54
votes
Accepted
What's special about 14.31818 MHz?
It is exactly 4× the NTSC color-burst frequency of 3.579545 MHz. Since it is (well, used to be) used in huge quantities in commercial color TV sets, it is both commonly available, and ...
- 176k
40
votes
Accepted
Why is crystal frequency often multiplied inside a microcontroller?
As crystal frequency gets higher, dimensions get smaller (especially thickness). Above roughly 50 MHz, AT-cut crystals become fragile, and manufacturers use third-overtone cuts for higher frequencies.....
- 27k
32
votes
How can you achieve decent clock accuracy in microcontrollers (e.g., max 1 second drift in a year)? How do digital watch manufacturers accomplish it?
Have you tried comparing with what accuracy you actually get from a digital watch? I don't remember them delivering "1 second per year" accuracy.
The crystal you linked was 50ppm. The ones ...
- 46.9k
31
votes
Accepted
Why not put a crystal oscillator inside the microcontroller?
A crystal is very different in physical construction from an MCU, and most crystals are annoyingly large to integrate into a chip package. It's possible, however, to integrate other types of (smaller) ...
- 20.3k
30
votes
Accepted
How does an ARM MCU run faster than the external crystal?
This doesn't have anything to do with the core being an ARM processor; it's about how the clocking circuitry works:
In many systems like microcontrollers, RF chips, audio chips, … you need to ...
- 98.3k
26
votes
Accepted
When should one use an external crystal for this MCU given that the internal oscillator is much faster?
The internal oscillator is much less stable than an external crystal oscillator.
If I'm reading the datasheet correctly, the internal 48 MHz oscillator is only factory calibrated to within 2.9% of ...
- 14.5k
25
votes
Accepted
Why does a mouse need a crystal?
That MOS 5717 thing is most likely a microcontroller or some part that executes code. It needs a clock to run.
However, just a clock for a micro doesn't need crystal accuracy. That is probably for ...
- 314k
23
votes
Accepted
Crystal oscillator load capacitance, again
Is there a one true answer to the question? It all seems very
frustrating to me. Why doesn't an oscillator start?
A crystal oscillator will fail to start when the crystal and the capacitors ...
- 467k
23
votes
Accepted
Why is signal on XTAL1 and XTAL2 a sine wave (not square)?
This circuit is not a digital circuit. In fact, it is a fairly mathematically complicated non-linear analog circuit with automatic gain control with self-sustainable oscillation mode. It is called a "...
- 41.8k
23
votes
Why not put a crystal oscillator inside the microcontroller?
A crystal is fabricated on a completely different process and from a completely different material than a microcontroller is. You can't integrate them both into a single chip, so you would have to ...
- 36.3k
22
votes
How can we test the lifespan of electrical components?
It is excellent to test your product like you describe but it is not the easiest way to provide a rationale for the lifespan. The following method is the one I follow in the medical devices industry :
...
- 701
21
votes
Why does a mouse need a crystal?
The MOS 5717 has quite a bit of independent work to do on its own to create the mouse functionality from a set of optical encoders.
The description from the datasheet states it the best as,
The ...
- 3,042
19
votes
What frequency stability crystal do we need?
Too long to read all that stuff so, here's the short story
From comments under the question, Trevor says this: -
we think the caps are correct. We have 2x 15pf to ground. The crystal we're using is ...
- 467k
17
votes
Accepted
Different (15 and 10 pF) load capacitors on 32.768 Hz quartz crystal
The question is is it going to be any better?
It sounds to me like it will be a bit better in that it will reduce the error between the two clocks - the capacitors do that - they trim the phase ...
- 467k
17
votes
Accepted
Trying to get a 32.768 kHz crystal to oscillate
Rereading, I'm pretty sure I need 25pF caps instead as I misinterpreted how load capacitance worked.
You need 25pF caps less any stray board and chip capacitance. You'll probably end up at around 18-...
- 46.7k
16
votes
How can I calibrate a retail digital clock that uses a 32.768 kHz crystal?
First, refer to this post for details of (we hope) the underlying oscillator schematic. You'll probably just see the crystal and the caps, with traces going into a chip.
Verify that you've found the ...
- 46.7k
16
votes
Why not put a crystal oscillator inside the microcontroller?
Here's a crystal oscillator (source):
In order to work, a crystal must be suspended in free air (or inert gas) by the edges. It works like a tuning fork, it is a mechanical resonator with electrical ...
- 80.3k
15
votes
32 kHz crystal not operating as expected
There are two main things going on:
You don't have enough load capacitance.
You don't understand load capacitance.
Imagine one side of a crystal being driven with a sine wave at the crystal ...
- 314k
15
votes
What frequency stability crystal do we need?
If you have a suitable means of measuring the frequency, you can adjust one of the load caps with a trimcap to get closer to the ideal. However you will need some means of measuring without altering ...
15
votes
Why can I apply any excitation to a crystal (50, 60, 150 Hz) and any voltage and it will always output its natural frequency?
A crystal, on its own, is nothing more than a passive filter with a very particular frequency response that makes it very selective to frequency; it has two so-called resonant frequencies:
the "...
- 20.3k
14
votes
Accepted
32 kHz crystal not operating as expected
The MCU maker is likely at fault. There is absolutely no excuse for not designing a modern MCU RTC oscillator to reliably function with any typical commercially available 32kHz crystal.
Unfortunately,...
- 1,845
14
votes
How can I calibrate a retail digital clock that uses a 32.768 kHz crystal?
Since the clock is running fast, you could try increasing the capacitance on the crystal by using a "gimmick", which is a capacitor made by two insulated wires twisted together. Magnet wire ...
- 7,555
13
votes
Why can I apply any excitation to a crystal (50, 60, 150 Hz) and any voltage and it will always output its natural frequency?
If we hit a crystal with an impulse, it will 'ring' at its natural frequency. Rather as a bell, struck with a hammer. This is the transient behaviour, of crystal, or bell.
If we apply a 50 Hz ...
- 171k
12
votes
Strange extra frequency in crystal oscillator
This really looks like a sampling artifact on your end, not something the crystal is doing. Expand the scope time scale (lower time/division) until you only have a cycle or two per division at most. ...
- 314k
12
votes
Accepted
Two Crystals & Two PIC18F Microcontrollers on same PCB
There are advantages to using an external crystal oscillator module in a case like this and PIC18F MCUs are designed to accept such arrangements by driving one side of their crystal oscillator pins.
...
- 78.4k
12
votes
How do crystal oscillators start?
The noise level of an unpowered XTAL oscillator is approximately computable; in this circuit, even with NO power, the 1Kohm resistor provides broadband electrical random noise:
simulate this circuit &...
- 34.6k
12
votes
Crystal oscillator with inverter gate, crystal engraved with "1 MHz" outputs 1.68 MHz
I am, frankly, not sure why your circuit works for you at all, with any crystal. Presumably it's intended to work at or near the crystal's series resonance. In my experience, if you get a crystal ...
- 46.7k
12
votes
How can I calibrate a retail digital clock that uses a 32.768 kHz crystal?
You can add a TCXO such as Maxim's DS32kHz to the board if you like.
Glue the chip down, preferably on a ground pour (dead bug style- legs in the air).
Remove R214 (0Ω resistor).
There are four ...
12
votes
How can you achieve decent clock accuracy in microcontrollers (e.g., max 1 second drift in a year)? How do digital watch manufacturers accomplish it?
The 16 MHz crystal part in your link has 30 ppm initial tolerance and 50 ppm temperature stability. It has also specified aging of 3 ppm during first year and 1 ppm per year afterwards.
To put that ...
- 164k
12
votes
Why is crystal frequency often multiplied inside a microcontroller?
A crystal is rarely used to directly provide a clock at its native frequency. Its main purpose is to provide a system timebase, so that everything derived from this crystal will agree on the exact ...
- 22.7k
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