{"id":5976,"date":"2021-05-21T05:31:55","date_gmt":"2021-05-21T05:31:55","guid":{"rendered":"http:\/\/fastbitlab.com\/?p=5976"},"modified":"2023-02-08T16:47:46","modified_gmt":"2023-02-08T11:17:46","slug":"freertos-lecture-27-exercise-understanding-mcu-clock-configuration-part-2","status":"publish","type":"post","link":"https:\/\/fastbitlab.com\/blog\/freertos-lecture-27-exercise-understanding-mcu-clock-configuration-part-2\/","title":{"rendered":"FreeRTOS Lecture 27 – Exercise: Understanding MCU clock configuration Part-2"},"content":{"rendered":"
\n
\n
\n
\n

 <\/p>\n<\/div>\n

\n

Exercise: Understanding MCU clock configuration Part-2<\/span><\/h1>\n
\n
\n
\n

 <\/p>\n

<\/div>\n

 <\/p>\n<\/div>\n<\/div>\n<\/div>\n

From Figure 1, you can see that the reset handler begins from line 42. This is the first function or subroutine, which will be called when you reset the microcontroller.<\/span><\/p>\n

\"Understanding
Figure 1. Startup code.<\/span><\/figcaption><\/figure>\n

 <\/p>\n

If you trace this reset function, you can see that it is actually called SystemInit (Figure 2). Branch bl branch to SystemInit. The SystemInit is implemented in the system-stm32F4xx.c (Figure 3). This is added by the OpenSTM32 system workbench itself when you create a project.<\/span><\/p>\n

\"Understanding
Figure 2. Call for SystemInit in reset handler.<\/span><\/figcaption><\/figure>\n

 <\/p>\n

\"
Figure 3. system-stm32F4xx.c file.<\/span><\/figcaption><\/figure>\n

 <\/p>\n

Now let\u2019s analyze function carefully. Let\u2019s see where it is turning on the PLL. It actually turns on the PLL in the configure the system clock source, PLL multiplier, and divider factors section shown in Figure 4. So, let’s trace SetSysClock() function.<\/span><\/p>\n

\"Understanding
Figure 4. Function to configure the PLL.<\/span><\/figcaption><\/figure>\n

 <\/p>\n

The implementation of the SetSysClock function is as shown in Figure 5. Now let\u2019s check this function. The statement to turn on the main PLL is given in the section called to enable the main PLL (Figure 6). That means this project is by default turning on the PLL. The SetSysClock function is doing so many PLL configurations in order to raise the system clock to 180 MHz. This PLL configuration is not required in our project because our project is a very simple project to print \u201cHello World\u201d. So, why do we need 180 MHz of system clock? That\u2019s not needed.<\/span><\/p>\n

\"Understanding
Figure 5. Implementation of SetSysClock.<\/span><\/figcaption><\/figure>\n

 <\/p>\n

\"Statement
Figure 6. Statement to turn on the main PLL.<\/span><\/figcaption><\/figure>\n

 <\/p>\n

Now let\u2019s have a default configuration, i.e., 16 MHz of the system clock. For that, you have to turn off the PLL. Let\u2019s reverse all the configurations or settings, which is done by the SetSysClock. <\/span><\/p>\n

Steps to get into the default configuration is as follows:<\/span><\/p>\n