What is output of quartz crystal across MCU

[Vindhyachal.takniki]

1.What is output type of quartz crystal we connect across microcontroller? Is it sine, square, clipped sine?

2. I have 16Mhz crystal connected across(MCU) with two capacitors to GND from each leg. MY DSO is 60Mhz & probe is at 10x

3. I tried probing across one leg of probe at Crystal In & another at cyrstal output. It seems sine to me? Or is due to my DSO is less freq? I think DSO should be atleast 5 times for square & 10 times for sine wave.

4. Three probe I tried:

a) One probe at XTIN & another at XTout

b) One probe at XTIN & another at GND

c) One probe at XTOUT & another at GND

[MosherIV]

Hi

It depends on the drive circuit for the Xtal.
Most often for built in micro controllers, the Xtal output is sine wave like but not true sine, it is distorted - often mix of sine and square.

Directly probing the Xtal in circuit will affect the xtal due to the probe capacitance. I only do it to check the oscillator is running.
Most microcontrollers have osc output if you really want to see what freq the xtal is running at.

[Ian.M]

Nearly all MCUs and digital chips that include a crystal oscillator circuit use a Pierce oscillator.  You can bet if its got two pins for a crystal or resonator, it will use the Pierce circuit and one pin will be an output to drive the crystal and the other the feedback input.

An ordinary x10 passive scope probe is unsuitable for measurements on unbuffered crystal oscillator circuits, as even a leading brand, high bandwidth one will present a capacitive load of about 10pF at the tip.  Lower bandwidth and 'off-brand' probes may present considerably higher capacitive loads, 15pF being fairly typical.  This is comparable to the typical load capacitance of the crystal, so frequency pulling is unavoidable, and the additional loading may cause the oscillator to stop, or a marginal oscillator that doesn't run reliably to start.  You can check the frequency and get a rough idea of the amplitude without grossly disturbing it by probing the driven side of the crystal, with a very small capacitor (approx 1pF) in series with the probe tip.  If the series capacitor is exactly 1/9 of the probe tip capacitance, the probe will then act as an AC coupled x100 probe.  To measure the amplitude with reasonable accuracy and be confident the waveform you are seeing is not an artifact of your probing technique, you need a calibrated low capacitance active probe, with *lots* of bandwidth (see below) and a test jig that minimizes the length of the probe ground connection.

Nyquist's criterion states that a repetitive waveform can be correctly reconstructed provided that the sampling frequency is greater than double the highest frequency to be sampled.  Its generally accepted that to reproduce a recognizable squarewave, you need all its harmonics up to at least its fifth.  Therefore, to investigate the output waveform of a 16MHz oscillator, you'd need a scope with over 80MHz analog bandwidth and over 160Msamp/sec equivalent sample rate.  N.B. you'd need considerably higher bandwidth and equivalent sample rate to distinguish between a clipped sinewave and a slewrate limited squarewave

[David Hess]

The output pin from the microcontroller will be a square wave and is low impedance so can be measured as usual.

The other side of the crystal is a high impedance sine wave so measuring it requires a low capacitance probe.  When I have measured it, I added a JFET source follower into the circuit to buffer the crystal from the oscilloscope probe.

[amyk]

You can often pick up the signal from the oscillator, if it is oscillating, by not making contact but simply putting the probe tip in the vicinity of the crystal terminals.

[Vindhyachal.takniki]

So if we dont connect any probe, since it effects the meausrement and distort the signal by loading.

What exactly is shape of signal at XTIN & XTOUT of MCU when a crystal is connected acrss it with each pin having capacitor to GND?



Is it sine, square, distorted sine?

[MosherIV]

Distorted sine

[ambrosia heart]

So if we dont connect any probe, since it effects the meausrement and distort the signal by loading.

What exactly is shape of signal at XTIN & XTOUT of MCU when a crystal is connected acrss it with each pin having capacitor to GND?



Is it sine, square, distorted sine?

If it is a microprosser, it is 95% sine in one leg, the other not.   If it is a decoder chip, it is a sine wave.
See photos.

[ambrosia heart]

3 more photos
If you want to find out which chips are, download  onkyo service manual to find them out.

[T3sl4co1l]

Generally two kinds:

1. High gain, possibly with schmitt trigger.  Square wave output, standard logic output pin.  Needs a series termination resistor to drive the crystal and limit drive power (typically 1.5kΩ or so).  This requires additional parts, but can be used with pretty much anything you can hook onto it: crystals, ceramic resonators, even LC resonators.

2. Limited gain, usually a single stage inverter.  No series resistor is needed.  Gain depends on size of the inverter, which can be too low for some crystals.  ST MCUs for example tend to be on the small side, so need a higher ESR crystal (and smaller loading capacitors) than others do.  The drive strength may also be selectable from software (e.g. TI MSP430s often do), or from hardware (e.g., ATMEGA fuse settings).

The signal level is small (100s mV?), limited by the transfer function of the inverter: gain is highest for small signals, and drops at larger signal levels.  This also causes odd mode distortion, which is why the waveform isn't quite sinusoidal, but a bit squashed.

Tim