random_stm32.c

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#include <sec/random_p.h>

#include <cpu/power.h>

#include <drv/clock_cm3.h>

#include <io/cm3.h>

struct stm32_adc *adc = (struct stm32_adc *)ADC1_BASE;

/*
 * Return the cpu core temperature in raw format
 */
INLINE uint16_t hw_readRawTemp(void)
{
    /* We sample only from one channel */
    adc->SQR1 |= BV(SQR1_SQ_LEN_SHIFT);
    adc->SQR3 = (ADC_TEMP_CH & SQR3_SQ_MASK);

    /* Start convertion */
    adc->CR2 |= CR2_EXTTRIG_SWSTRT_SET;

    /* Wait in polling until conversion is done */
    while (!(adc->SR & BV(SR_EOC)))
        cpu_relax();

    /* Return the last converted data */
    return (uint16_t)adc->DR;
}

INLINE void hw_initIntTemp(void)
{
    RCC->APB2ENR |= RCC_APB2_ADC1;

    /* Reset registry */
    adc->CR1 = 0;
    adc->CR2 = 0;
    adc->SQR1 = 0;
    adc->SQR2 = 0;
    adc->SQR3 = 0;

    /*
     * Configure ADC
     *  - Regular mode
     *  - Wake up adc
     *  - Wake up temperature and Vrefint
     */
    adc->CR2 |= BV(CR2_ADON) | ADC_EXTERNALTRIGCONV_NONE | BV(CR2_TSVREFE);

    /* Set 17.1usec sampling time*/
    adc->SMPR1 |= ((ADC_SAMPLETIME_239CYCLES5 << SMPR1_CH17) | (ADC_SAMPLETIME_239CYCLES5 << SMPR1_CH16));
}


void random_pull_entropy(uint8_t *entropy, size_t len)
{
    // We use the internal temperature sensor of LM3S as a source of entropy.
    // The last bit of the acquisition is very variable and with a decent distribution
    // to consider it "entropic". It does not really matter because it will
    // go through a randomness extractor anyway.
    hw_initIntTemp();

    for (size_t j=0; j<len; j++)
    {
        uint8_t accum = 0;
        for (int b=0; b<8; ++b)
            if (hw_readRawTemp() & 1)
                accum |= 1<<b;

        *entropy++ = accum;
    }
}