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neorv32/rtl/core/neorv32_trng.vhd

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-- #################################################################################################
-- # << NEORV32 - True Random Number Generator (TRNG) >> #
-- # ********************************************************************************************* #
-- # This processor module instantiates the "neoTRNG" true random number generator. An optional #
-- # "random pool" FIFO can be configured using the TRNG_FIFO generic. #
-- # See the neoTRNG documentation for more information: https://github.com/stnolting/neoTRNG #
-- # ********************************************************************************************* #
-- # BSD 3-Clause License #
-- # #
-- # The NEORV32 RISC-V Processor, https://github.com/stnolting/neorv32 #
-- # Copyright (c) 2024, Stephan Nolting. All rights reserved. #
-- # #
-- # Redistribution and use in source and binary forms, with or without modification, are #
-- # permitted provided that the following conditions are met: #
-- # #
-- # 1. Redistributions of source code must retain the above copyright notice, this list of #
-- # conditions and the following disclaimer. #
-- # #
-- # 2. Redistributions in binary form must reproduce the above copyright notice, this list of #
-- # conditions and the following disclaimer in the documentation and/or other materials #
-- # provided with the distribution. #
-- # #
-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to #
-- # endorse or promote products derived from this software without specific prior written #
-- # permission. #
-- # #
-- # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS #
-- # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF #
-- # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE #
-- # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, #
-- # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #
-- # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED #
-- # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING #
-- # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED #
-- # OF THE POSSIBILITY OF SUCH DAMAGE. #
-- #################################################################################################
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
library neorv32;
use neorv32.neorv32_package.all;
entity neorv32_trng is
generic (
IO_TRNG_FIFO : natural range 1 to 2**15 -- RND fifo depth, has to be a power of two, min 1
);
port (
clk_i : in std_ulogic; -- global clock line
rstn_i : in std_ulogic; -- global reset line, low-active, async
bus_req_i : in bus_req_t; -- bus request
bus_rsp_o : out bus_rsp_t; -- bus response
irq_o : out std_ulogic -- CPU interrupt
);
end neorv32_trng;
architecture neorv32_trng_rtl of neorv32_trng is
-- neoTRNG Configuration ------------------------------------------------------------------------
constant num_cells_c : natural := 3; -- total number of ring-oscillator cells
constant num_inv_start_c : natural := 5; -- number of inverters in first cell, has to be odd
-- ----------------------------------------------------------------------------------------------
-- use simulation mode (pseudo-RNG)? --
constant sim_mode_c : boolean := is_simulation_c; -- is this a simulation?
-- control register bits --
constant ctrl_data_lsb_c : natural := 0; -- r/-: Random data byte LSB
constant ctrl_data_msb_c : natural := 7; -- r/-: Random data byte MSB
--
constant ctrl_fifo_size0_c : natural := 16; -- r/-: log2(FIFO size) bit 0
constant ctrl_fifo_size1_c : natural := 17; -- r/-: log2(FIFO size) bit 1
constant ctrl_fifo_size2_c : natural := 18; -- r/-: log2(FIFO size) bit 2
constant ctrl_fifo_size3_c : natural := 19; -- r/-: log2(FIFO size) bit 3
--
constant ctrl_irq_fifo_nempty : natural := 25; -- r/w: IRQ if fifo is not empty
constant ctrl_irq_fifo_half : natural := 26; -- r/w: IRQ if fifo is at least half-full
constant ctrl_irq_fifo_full : natural := 27; -- r/w: IRQ if fifo is full
constant ctrl_fifo_clr_c : natural := 28; -- -/w: Clear data FIFO (auto clears)
constant ctrl_sim_mode_c : natural := 29; -- r/-: TRNG implemented in pseudo-RNG simulation mode
constant ctrl_en_c : natural := 30; -- r/w: TRNG enable
constant ctrl_valid_c : natural := 31; -- r/-: Output data valid
-- Component: neoTRNG true random number generator --
component neoTRNG
generic (
NUM_CELLS : natural := 3; -- number of ring-oscillator cells
NUM_INV_START : natural := 5; -- number of inverters in first cell, has to be odd
SIM_MODE : boolean := false -- enable simulation mode (use pseudo-RNG)
);
port (
clk_i : in std_ulogic; -- module clock
rstn_i : in std_ulogic; -- module reset, low-active, async, optional
enable_i : in std_ulogic; -- module enable (high-active)
data_o : out std_ulogic_vector(7 downto 0); -- random data byte output
valid_o : out std_ulogic -- data_o is valid when set
);
end component;
-- control --
signal enable : std_ulogic;
signal fifo_clr : std_ulogic;
signal irq_fifo_nempty : std_ulogic;
signal irq_fifo_half : std_ulogic;
signal irq_fifo_full : std_ulogic;
-- data FIFO --
type fifo_t is record
we : std_ulogic; -- write enable
re : std_ulogic; -- read enable
clear : std_ulogic; -- sync reset, high-active
wdata : std_ulogic_vector(7 downto 0); -- write data
rdata : std_ulogic_vector(7 downto 0); -- read data
avail : std_ulogic; -- data available?
half : std_ulogic; -- at least half full?
free : std_ulogic; -- space left?
end record;
signal fifo : fifo_t;
begin
-- Bus Access ----------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
bus_access: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
bus_rsp_o.ack <= '0';
bus_rsp_o.err <= '0';
bus_rsp_o.data <= (others => '0');
enable <= '0';
fifo_clr <= '0';
irq_fifo_nempty <= '0';
irq_fifo_half <= '0';
irq_fifo_full <= '0';
elsif rising_edge(clk_i) then
-- bus handshake --
bus_rsp_o.ack <= bus_req_i.stb;
bus_rsp_o.err <= '0';
bus_rsp_o.data <= (others => '0');
-- defaults --
fifo_clr <= '0'; -- auto-clear
if (bus_req_i.stb = '1') then
-- write access --
if (bus_req_i.rw = '1') then
enable <= bus_req_i.data(ctrl_en_c);
fifo_clr <= bus_req_i.data(ctrl_fifo_clr_c);
irq_fifo_nempty <= bus_req_i.data(ctrl_irq_fifo_nempty);
irq_fifo_half <= bus_req_i.data(ctrl_irq_fifo_half);
irq_fifo_full <= bus_req_i.data(ctrl_irq_fifo_full);
-- read access --
else
bus_rsp_o.data(ctrl_data_msb_c downto ctrl_data_lsb_c) <= fifo.rdata;
--
bus_rsp_o.data(ctrl_fifo_size3_c downto ctrl_fifo_size0_c) <= std_ulogic_vector(to_unsigned(index_size_f(IO_TRNG_FIFO), 4));
--
bus_rsp_o.data(ctrl_irq_fifo_nempty) <= irq_fifo_nempty;
bus_rsp_o.data(ctrl_irq_fifo_half) <= irq_fifo_half;
bus_rsp_o.data(ctrl_irq_fifo_full) <= irq_fifo_full;
bus_rsp_o.data(ctrl_sim_mode_c) <= bool_to_ulogic_f(sim_mode_c);
bus_rsp_o.data(ctrl_en_c) <= enable;
bus_rsp_o.data(ctrl_valid_c) <= fifo.avail;
end if;
end if;
end if;
end process bus_access;
-- neoTRNG True Random Number Generator ---------------------------------------------------
-- -------------------------------------------------------------------------------------------
neoTRNG_inst: neoTRNG
generic map (
NUM_CELLS => num_cells_c,
NUM_INV_START => num_inv_start_c,
SIM_MODE => sim_mode_c
)
port map (
clk_i => clk_i,
rstn_i => rstn_i,
enable_i => enable,
data_o => fifo.wdata,
valid_o => fifo.we
);
-- Data FIFO ("Random Pool") --------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
rnd_pool_fifo_inst: entity neorv32.neorv32_fifo
generic map (
FIFO_DEPTH => IO_TRNG_FIFO, -- number of fifo entries; has to be a power of two; min 1
FIFO_WIDTH => 8, -- size of data elements in fifo
FIFO_RSYNC => true, -- sync read
FIFO_SAFE => true, -- safe access
FULL_RESET => false -- no HW reset, try to infer BRAM
)
port map (
-- control --
clk_i => clk_i, -- clock, rising edge
rstn_i => rstn_i, -- async reset, low-active
clear_i => fifo.clear, -- sync reset, high-active
half_o => fifo.half, -- at least half full
-- write port --
wdata_i => fifo.wdata, -- write data
we_i => fifo.we, -- write enable
free_o => fifo.free, -- at least one entry is free when set
-- read port --
re_i => fifo.re, -- read enable
rdata_o => fifo.rdata, -- read data
avail_o => fifo.avail -- data available when set
);
fifo.clear <= '1' when (enable = '0') or (fifo_clr = '1') else '0';
fifo.re <= '1' when (bus_req_i.stb = '1') and (bus_req_i.rw = '0') else '0';
-- FIFO-level interrupt generator --
irq_generator: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
irq_o <= '0';
elsif rising_edge(clk_i) then
irq_o <= enable and (
(irq_fifo_nempty and fifo.avail) or -- IRQ if FIFO not empty
(irq_fifo_half and fifo.half) or -- IRQ if FIFO at least half full
(irq_fifo_full and (not fifo.free))); -- IRQ if FIFO full
end if;
end process irq_generator;
end neorv32_trng_rtl;
-- ############################################################################################################################
-- ############################################################################################################################
-- #################################################################################################
-- # << neoTRNG V3 - A Tiny and Platform-Independent True Random Number Generator >> #
-- # ********************************************************************************************* #
-- # The neoTNG true-random generator uses free-running ring-oscillators to generate "phase noise" #
-- # that is used as entropy source. The ring-oscillators are based on plain inverter chains that #
-- # are decoupled using individually-enabled latches in order to prevent the synthesis from #
-- # trimming parts of the logic. Hence, the TRNG provides a platform-agnostic architecture that #
-- # can be implemented for any FPGA without requiring primitive instantiation or technology- #
-- # specific attributes or synthesis options. #
-- # #
-- # The random output from each entropy cells is synchronized and XOR-ed with the other cell's #
-- # outputs before it is and fed into a simple 2-bit "von Neumann randomness extractor" #
-- # (extracting edges). 64 de-biased bits are "combined" using a LFSR-style shift register (in #
-- # order to improve spectral distribution) to provide one final random data byte. #
-- # ********************************************************************************************* #
-- # BSD 3-Clause License #
-- # #
-- # Copyright (c) 2023, Stephan Nolting. All rights reserved. #
-- # #
-- # Redistribution and use in source and binary forms, with or without modification, are #
-- # permitted provided that the following conditions are met: #
-- # #
-- # 1. Redistributions of source code must retain the above copyright notice, this list of #
-- # conditions and the following disclaimer. #
-- # #
-- # 2. Redistributions in binary form must reproduce the above copyright notice, this list of #
-- # conditions and the following disclaimer in the documentation and/or other materials #
-- # provided with the distribution. #
-- # #
-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to #
-- # endorse or promote products derived from this software without specific prior written #
-- # permission. #
-- # #
-- # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS #
-- # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF #
-- # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE #
-- # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, #
-- # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #
-- # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED #
-- # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING #
-- # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED #
-- # OF THE POSSIBILITY OF SUCH DAMAGE. #
-- # ********************************************************************************************* #
-- # neoTRNG - https://github.com/stnolting/neoTRNG (c) Stephan Nolting #
-- #################################################################################################
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity neoTRNG is
generic (
NUM_CELLS : natural := 3; -- number of ring-oscillator cells
NUM_INV_START : natural := 5; -- number of inverters in first cell, has to be odd
SIM_MODE : boolean := false -- enable simulation mode (use pseudo-RNG)
);
port (
clk_i : in std_ulogic; -- module clock
rstn_i : in std_ulogic; -- module reset, low-active, async, optional
enable_i : in std_ulogic; -- module enable (high-active)
data_o : out std_ulogic_vector(7 downto 0); -- random data byte output
valid_o : out std_ulogic -- data_o is valid when set
);
end neoTRNG;
architecture neoTRNG_rtl of neoTRNG is
-- entropy generator cell --
component neoTRNG_cell
generic (
NUM_INV : natural; -- number of inverters, has to be odd
SIM_MODE : boolean -- use LFSR instead of physical entropy source
);
port (
clk_i : in std_ulogic; -- clock
rstn_i : in std_ulogic; -- reset, low-active, async, optional
en_i : in std_ulogic; -- enable chain input
en_o : out std_ulogic; -- enable chain output
rnd_o : out std_ulogic -- random data (sync)
);
end component;
-- entropy cell interconnect --
signal cell_en_in : std_ulogic_vector(NUM_CELLS-1 downto 0); -- enable sreg input
signal cell_en_out : std_ulogic_vector(NUM_CELLS-1 downto 0); -- enable sreg output
signal cell_rnd : std_ulogic_vector(NUM_CELLS-1 downto 0); -- cell random output
signal rnd_raw : std_ulogic; -- combined raw random data
-- de-biasing --
signal debias_sreg : std_ulogic_vector(1 downto 0); -- sample buffer
signal debias_state : std_ulogic; -- process de-biasing every second cycle
signal debias_valid : std_ulogic; -- result bit valid
signal debias_data : std_ulogic; -- result bit
-- sampling control --
signal sample_en : std_ulogic; -- global enable
signal sample_sreg : std_ulogic_vector(7 downto 0); -- shift register / de-serializer
signal sample_cnt : std_ulogic_vector(6 downto 0); -- bits-per-sample (64) counter
begin
-- Sanity Checks --------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
assert false report
"[neoTRNG NOTE] << neoTRNG V3 - A Tiny and Platform-Independent True Random Number Generator >>" severity note;
assert ((NUM_INV_START mod 2) /= 0) report
"[neoTRNG ERROR] Number of inverters in first cell <NUM_INV_START> has to be odd!" severity error;
-- Entropy Source -------------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
entropy_source:
for i in 0 to NUM_CELLS-1 generate
neoTRNG_cell_inst: neoTRNG_cell
generic map (
NUM_INV => NUM_INV_START + 2*i, -- increasing cell length
SIM_MODE => SIM_MODE
)
port map (
clk_i => clk_i,
rstn_i => rstn_i,
en_i => cell_en_in(i),
en_o => cell_en_out(i),
rnd_o => cell_rnd(i)
);
end generate;
-- enable shift register chain --
cell_en_in(0) <= sample_en;
cell_en_in(NUM_CELLS-1 downto 1) <= cell_en_out(NUM_CELLS-2 downto 0);
-- combine cell outputs --
combine: process(cell_rnd)
variable tmp_v : std_ulogic;
begin
tmp_v := '0';
for i in 0 to NUM_CELLS-1 loop
tmp_v := tmp_v xor cell_rnd(i);
end loop;
rnd_raw <= tmp_v;
end process combine;
-- John von Neumann Randomness Extractor (De-Biasing) -------------------------------------
-- -------------------------------------------------------------------------------------------
debiasing_sync: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
debias_sreg <= (others => '0');
debias_state <= '0';
elsif rising_edge(clk_i) then
debias_sreg <= debias_sreg(0) & rnd_raw;
-- start operation when last cell is enabled and process in every second cycle --
debias_state <= (not debias_state) and cell_en_out(NUM_CELLS-1);
end if;
end process debiasing_sync;
-- edge detector - check groups of two non-overlapping bits from the random stream --
debiasing_comb: process(debias_state, debias_sreg)
variable tmp_v : std_ulogic_vector(2 downto 0);
begin
tmp_v := debias_state & debias_sreg(1 downto 0);
case tmp_v is
when "101" => debias_valid <= '1'; -- rising edge
when "110" => debias_valid <= '1'; -- falling edge
when others => debias_valid <= '0'; -- no valid data
end case;
end process debiasing_comb;
-- edge data --
debias_data <= debias_sreg(0);
-- Sampling Control -----------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
sampling_control: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
sample_en <= '0';
sample_cnt <= (others => '0');
sample_sreg <= (others => '0');
elsif rising_edge(clk_i) then
sample_en <= enable_i;
if (sample_en = '0') or (sample_cnt(sample_cnt'left) = '1') then -- start new iteration
sample_cnt <= (others => '0');
sample_sreg <= (others => '0');
elsif (debias_valid = '1') then -- LFSR-style sample shift register to inter-mix random stream
sample_cnt <= std_ulogic_vector(unsigned(sample_cnt) + 1);
sample_sreg <= sample_sreg(6 downto 0) & (sample_sreg(7) xor debias_data);
end if;
end if;
end process sampling_control;
-- TRNG output stream --
data_o <= sample_sreg;
valid_o <= sample_cnt(sample_cnt'left);
end neoTRNG_rtl;
-- ############################################################################################################################
-- ############################################################################################################################
-- #################################################################################################
-- # << neoTRNG V3 - A Tiny and Platform-Independent True Random Number Generator >> #
-- # ********************************************************************************************* #
-- # neoTRNG entropy source cell, based on a simple ring-oscillator constructed from an odd number #
-- # of inverter. The inverters are decoupled using individually-enabled latches to prevent the #
-- # synthesis from removing parts of the oscillator chain - hardware hack! ;) #
-- # ********************************************************************************************* #
-- # BSD 3-Clause License #
-- # #
-- # Copyright (c) 2023, Stephan Nolting. All rights reserved. #
-- # #
-- # Redistribution and use in source and binary forms, with or without modification, are #
-- # permitted provided that the following conditions are met: #
-- # #
-- # 1. Redistributions of source code must retain the above copyright notice, this list of #
-- # conditions and the following disclaimer. #
-- # #
-- # 2. Redistributions in binary form must reproduce the above copyright notice, this list of #
-- # conditions and the following disclaimer in the documentation and/or other materials #
-- # provided with the distribution. #
-- # #
-- # 3. Neither the name of the copyright holder nor the names of its contributors may be used to #
-- # endorse or promote products derived from this software without specific prior written #
-- # permission. #
-- # #
-- # THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS #
-- # OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF #
-- # MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE #
-- # COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, #
-- # EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE #
-- # GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED #
-- # AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING #
-- # NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED #
-- # OF THE POSSIBILITY OF SUCH DAMAGE. #
-- # ********************************************************************************************* #
-- # neoTRNG - https://github.com/stnolting/neoTRNG (c) Stephan Nolting #
-- #################################################################################################
library ieee;
use ieee.std_logic_1164.all;
entity neoTRNG_cell is
generic (
NUM_INV : natural; -- number of inverters, has to be odd
SIM_MODE : boolean -- use LFSR instead of physical entropy source
);
port (
clk_i : in std_ulogic; -- clock
rstn_i : in std_ulogic; -- reset, low-active, async, optional
en_i : in std_ulogic; -- enable chain input
en_o : out std_ulogic; -- enable chain output
rnd_o : out std_ulogic -- random data (sync)
);
end neoTRNG_cell;
architecture neoTRNG_cell_rtl of neoTRNG_cell is
signal rosc : std_ulogic_vector(NUM_INV-1 downto 0); -- ring oscillator element: inverter + latch
signal sreg : std_ulogic_vector(NUM_INV-1 downto 0); -- enable shift register
signal sync : std_ulogic_vector(1 downto 0); -- output synchronizer
begin
-- Physical Entropy Source: Ring Oscillator -----------------------------------------------
-- -------------------------------------------------------------------------------------------
-- Each cell is based on a simple ring oscillator with an odd number of inverters. Each
-- inverter is followed by a latch that provides a reset (to start in a defined state) and
-- a latch-enable to make the latch transparent. Switching to transparent mode is done one by
-- one by the enable shift register (see notes below).
sim_mode_false:
if SIM_MODE = false generate
-- ring oscillator --
ring_osc:
for i in 0 to NUM_INV-1 generate
ring_osc_start:
if (i = 0) generate -- inverting latch
rosc(i) <= '0' when (en_i = '0') else (not rosc(NUM_INV-1)) when (sreg(i) = '1') else rosc(i);
end generate;
ring_osc_chain:
if (i > 0) generate -- inverting latch
rosc(i) <= '0' when (en_i = '0') else (not rosc(i-1)) when (sreg(i) = '1') else rosc(i);
end generate;
end generate;
end generate;
-- Simulation-Only Entropy Source: Pseudo-RNG ---------------------------------------------
-- -------------------------------------------------------------------------------------------
-- The pseudo-RNG is meant for functional rtl simulation only. It is based on a simple LFSR.
-- Do not use this option for "real" implementations!
sim_mode_true:
if SIM_MODE = true generate
assert false report
"[neoTRNG WARNING] Implementing non-physical pseudo-RNG!" severity warning;
sim_lfsr: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
rosc <= (others => '0');
elsif rising_edge(clk_i) then
if (en_i = '0') then
rosc <= (others => '0');
else -- sequence might NOT be maximum-length!
rosc(rosc'left downto 1) <= rosc(rosc'left-1 downto 0);
rosc(0) <= not (rosc(NUM_INV-1) xor rosc(0));
end if;
end if;
end process sim_lfsr;
end generate;
-- Output Synchronizer --------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
-- Sample the actual entropy source (= phase noise) and move it to the system's clock domain.
synchronizer: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
sync <= (others => '0');
elsif rising_edge(clk_i) then
sync <= sync(0) & rosc(NUM_INV-1);
end if;
end process synchronizer;
-- cell output --
rnd_o <= sync(1);
-- Enable Shift-Register ------------------------------------------------------------------
-- -------------------------------------------------------------------------------------------
-- Using individual enable signals from a shift register for each inverter in order to prevent
-- the synthesis tool from removing all but one inverter (since they implement "logical
-- identical functions"). This makes the TRNG platform independent as we do not require tool-/
-- technology-specific primitives, attributes or other options.
en_shift_reg: process(rstn_i, clk_i)
begin
if (rstn_i = '0') then
sreg <= (others => '0');
elsif rising_edge(clk_i) then
sreg <= sreg(sreg'left-1 downto 0) & en_i;
end if;
end process en_shift_reg;
-- output for global enable chain --
en_o <= sreg(sreg'left);
end neoTRNG_cell_rtl;
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