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System Core clock

Question asked by sd.pranathi on Jan 4, 2013
Latest reply on Jan 21, 2013 by Clive One
Hello all,
I am using STM32F2.Pll as clock source which uses HSE as PLL clock source. I want to set my System core clock to 24Mhz and i have done.
And I read one of the comment in stm32f2xx_rcc.c which says
"SystemCoreClockUpdate(): Updates the variable SystemCoreClock and must
  *                                 be called whenever the core clock is changed
  *                                 during program execution."
Q1:
Since i have changed my core clock is it necessary to use update it or is it just during program execution.
 I read that AHB is derived from HCLK and PCLK1 and PCLK2 are derived from AHB.
I am using SPI1 which uses APB2 clock.
Q2:
I am using APB2 prescalar as 2.
But I cannot see any clock on my scope when i probe the SCLK of SPI1. Even if send and receive many bytes .

How does system clock effect my SCLK?
Sorry for my stupid questions m a newbie to clocks and controllers

Thanks

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