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SoC designs use an increasingly complex collection of voltages both for interfacing with external components and in an internal hierarchy of voltage islands in order to reduce power (see the ARM ® -Synopsys Low-Power Methodology Manual, LPMM, in the bibliography for further detail). The internal voltage islands are usually inferred by EDA tools, based on the power-intent captured in side-files written in formats like the universal power format (UPF). Internal voltage islands therefore do not generally appear in the design RTL and do not get mapped onto the FPGA- based prototype. If the RTL delivered to the prototyping team already has voltage control logic, retention registers etc. then these will need to be tied to inactive state (see chapter 7). It is very unlikely that the voltage regions on the FPGA board will correspond with the regions within the SoC itself.
External voltage needs must still be mapped correctly, however. FPGA IO pins are arranged in banks that support different voltage thresholds and are powered by different voltage rings. It is important that the platform can supply any of the required voltages and route them to the relevant FPGA power pins. Voltage inflexibility additionally constrains IO usage and placement so that voltage rather than performance or connectivity sometimes governs partitioning choices with the result of compromising other priorities. Indeed, new voltage requirements can be a reason that existing boards cannot be reused in new projects, so building in flexibility is a good investment for helping to ensure longer life of the boards across multiple projects.
Figure 53 shows a screenshot from a utility which is used to remotely set-up the voltage of different regions on a HAPS-64 board.
Figure 53: On-board IO voltage configuration
Here we can see that the FPGA pins and daughter-card connectors are grouped the into different regions which can be set to chosen voltages (in reality, there is good use of color to clarify the user interface). The region voltage is slected by use of pull-down menus and we can see that “OFF” is also a valid option.
This configuration tool then configures programmable switches on the board which setup the reference and rail voltage for the FPGA devices themselves. Although this is an example from a commercial board, it is possible to create such utilities in- house and might be worth the effort if many copies of a board are to be made and used in a wide variety of projects.
Manufacturer:Xilinx
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