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stm32xx: Fix RTC drift when using HSE

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When using HSE to clock RTC NuttX internal time is gaining 5.5 second per
minute. Problem was NuttX using 7182 for one of the RTC division factors,
it should have been 7812. The incorrect factors used are 7182 and 0xff.

These are used in 3-4 places within Nuttx and other places as 7812 and 0xff.
However, the STMicro app note AN4759 suggests using 7999 and 124, which is
what I've used.

Explanation: These 2 factors are used to divide the HSE clock (which at this
point is 1 MHz) to 1 Hz for the RTC hardware.
To test the 2 factors, add 1 to both numbers and multiply them together.
The result needs to be as close as possible to 1 MHz.
The suggested values of 7999 and 124 => 8000*125 = 1,000,000, the prime
factors. So, the best fix for Nuttx would be these values.

Issue discovered and fixed by Peter Moody
This commit is contained in:
Alan Carvalho de Assis 2022-06-21 10:10:09 -03:00 • committed by Petro Karashchenko
parent 9481456fde
commit 19fd0a5587
4 changed files with 16 additions and 16 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

8
arch/arm/src/stm32/stm32_rtcc.c
View file

@ -415,12 +415,12 @@ static int rtc_setup(void)
/* Configure RTC pre-scaler with the required values */
#ifdef CONFIG_STM32_RTC_HSECLOCK
/* For a 1 MHz clock this yields 0.9999360041 Hz on the second
* timer - which is pretty close.
/* STMicro app note AN4759 suggests using 7999 and 124 to
* get exactly 1MHz when using the RTC at 8MHz.
*/
putreg32(((uint32_t)7182 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)0x7f << RTC_PRER_PREDIV_A_SHIFT),
putreg32(((uint32_t)7999 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)124 << RTC_PRER_PREDIV_A_SHIFT),
STM32_RTC_PRER);
#else
/* Correct values for 32.768 KHz LSE clock and inaccurate LSI clock */

8
arch/arm/src/stm32/stm32f40xxx_rtcc.c
View file

@ -483,12 +483,12 @@ static int rtc_setup(void)
/* Configure RTC pre-scaler with the required values */
#ifdef CONFIG_STM32_RTC_HSECLOCK
/* For a 1 MHz clock this yields 0.9999360041 Hz on the second
* timer - which is pretty close.
/* STMicro app note AN4759 suggests using 7999 and 124 to
* get exactly 1MHz when using the RTC at 8MHz.
*/
putreg32(((uint32_t)7182 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)0x7f << RTC_PRER_PREDIV_A_SHIFT),
putreg32(((uint32_t)7999 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)124 << RTC_PRER_PREDIV_A_SHIFT),
STM32_RTC_PRER);
#else
/* Correct values for 32.768 KHz LSE clock and inaccurate LSI clock */

8
arch/arm/src/stm32f7/stm32_rtc.c
View file

@ -490,12 +490,12 @@ static int rtc_setup(void)
/* Configure RTC pre-scaler with the required values */
#ifdef CONFIG_STM32F7_RTC_HSECLOCK
/* For a 1 MHz clock this yields 0.9999360041 Hz on the second
* timer - which is pretty close.
/* STMicro app note AN4759 suggests using 7999 and 124 to
* get exactly 1MHz when using the RTC at 8MHz.
*/
putreg32(((uint32_t)7182 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)0x7f << RTC_PRER_PREDIV_A_SHIFT),
putreg32(((uint32_t)7999 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)124 << RTC_PRER_PREDIV_A_SHIFT),
STM32_RTC_PRER);
#else
/* Correct values for 32.768 KHz LSE clock and inaccurate LSI clock */

8
arch/arm/src/stm32h7/stm32_rtc.c
View file

@ -490,12 +490,12 @@ static int rtc_setup(void)
/* Configure RTC pre-scaler with the required values */
#ifdef CONFIG_STM32H7_RTC_HSECLOCK
/* For a 1 MHz clock this yields 0.9999360041 Hz on the second
* timer - which is pretty close.
/* STMicro app note AN4759 suggests using 7999 and 124 to
* get exactly 1MHz when using the RTC at 8MHz.
*/
putreg32(((uint32_t)7182 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)0x7f << RTC_PRER_PREDIV_A_SHIFT),
putreg32(((uint32_t)7999 << RTC_PRER_PREDIV_S_SHIFT) |
((uint32_t)124 << RTC_PRER_PREDIV_A_SHIFT),
STM32_RTC_PRER);
#else
/* Correct values for 32.768 KHz LSE clock and inaccurate LSI clock */