Posted on November 11, 2012 at 01:58We don't tend to get things deleted around here, and it might show up in a forum or external search.
In case anyone happens upon the thread, the CMSIS implementation typically calls the SystemInit() function prior to main(), or CRT startup code, from startup_stm32f4xx.s.
SystemInit() resides in system_stm32fxx.c, along with all the PLL and clock settings and configuration code.
For clocks you can either manually decode RCC register content, and extrapolate via HSE_VALUE, or knowing HSI is 16 MHz.
Or you can us library functions like
RCC_ClocksTypeDef RCC_ClocksStatus;
RCC_GetClocksFreq(&RCC_ClocksStatus);
Where
typedef struct
{
uint32_t SYSCLK_Frequency; /*!< SYSCLK clock frequency expressed in Hz */
uint32_t HCLK_Frequency; /*!< HCLK clock frequency expressed in Hz */
uint32_t PCLK1_Frequency; /*!< PCLK1 clock frequency expressed in Hz */
uint32_t PCLK2_Frequency; /*!< PCLK2 clock frequency expressed in Hz */
}RCC_ClocksTypeDef;
Subject: Keil, initial clock configuration.
I'm writing my own USART code to work with USART1. I need to know what the start-up code that keil gives you sets the clock initially at. I can change it manually but id like to know where the code is. Also USART1 is running off of APB2 clock, how do I figure out what the clock speed is so I can figure out what I should write in the USART_BRR register?
As I understand it, APBx clock comes from the system clock (HSI,HSE,PLL), then through AHB prescaler (if is has one) then through the APBx prescaler. Is this correct?
I would like to edit the keil start up code to enable the HSI with PLL to make system clock 168Mhz, then have APB2 clock run at 12Mhz (There is a table for BRR register at this speed).
I'm using the system_stm32f4xx.c and startup_stm32f4xx.c.
Thanks for the advise.
