Hot answers tagged

61

If you think of a crystal as being a tiny bell, it's easy to see how, if you hit it with a tiny little hammer, it would ring with a pure tone just like a big bell would if you hit the big bell with a small hammer. That's exactly what a crystal does, but the trick is that it's made of piezoelectric material which makes electricity when you hit it and changes ...


58

The frequency of a real time clock varies with the application. The frequency 32768 Hz (32.768 KHz) is commonly used, because it is a power of 2 (215) value. And, you can get a precise 1 second period (1 Hz frequency) by using a 15 stage binary counter. Practically, in majority of the applications, particularly digital, the current consumption has to be as ...


51

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 particularly useful when you want to generate a signal to be displayed on such a TV.


50

Both ceramic resonators and quartz crystals work on the same principle: the vibrate mechanically when an AC signal is applied to them. Quartz crystals are more accurate and temperature stable than ceramic resonators. The resonator or crystal itself has two connections. On the left the crystal, right the ceramic resonator. Like you say the oscillator ...


39

The devices with two pins are not oscillators, they are resonators (crystals), which can be used in an oscillator circuit (such as a Pierce oscillator), and if used with the correct circuit will oscillate at (or near) the marked frequency. The Pierce oscillator circuit, shown below, uses two capacitors (load capacitors, C1/C2), the crystal (X1), and an ...


34

May not oscillate. May not oscillate reliably. May have low to very low output. May oscillate on wrong frequency (may be an undertone or an overtone). For extra points may jump between modes (not usual) May start up very slowly. And/or not always. May produce less sinusoidal waveform than expected. May shatter crystal (not usual in modern applications). ...


32

Your placement is fine. Your routing of the crystal signal traces is fine. Your grounding is bad. Fortunately, doing it better actually makes your PCB design easier. There will be significant high frequency content in the microcontroller return currents and the currents thru the crystal caps. These should be contained locally and NOT allowed to flow ...


29

I'd buy into the answer on the Arduino Forum: The original ATmega8 Arduino ran at 16MHz, which was the top rated clock speed for the ATmega8 cpu used. When "upgraded" to ATmega168 (with a 20MHz top cpu speed), the clock was left at 16MHz (probably) because the designers thought that more people/code would have backward compatibility issues with a ...


28

Crystal oscillators are much more accurate, they are small, have low temperature coefficients and low drift at a low cost.


28

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 generate a faster clock that is an exact multiple of some reference clock (for example, an external crystal). You do that by having a voltage-controlled oscillator (...


25

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 communication. USB requires a fairly high-accuracy clock. A mouse doesn't need to track real time, and there is no reason for it to be in sync with the CPU ...


23

The number 32768 is a power of 2, i.e. it is 2^15. If you have a 32.768kHz clock frequency it is easy to divide it to an 1Hz frequency using binary frequency dividers, a.k.a. binary counters, i.e. chains of flip-flops. Having a 1Hz frequency means you have a clock signal which provides 1s time resolution: count the seconds with a counter, do the math and ...


23

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 the specified frequency - not even good enough for RS-232. There are ways to synchronize it to an external clock, I think it's designed to be used in a USB device ...


23

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 "Pierce oscillator". The frequency of oscillations is defined by a sharp slope of the electromechanical resonator (crystal), while the gain control is based on ...


21

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 5717 is a custom CMOS mouse controller for the Commodore 64. It will be housed in the body of a two button mouse, enabling it to be plugged into ...


19

A crystal does not oscillate on its own. You don't simply apply power and get oscillations out. Think of a crystal as a very accurate and sharp frequency filter. You put it in the feedback path of a amplifier in the right way, and it causes the circuit to oscillate at the crystal's resonant frequency. It's the circuit that causes the oscillations. They ...


18

If the AVR or any MCU has a way to output the clock to an external pin, even if it divides it down, I would measure that pin. Measuring at a clock pin will interfere with the clock frequency due to capacitance of the scope probe. This way it’s buffered.


18

Two main reasons I can think of are... You want to run at a higher clock speed than available with the internal oscillator. For the Atmega16 chip you mentioned, the internal oscillator has maximum speed of 8MHz, but you can run the chip at up to 16MHz with an external crystal. You need precise and accurate timing. With some effort you can calibrate the ...


17

A quartz crystal is a mechanical resonator with particularly stable properties. Quartz is a very stable material -- it doesn't 'age', or change much with temperature. It is also possible to prepare quartz to be very pure and have consistent properties. Quartz is also slightly piezoelectric -- an electric field causes a deflection, and a deflection generates ...


16

Actually crystal oscillators can easily go up to 10's of MHz. Above that in most cases a PLL (Phase Locked Loop) is used, which is an oscillator that is not very accurate in itself, but can be tuned (its frequency can be adjusted somewhat). The frequency of this high-frequency oscillator is divided by a suitable factor (dividing a signal by a power of 2 is ...


15

To answer your question, a resonator is essentially a low-budget crystal. An oscillator is an amplifier circuit, with feedback so that it oscillates, and a "frequency determining element" that keeps it oscillating at the desired frequency. A crystal can be made for a precise frequency, and it will drift very little if the temperature or stray capacitance ...


15

You are actually building an oscillator circuit, using a crystal, some capacitors, and the internal circuitry of your microcontroller. If you don't use the caps, your clocking won't work. The values are calculated based on the properties of the crystal, as described in http://ww1.microchip.com/downloads/en/appnotes/00826a.pdf, page 9 Essentially, the load ...


14

The main reason is that a 1 Hz crystal would have to be physically very big. A crystal is a piece of quartz that mechanically vibrates at the specific frequency. Since quarts exhibits a fairly strong piezo-electric effect, those vibrations also cause electrical signals and vice versa. Getting a physically small crystal down to 33 kHz resonant frequency ...


14

Your problem is almost certainly not a direct issue with the solder itself but with the flux in the solder. Many fluxes leave a conductive film on the board which must be removed for circuits such as yours. Note that RTC chips try to be extremely low power so as to preserve the battery as much as possible. As such, the biasing of the 32.768KHz crystal for ...


14

Your layout looks a lot worse in the screen shot than it actually is with the ground pour, however I'd still try to isolate the ground return from the load caps and have it go right back to the ground on the chip. I'd also like to see a ground pour under the caps and crystal (maybe you have one) and connected to the same ground point as the load caps. That'...


14

It's primarily due to cost. These particular crystals are dirt cheap due to the watch industry. This answer provides more detail, here's an excerpt: There are 1.2 billion watches sold each year. The majority of them are inexpensive digital watches, requiring a small, 32kHz crystal. ... As a result, these crystals are extraordinarily inexpensive... [...


14

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 response of the filter formed around the crystal, the output impedance of the silicon and the capacitor at the output. I'm thinking Pierce oscillator topologies here ...


14

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 frequency. This signal is low impedance. The load capacitance is that capacitance you put on the other side of the crystal to cause a 180° phase shift. The ...


13

One reason may be more mechanical than electrical. Crystals are mechanically vulnerable, and may easily be damaged by shock and (ironically) vibration. Fixing the housing to a larger mass may reduce the effects of those. Note that, while most likely there will be electrical contact between case and ground in the second picture, I would not rely on it ...


13

You could do a 3:1 gear ratio and do the divisor at 32768. 32768 = 10,923 + 10,923 + 10,922 which indicates a state machine that first counts to 10,923 repeats and then drops a count, it would be accurate every 3 seconds. The worst absolute error you would see is 31 PPM which is about what the crystal can do (depending on your crystal).


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