SCM learning notes 5--stm32 clock system (based on Baiwen STM32F103 series tutorials)

The fifth chapter STM32 The clock system

Section 1 STM32 Clock tree

Clock is of great significance to single chip microcomputer , Single chip computer can't work without clock ,STM32 Any peripheral of without a clock , Can't work , So if you want a peripheral to work , The first step is to turn on his clock .

because STM32 It's very complicated , There are many peripherals , But in actual use, only a limited number of peripherals will be used , And different peripherals need different clock frequencies , Some high speed , Some low speed . So in order to reduce power consumption , Improve the anti-jamming ability , More complicated MCU All of them adopt a variety of clock sources to solve these problems .

from STM32 From the clock tree , I will STM32 Our clocks fall into three categories ：

One 、 Crystal oscillator clock , That is, the system clock in the red box is input to the source clock , Used to configure the system clock , share 4 individual .

1、 Internal high speed clock （High Speed Internal clock signal）HSI：MCU High speed clock signal provided internally ,RC oscillator ,8MHz frequency . There is no additional cost when using , But the accuracy is relatively poor .

2、 External high-speed clock （High Speed External clock signal）HSE： Provided by external crystal oscillator circuit , The input frequency range is required to be 4-16MHz. High accuracy , But it will increase the cost .

3、 External low speed clock （Low Speed External clock signal）LSE： Provided by external crystal oscillator circuit , It's the frequency input 32.768KHz. Commonly used in RTC Real time clock .

4、 Internal low speed clock （Low Speed Internal clock signal）LSI： Internal low speed RC The oscillator provides , frequency 40KHz. The watchdog clock can only be LSI, meanwhile LSI It can also be used as RTC The clock source of .

Be careful ：HSE The pin of is OSC_OUT and OSC_IN These two pin chips are led out independently , It can be connected to external crystal oscillator circuit ; and LSE The pin of OSC32_IN and OSC32_OUT The two pins are not independent , But in PC14 and PC15 On ,OSC32_IN–>PC14 ; OSC32_OUT–>PC15

about STM32F103 Series of MCU, Both need a high-speed clock source and a low-speed clock source , Internal clock source saves cost , You don't need to design your own clock circuit ; External clock source improves accuracy , You need to design your own clock circuit .

Two 、 In the green box The system clock , It's the working clock of the chip .

3、 ... and 、 All in the blue box Peripheral clock .

I said before , Any peripheral must turn on its clock before it works , So how to turn on his clock .

It can be seen from the analysis above that , The peripheral clock is attached to APB1 and APB2 Upper , and APB1、APB2 yes AHB From bus frequency division ,AHB The clock of the bus is directly connected with the system clock , So turn on the system clock , The peripheral clock will turn on . So where does the system clock come from .

In the second quarter System clock source

The picture above shows , The system clock source has three paths in total ：

1、 Directly from the internal high-speed clock , The system clock is 8MHz. If the program does not configure the system clock , After the system is powered on , The default system clock source is the internal high-speed clock , That is to say 8MHz.

2、 Input through PLL .

As you can see from the picture above , The clock source of PLL also has two parts , One is from the internal high-speed clock 2 frequency division , The other is from the external high-speed clock .

From the internal high-speed clock 2 frequency division , PLL input clock is 4MHz, Through frequency doubling , The maximum output is 4*16 = 64MHz.

From external high-speed clock ,（STM32F103 The external high-speed clock of is 8MHz） PLL direct input 8MHz, Through frequency doubling , The maximum output is 8*9 = 72MHz. Because the maximum value of the system clock is 72MHz, So the PLL here is the largest 9 frequency doubling .

3、 Directly from the external high-speed clock , The system clock is 4-16MHz.

In the third quarter Peripheral clock source

above-mentioned , All peripherals are attached to APB1 and APB2 On the bus ,APB1 and APB2 Your clock comes from AHB frequency division , and AHB The clock comes from the system clock frequency division .

in other words , After getting the system clock , In the configuration AHB and APB1、APB2 The pre frequency division coefficient of can determine the peripheral clock .

The fourth quarter, Real time clock and independent watchdog clock sources

The clock source of the real-time clock has three paths .

1、 By external high-speed clock 128 Frequency division .

2、 External low speed clock （ Usually choose this ）.

3、 Internal low speed clock .

The clock of the independent watchdog only comes from the internal low-speed clock .

Section 5 To configure STM32 The system clock

5.1 Configure the system clock process

After the introduction above , We are right. STM32 The source of all clocks in has been clearly recognized , Next, we can configure the clock .

Configuring the clock is generally divided into the following steps ：

1、 Determine the source of the system clock , yes HSI still HSE Or PLL input .

2、 Determine the relevant frequency division 、 Frequency doubling coefficient —— Here the system clock is set .

3、 To configure AHB、APB1、APB2 The division coefficient of —— Here, the peripheral clock is set .

After these three steps ,STM32 The clock of is configured , What clock is needed later , Just call the corresponding function to start the corresponding clock .

In general , We all want the system to work at the maximum allowable clock frequency , So generally STM32F103 The clock is configured as 72MHz. It said , The system clock should be configured as 72MHz, It can only be achieved by PLL output 72MHz Working frequency of .

In this mode , The system clock and peripheral clock of the single chip microcomputer can reach the maximum value allowed by the system , Therefore, the system clock is usually configured in this way .

5.2 Use HAL Library configuration clock

HAL Library SystemInit Functions are not like the standard library SystemInit Function to initialize and configure the clock , So use HAL library We must write our own clock configuration function .

5.2.1 Related structures

stay HAL In the library , Two structures are needed to configure the system clock .

On the left RCC_OscInitTypeDef The structure describes the crystal oscillator source . The members inside are used to indicate the state of the external high-speed clock 、 frequency division 、 Internal high-speed clock status and oscillator type and PLL Properties of PLL . This structure is to set the clock source of the clock tree and the relevant information of the PLL , In the red box below .

The structure on the right RCC_ClkInitTypeDef It describes the type of open clock , Is it the system clock or AHB The clock is still APB1、APB2 The clock of ; also AHB、APB1、APB2 Clock prescaler , The part in the SCM clock tree is the part in the red box in the figure below .

5.2.2 Correlation function

stay HAL In the library , The functions used for clock configuration are generally as follows .

Unmarked 5 Functions are generally used less , No introduction here .

5.2.3 Configure the system clock code

This is the code provided by Baiwen .

Be careful ：HAL_RCC_ClockConfig(RCC_ClkInitTypeDef *RCC_ClkInitStruct, uint32_t FLatency) The second parameter is to set FLASH Reading and writing speed , CPU The higher the frequency is. , The larger this parameter can be selected .STM32 Of FLASH In the manual , About FLASH_ACR The register of LATENCY Description of bit ：
0 wait state if 0MHz < SYSCLK <= 24MHz
1 wait state if 24MHz < SYSCLK <= 48MHz
2 wait state if 48MHz < SYSCLK <= 72MHz

The following is the clock configuration process described earlier , Write your own clock configuration code .

About RCC_OscInitTypeDef Members of the structure , You don't need to assign all the configuration , Keep the default value . For unnecessary things, such as HSE As the input source of the system clock , that HSI The configuration of does not need to be assigned as off , Keep the default value , They are all closed by default . The same is true for other structures , When some default values are the values we need to set , There is no need to assign values to the structure members .

So we just need to set up 1、 The crystal oscillator source is HSE;2、HSE State on ;3、HSE1 frequency division （ stay STM32F103 in , Only 1 Frequency division, so it's OK not to set ）4、PLL The input source is HSE;5、PLL open ;6、PLL by 9 frequency doubling . The sequence is shown in the figure below .

Configure according to the above figure ,HSE1 It's divided into 8MHz As PLL Input ,PLL9 Frequency doubling is 72MHz As a system clock ,AHB1 The clock is the system clock 1 Divide the frequency to get , yes 72MHz,APB1 The clock is AHB The clock 3 frequency division , by 36MHz,APB2 The clock is AHB The clock 1 Frequency division comes from 72MHz.

The system clock = 72MHz;

AHB The clock = 72MHz;

APB1 The clock = 36MHz;

APB2 The clock = 72MHz;

call HAL Library function , See if the actual clock is the same as the theoretical value .

You can see , The actual value of the four clocks is the same as the theoretical value of the configuration .