src/rp2_common/pico_lwip/random.c

/*
 * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
 *
 * SPDX-License-Identifier: BSD-3-Clause
 */

#include "pico.h"
#include "hardware/structs/rosc.h"

static uint8_t pico_lwip_random_byte(int cycles) {
    static uint8_t byte;
    assert(cycles >= 8);
    assert(rosc_hw->status & ROSC_STATUS_ENABLED_BITS);
    for(int i=0;i<cycles;i++) {
        // picked a fairly arbitrary polynomial of 0x35u - this doesn't have to be crazily uniform.
        byte = ((byte << 1) | rosc_hw->randombit) ^ (byte & 0x80u ? 0x35u : 0);
        // delay a little because the random bit is a little slow
        busy_wait_at_least_cycles(30);
    }
    return byte;
}

unsigned int pico_lwip_rand(void) {
    uint32_t value = 0;
    for (int i = 0; i < 4; i++) {
        value = (value << 8u) | pico_lwip_random_byte(32);
    }
    return value;
}