microwatt/fpga/fpga-random.xdc

set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets soc0/processor/execute1_0/random_0/ro_reg*]
set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets soc0/processor/execute1_0/random_0/p_*]
set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets soc0/processor/execute1_0/random_0/D*]
Add random number generator and implement the darn instruction This adds a true random number generator for the Xilinx FPGAs which uses a set of chaotic ring oscillators to generate random bits and then passes them through a Linear Hybrid Cellular Automaton (LHCA) to remove bias, as described in "High Speed True Random Number Generators in Xilinx FPGAs" by Catalin Baetoniu of Xilinx Inc., in: https://pdfs.semanticscholar.org/83ac/9e9c1bb3dad5180654984604c8d5d8137412.pdf This requires adding a .xdc file to tell vivado that the combinatorial loops that form the ring oscillators are intentional. The same code should work on other FPGAs as well if their tools can be told to accept the combinatorial loops. For simulation, the random.vhdl module gets compiled in, which uses the pseudorand() function to generate random numbers. Synthesis using yosys uses nonrandom.vhdl, which always signals an error, causing darn to return 0xffff_ffff_ffff_ffff. This adds an implementation of the darn instruction. Darn can return either raw or conditioned random numbers. On Xilinx FPGAs, reading a raw random number gives the output of the ring oscillators, and reading a conditioned random number gives the output of the LHCA. Signed-off-by: Paul Mackerras <paulus@ozlabs.org> 4 years ago			`set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets soc0/processor/execute1_0/random_0/ro_reg*]`
			`set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets soc0/processor/execute1_0/random_0/p_*]`
			`set_property ALLOW_COMBINATORIAL_LOOPS TRUE [get_nets soc0/processor/execute1_0/random_0/D*]`