Select System Clock SYSCLK in system_stm32f10x.c

[RomainR.]

Posted on July 05, 2011 at 23:43

Hi All,

I do not understand

the process of

frequency selection

HSE

in the following lines

:

# if defined (

STM32F10X_LD_VL

) | |

(defined

STM32F10X_MD_VL

)

/ *

#Define

SYSCLK_FREQ_HSE

HSE_Value

* /

#define

SYSCLK_FREQ_24MHz

24000000

#else

I understand after reset, HSI clock source is selected but

#Define

SYSCLK_FREQ_HSE

HSE_Value

> will select external 8MHz crystal

and

#define

SYSCLK_FREQ_24MHz

24000000

> will select wich system clock source HSI or HSE ?

Thank for help.

Romain

[Tesla DeLorean]

Posted on July 06, 2011 at 07:22

No.

The 24 MHz value is what SYSCLK will be out of the PLL (3x).

SYSCLK_FREQ = 24MHz

Or if you want it directly from the external source

SYSCLK_FREQ = HSE

You would normally define the HSE frequency used in your system using the -DHSE_Value on the compiler command line or as a ''#define HSE_Value'' in stm32f10x_conf.h

[RomainR.]

Posted on July 06, 2011 at 12:52

Hi,

Ok, 24MHz is SYSCLK out of PLL but from witch source ?

For my project, i need to use HSE 8MHz external crystal througt PLL with 3x to obtain SYSCLK = 24MHz

How should i do with system_stm32f10x.c functions ?

Thanks

[RomainR.]

Posted on July 06, 2011 at 14:54

Ok, resolved.

In function static void SetSysClockTo24(void).

The source is HSE and PLL configuration:  = (HSE / 2) * 6 = 24 MHz

[Tesla DeLorean]

Posted on July 06, 2011 at 15:50

In function static void SetSysClockTo24(void).

 

The source is HSE and PLL configuration:  = (HSE / 2) * 6 = 24 MHz

 

 

Yes, all the settings would be made in

STM32F10x_StdPeriph_Lib_V3.5.0\Libraries\CMSIS\CM3\DeviceSupport\ST\STM32F10x\system_stm32f10x.c

I don't know if they've improved the code, but in many cases it didn't take into consideration the HSE_Value for the oscillator, and assumed it was always 8 MHz.

SystemCoreClockUpdate() seemed to be a more agnostic routine, as it determined the current system state and considered the frequencies.

If your system has specific requirements, (using HSI to get to 24 MHz) it might make sense to re-code or tailor the routines. I've always prefered general solutions to these kinds of things, using a couple of system wide defines, and computing PLL settings dynamically rather than hard coding specific settings for multipliers/dividers.