The crystal (+C1/C2) is a very narrow bandwidth resonator/filter. Only the fundamental frequency can fit through it.

So it is a sine wave.

Square waves are made square, by all the odd harmonics filling in the gaps until the sine becomes square. No Harmonics = Not Square

[Note crystals do in fact have "harmonics" called overtones, but they are slightly off frquency from each other, so the harmonics of the fundamental do not quite hit the 3rd overtone etc]

Another view is that the crystal is like the wheels of a bicycle rolling down the road. The CMOS invertor driving it, is like your feet and legs. Now you could "stab" at the pedals, and try to make the movement a square wave if you wanted. But the pedals are just going to go round and round smoothly regardless, because the flywheel effect is so large. The crystal is like an enormous flywheel smoothly and sinusoidally rolling around.

The crystal really is like a heavy flywheel. If you suddenly disconnect the drive, the signal will take thousands of cycles to die away. When you turn the oscillator on, it takes thousands of cycles to start, slowly building up amplitude. This is why your processor has an "oscillator startup timer"

The crystal (+C1/C2) is a very narrow bandwidth resonator/filter. Only the fundamental frequency can fit through it.

Sine waves are a single pure frequncy, So it is a sine wave.

Square waves are made square, by all the odd harmonics filling out the hump until the sine becomes square. No Harmonics = Not Square

[Note crystals do in fact have "harmonics" called overtones, but they are slightly off frquency from each other, so the harmonics of the fundamental do not quite hit the 3rd overtone etc]

Another view is that the crystal is like the wheels of a bicycle rolling down the road. The CMOS invertor driving it, is like your feet and legs. Now you could "stab" at the pedals, and try to make the movement a square wave if you wanted. But the pedals are just going to go round and round smoothly regardless, because the flywheel effect is so large. The crystal is like an enormous flywheel smoothly and sinusoidally rolling around.

The crystal really is like a heavy flywheel. If you suddenly disconnect the drive, the signal will take thousands of cycles to die away. When you turn the oscillator on, it takes thousands of cycles to start, slowly building up amplitude. This is why your processor has an "oscillator startup timer"

The crystal (+C1/C2) is a very narrow bandwidth resonator/filter. Only the fundamental frequency can fit through it.

So it is a sine wave.

Square waves are made square, by all the odd harmonics filling in the gaps until the sine becomes square. No Harmonics = Not Square

[Note crystals do in fact have "harmonics" called overtones, but they are slightly off frquency from each other, so the harmonics of the fundamental do not quite hit the 3rd overtone etc]

Another view is that the crystal is like the wheels of a bicycle rolling down the road. The CMOS invertor driving it, is like your feet and legs. Now you could "stab" at the pedals, and try to make the movement a square wave if you wanted. But the pedals are just going to go round and round smoothly regardless, because the flywheel effect is so large. The crystal is like an enormous flywheel smoothly and sinusoidally rolling around.

The crystal (+C1/C2) is a very narrow bandwidth resonator/filter. Only the fundamental frequency can fit through it.

So it is a sine wave.

Square waves are made square, by all the odd harmonics filling in the gaps until the sine becomes square. No Harmonics = Not Square

[Note crystals do in fact have "harmonics" called overtones, but they are slightly off frquency from each other, so the harmonics of the fundamental do not quite hit the 3rd overtone etc]

Another view is that the crystal is like the wheels of a bicycle rolling down the road. The CMOS invertor driving it, is like your feet and legs. Now you could "stab" at the pedals, and try to make the movement a square wave if you wanted. But the pedals are just going to go round and round smoothly regardless, because the flywheel effect is so large. The crystal is like an enormous flywheel smoothly and sinusoidally rolling around.

The crystal really is like a heavy flywheel. If you suddenly disconnect the drive, the signal will take thousands of cycles to die away. When you turn the oscillator on, it takes thousands of cycles to start, slowly building up amplitude. This is why your processor has an "oscillator startup timer"