Reasons for using the external crystal oscillator and how to use the internal RC oscillator

Reason one

In the early years, the production process of the chip may not be able to make the crystal into the chip, but now it is ok. This problem is mainly determined by practicality and cost.

Reason two

The material of the chip and the crystal is different. The material of the chip (integrated circuit) is silicon, and the crystal is quartz (silicon dioxide). It can't be done together, but it can be packaged together. It can be realized now, but the cost is low. It is quite high.

Reason three

Once the crystal oscillator is packaged inside the chip, the frequency is also fixed. If you want to change the frequency, it is basically impossible. If you put it outside, you can freely change the crystal to provide different frequencies to the chip. Some people say that there is a PLL inside the chip, what is the crystal frequency of the chip? It is not necessary to use the PLL multiplier/frequency division. Then there is a problem of returning to the cost. The 100M crystal oscillator is integrated into the chip, but I can't use it. So high frequency, I just want to use 10M frequency, then why should I buy a chip that integrates 100M crystal oscillator, which is expensive and wasteful.

What we usually call "on-chip clock", is there actually no crystal oscillator at all, there is an RC oscillator circuit.

It can be seen from the figure that there are three ways to supply the system clock, HSI, HSE, and PLL. If the internal clock is selected as the system clock, the multiplication frequency is less than 72Mhz, and at most 8Mhz/2*16 = 64Mhz.

If you are using an internal RC oscillator instead of an external crystal, please proceed as follows:

1) For 100-pin or 144-pin products, OSC_IN should be grounded and OSC_OUT should be left floating. 2) For products with less than 100 feet, there are 2 ways to connect: i) OSC_IN and OSC_OUT are grounded through 10K resistors respectively. This method improves EMC performance. Ii) Remap OSC_IN and OSC_OUT to PD0 and PD1, respectively, and configure PD0 and PD1 as push-pull outputs and output '0'. This method can reduce power consumption and (relative to i above) save 2 external resistors.

The clock is the pulse of the STM32 microcontroller and is the driving source of the microcontroller. The corresponding clock must be turned on using any of the peripherals. The advantage of this is that if a peripheral is not used, its clock is turned off, which can reduce the power consumption of the system, achieve energy saving, and achieve low power consumption.

The clock of the STM32 microcontroller can be provided by the following three clock sources:

1, HSI: high-speed internal clock signal STM32 microcontroller with a clock (8M frequency), poor accuracy

2. HSE: High-speed external clock signal with high precision.

3. LSE: Low-speed external crystal 32.768kHz mainly provides an accurate clock source and is generally used as an RTC clock.