Why do we need external crystal oscillator if already having internal RC oscillator ?

Thread Starter

mishra87

Joined Jan 17, 2016
906
Hello friend,

I am hunting the answer of below question.
Why do we need external crystal oscillator if already having internal RC oscillator ?

Regards,
 

Papabravo

Joined Feb 24, 2006
16,132
I still do not convince.
STM32F405 datasheet says High-speed internal (HSI) RC oscillator (pade-103) is 16MHz and with PLL it can extended upto 168MHz so do we need external clock ?
https://www.st.com/resource/en/datasheet/dm00037051.pdf

Regards,
It just doesn't matter whether you are convinced or not. I guess you did not understand my point. The designers of the part are providing more than what you "need" at a very low marginal cost increment. Since they can do that it makes the part more flexible. IMHO an RC clock might not have suitable characteristics over time and temperature with respect to an external crystal oscillator for example. For other applications the long term accuracy of the clock might be a complete "don't care".
 

KeithWalker

Joined Jul 10, 2017
1,783
The internal oscillator frequency is controlled with resistors and capacitors. An external crystal oscillator will give a much more accurate and stable timebase than the internal one. When high accuracy and low drift are not important, the internal oscillator will suffice.
 

schmitt trigger

Joined Jul 12, 2010
440
The best way to convince yourself is to do a simple exercise:

Check out the microcontroller’s data sheet and search for the internal oscillator’s initial frequency accuracy and its drift over time and temperature.

Repeat the same with a crystal oscillator. Compare its performance against the former.
 

Ian0

Joined Aug 7, 2020
2,206
Weird frequencies. If you need 11.2896MHz or 12.288MHz to divide down for digital audio you'll not get it from an internal oscillator.
 

sparky 1

Joined Nov 3, 2018
543
If you are doing a one operation it does not make sense to automate the operation.
If you have 1000 crystals and want to know the % deviation of the batch you may want a crystal input port.

This is usually for an engineer that needs more versatility in his test equipment or pressed for time.
This is just one example of using external crystal port as an input.
It is popular to use the programmable interface language in order to log test data.
 

nsaspook

Joined Aug 27, 2009
8,382

Ian0

Joined Aug 7, 2020
2,206
There were so many early computers that ran at 3.579545MHz or 4.433619MHz (NTSC or PAL TV crystals) because those crystals were cheaper than a 4MHz crystal.
 

Papabravo

Joined Feb 24, 2006
16,132
@Papabravo

Can you elaborate on your one line answer please

why do you say 5 years ?

The Op was Monday 18th jan 2021 , where dies 5 year s come from ? What am I missing here ?
https://forum.allaboutcircuits.com/threads/5-years-at-aac.175907/

The TS has been an AAC member for 5 years. It normally takes four years to get an undergraduate degree if one is a full time student. I know that there are part time students, but I do not think that is the case here. I could be wrong.
 

andrewmm

Joined Feb 25, 2011
1,468
Thank you , I did not see they had been here 5 years,
That's a much better answer.
There were so many early computers that ran at 3.579545MHz or 4.433619MHz (NTSC or PAL TV crystals) because those crystals were cheaper than a 4MHz crystal.
Slightly off topic, but,
Other reason things like the ZX80 and the Apple II used these crystals,
it meant the processor could be used to output to the home TV relatively easily

The ZX80 in particular, during the TV line the processor was processing the video, only processing your program in the blanking period.

https://en.wikipedia.org/wiki/ZX80#Video_output
 

andrewmm

Joined Feb 25, 2011
1,468
I think the answer you require has been touched on,

The RC oscillator built in to the chips is not very good in terms of accuracy or stability.
In particular in a chip , the C is difficult to make accurately,

So over temperature, variations in voltage and even chip to chip, the actual frequency of the RCD oscillator will change by many hundreds of PPM.

Even a cheap Xtal oscillator, is a few orders of magnitude more stable and repeatable than the RC.

Using the RC oscillator in the chip is cheap , and for quiet a few jobs, such as a washing machine, more than good enough.

For uses that are a bit more critical, or where reliability of frequency is needed, then strap on a crystal.
Most CPUs have the option to just strap a crystal across two pins and away it goes ( might need a few capacitors as well )
Adds less than a 50 cents to the BOM costs, but that can be a consideration,

Personally , our boards tend to cost many thousand , so the cost of a crystal for a quality product is lost.
In fact,
most of our boards we do not use a crystal, but a complete oscillator, it adds a dollar to the costs, but that's minimal.

The "problem" with some single crystal oscillators, is they have a tendency to not oscillating,
they are very susceptible to layout, dust / moisture , and the cost of fixing one board that dies is extreme compared to the extra component costs.

Hope this helps
 

BobaMosfet

Joined Jul 1, 2009
1,776
Hello friend,

I am hunting the answer of below question.
Why do we need external crystal oscillator if already having internal RC oscillator ?

Regards,
Because the external oscillator can be much faster than the internal one- which means more things can be done.
 
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