Clock system is the core processor, it is necessary to seriously study the system clock, and help us to understand STM32.
Since many STM32 peripherals, and some peripherals does not require high clock frequency, the same circuit, a clock faster the greater the power consumption, immunity to electromagnetic interference while the more colonies the more so for the MCU complex are generally taken multiple clock source method to solve these problems.
A, STM32 clock source
There are five clock source in the STM32, as HSI, HSE, LSI, LSE, PLL
A clock frequency point: high speed clock source and a low-speed clock source
High-speed clock source: HSI, HSE, PLL
Low-speed clock source: LSI, LSE
Sources can be divided according to an external clock source (external oscillator implemented) and internal clock source
External clock source: HSE, LSE
Internal clock source: HSI, PLL, LSI
Second, the specific presentation
1, LSE: low-speed external clock, then the frequency of 32.768kHz quartz crystal.
2, HSE: high speed external clock, may be connected to a quartz / ceramic resonator, or access an external clock source, the frequency range of 4MHz ~ 16MHz.
3, HSI: high-speed internal clock, the RC oscillator frequency of 8MHz, the precision is not high.
4, LSI: low speed internal clock, the RC oscillator frequency of 40kHz, provides a low power clock.
5, PLL: phase locked loop frequency output, the clock input source selected to HSI / 2, HSE or HSE / 2. Alternatively multiplier 2 to 16 times, but it must not exceed the maximum output frequency is 72MHz.
Fourth, the system clock SYSCLK
The clock may be derived from three sources:
1, HSI oscillator clock
2, HSE oscillator clock
3, PLL clock
Fifth, icon
