External vs Internal Clock

[Max01800]

Guys, maybe it's a dumb question, but in this microcontroller STM32F103C6, why do they differentiate between external and internal clock signals if both the oscillators are inside the microcontroller?

 

[G4BCH]

If you read section 5.3.6 and 5.3.7 it explains how to use external clock signals and the internal clock circuits.
External clocks are used when better stability is needed or to synchronise with another system

 

[KlausST]

why do they differentiate between external and internal clock signals if both the oscillators are inside

External clock .. is external, internal is internal.

It´s not where the inverter/amplifier sits, it depends where the frequency determining part is.
External clock: XTAL = externally connected. XTAL is the device that determines the clock frequency .. accuracy and stability.


Klaus

 

[Max01800]

External clock .. is external, internal is internal.

It´s not where the inverter/amplifier sits, it depends where the frequency determining part is.
External clock: XTAL = externally connected. XTAL is the device that determines the clock frequency .. accuracy and stability.


Klaus

So, for example, the oscillator LSE OSC is used just to stabilize the incoming waveform?

 

[KlausST]

Hi,

So, for example, the oscillator LSE OSC is used just to stabilize the incoming waveform?

No.
An oscillator consists of an amplifier and a frequency dependent feedback.
An XTAL does not oscillate without an amplifier.

The amplifier is built inside, the frequency determining feedback (XTAL) is built outside.
The amplifier alone does not determine the frequency. The frequency is determined by the external component: XTAL

Klaus

 

[D.A.(Tony)Stewart]

HSC is obvious master clock
LSI in a low speed (Independent)RC clock just for watchdog timer 40kHz
LSC is for a real-time-clock RTC.

There are hundreds of passive resonator options for MEMS or Crystals (XTAL) case, load capacitance, initial tolerance at room temp and stability over specified temp range. Since the cut on these resonators follows a downward parabolic curve the + error is small at room temp or perhaps wrist temp and large -ve error above and below room temp when you search for in stock [x] "32,768 kHz", whjch is a binary multiple of 1 Hz.

Adjustable cap. may be used to null initial tolerance errors near room temp or as desired. The "load cap" value is the series combination of two caps.

But for those who want better temperature compensated time of day, one would choose from

or if you want even better +/-3 ppm XO or Xtal Oscillator and just drive the input port. Looser specs for cheaper parts exist too.

I remember hiring an engineer from Philadelphia when I worked in RB, San Diego, Cali, who was a recent graduate and did not know what a crystal was. but he was a good test engineer and adapted to programming automated tests. Whereas I as Test Eng Mgr had to solve a critical field failure after boards were farmed out to a board shop for this new factory that were all failing in the field. So I took a grinder to this encapsulated crystal oscillator with a film tuning lid and found rust. Then I went to the QA guy to call the Fab shop and told them to wash the PCBA flux before putting on the Xtal. Elapsed time 2 hrs.

Problem solved.

 

[Max01800]

Hi,


No.
An oscillator consists of an amplifier and a frequency dependent feedback.
An XTAL does not oscillate without an amplifier.

The amplifier is built inside, the frequency determining feedback (XTAL) is built outside.
The amplifier alone does not determine the frequency. The frequency is determined by the external component: XTAL

Klaus

THANK YOU SO MUCH