main categories of clock types and associated clock structures grouped by their driver and use:
- High-Speed I/O Clocks
These clocks are associated with the high-speed SelectIO™ interface bit slice logic, generated by the PLL, and routed via dedicated, low-jitter resources to the bit slice logic for high-speed I/O interfaces. In general, this clocking structure is created and controlled by Xilinx IP, such as memory IP or the High Speed SelectIO Wizard, and is not user specified.
- General Clocks
These clocks are used in most clock tree structures and can be sourced by a GCIO package pin, an MMCM/PLL, or fabric logic cells (not generally suggested). The general clocking network must be driven by BUFGCE/BUFGCE_DIV/BUFGCTRL buffers, which are available in any clock region that contains an I/O column. Any given clock region can support up to 24 unique clocks, and most UltraScale devices can support over 100 clock trees depending on their topology, fanout, and load placement.
- Gigabit Transceiver (GT) Clocks
Transmit, receive, and reference clocks of gigabit transceivers (GTH or GTY) use dedicated clocking in the clock regions that include the GTs. You can use GT clocks to achieve the following:
- Drive the general clocking network using the BUFG_GT buffers to connect any loads in the fabric
- Share clocks across several transceivers in the same or different Quad