syscalls: fix wraparound of RTC time

This change removes the erroneous cast to uint32_t (which caused time to
wrap around after 1 hour) and splits the multiplication into two terms
to remove the wraparound after 13 days.

Ref. https://esp32.com/viewtopic.php?f=13&t=1908
This commit is contained in:
Ivan Grokhotkov 2017-05-19 11:37:16 +08:00
parent e7db29b2a8
commit cc8af68244

View file

@ -46,8 +46,23 @@
#ifdef WITH_RTC #ifdef WITH_RTC
static uint64_t get_rtc_time_us() static uint64_t get_rtc_time_us()
{ {
uint64_t ticks = rtc_time_get(); const uint64_t ticks = rtc_time_get();
return (uint32_t) ((ticks * esp_clk_slowclk_cal_get()) >> RTC_CLK_CAL_FRACT); const uint32_t cal = esp_clk_slowclk_cal_get();
/* RTC counter result is up to 2^48, calibration factor is up to 2^24,
* for a 32kHz clock. We need to calculate (assuming no overflow):
* (ticks * cal) >> RTC_CLK_CAL_FRACT
*
* An overflow in the (ticks * cal) multiplication would cause time to
* wrap around after approximately 13 days, which is probably not enough
* for some applications.
* Therefore multiplication is split into two terms, for the lower 32-bit
* and the upper 16-bit parts of "ticks", i.e.:
* ((ticks_low + 2^32 * ticks_high) * cal) >> RTC_CLK_CAL_FRACT
*/
const uint64_t ticks_low = ticks & UINT32_MAX;
const uint64_t ticks_high = ticks >> 32;
return ((ticks_low * cal) >> RTC_CLK_CAL_FRACT) +
((ticks_high * cal) << (32 - RTC_CLK_CAL_FRACT));
} }
#endif // WITH_RTC #endif // WITH_RTC