[riot-notifications] [RIOT] xtimer: Add support for arbitrary frequency hardware timers (32768 Hz support) (#3990)

[Joakim Nohlgård]

> + * (512 / 15625), which reduces the truncation caused by the integer widths */
> +
> +inline static uint32_t _xtimer_us_to_ticks(uint32_t us) {
> +    /* return (usec * 512) / 15625; */
> +    /*
> +     * Optimization to avoid division instructions or division libraries.
> +     * This code gives equivalent machine code on ARMv7-M (Cortex-M3, M4) as
> +     * generated by compiling the above code line.
> +     * at -O2 or higher with GCC 4.9. Cortex-M4 with optimization level -Os or
> +     * lower does not automatically perform this optimization.
> +     * Cortex-M0 (ARMv6-M) does not have hardware division and will insert a
> +     * call to the helper function __aeabi_uidiv which is quite complex and
> +     * slow, and uses an additional 200 bytes of ROM compared to the helper
> +     * function required for multiplying two 32 bit integers into a 64 bit result.
> +     */
> +    return ((uint64_t)(us << 9) * 0x431bde83ul) >> (12 + 32);

I was worrying about the big scary modular arithmetic magic constant would mess up the division if didn't shift first to get the proper fixed-point number.
I will update the PR.

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Reply to this email directly or view it on GitHub:
https://github.com/RIOT-OS/RIOT/pull/3990/files#r40773873
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