Question about Capacitors

[HerbertMunch]

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Can someone please explain why the author of this has chosen these specific cap values?
If I substituted other values would it still work?

Many thanks

 

[Krumlink]

I would check the PIC's Oscilliator Range that IT can handle. I believe that can handle 12 to 32 pf caps. I would check first.

 

[HerbertMunch]

Why the 470n connected to VUSB?
Why the 10n connected between VDD and VSS
Why the 100n Connected between usb1 and usb4?
What about the 1M oscillator resistor?

If I changed these values, what would happen?

Many thanks

 

[blueroomelectronics]

VUSB can be between 0.22uf and 0.47uf it's in the datasheet.
A decoupling cap is usually between 0.01uf and 0.1uf (between VDD, VSS)
The USB 100nf cap is optional, just more filtering
What it does need is a power supply filter cap 22uf thru 47uf would work.
Also recommended is a ferrite bead between the USB power source and VDD.

 

[HerbertMunch]

VUSB can be between 0.22uf and 0.47uf it's in the datasheet.
A decoupling cap is usually between 0.01uf and 0.1uf (between VDD, VSS)
The USB 100nf cap is optional, just more filtering
What it does need is a power supply filter cap 22uf thru 47uf would work.
Also recommended is a ferrite bead between the USB power source and VDD.

Thanks mate, but can you explain how you know that 0.01 - 0.1 uf is right?

Cheers

 

[chipi]

arnt these caps just smothers and protectors and the 1m res is that just a stabliser for the resinator

 

[Speakerguy]

.01uF to .1uF will give low impedance in the range of a few MHz to a few tens of MHz. Since that's roughly in the range of the max frequency of the pic, they are sure to provide low impedance at the high frequencies that the pic operates at. Lower frequencies are taken care of by a much larger on-board electrolytic. The .01uF needs to be located right next to the supply pins. The larger electrolytic can be farther away and can serve for multiple devices, whereas a local bypass cap (.1 to .01uF) is needed at the pins of every device.

I don't know why he has a 1Mohm on the crystal. I've never used one.

 

[HerbertMunch]

Ok nice one cheers mate.

 

[Hero999]

A large value resistor between 1M and 10M is normally used on CMOS crystal oscillators to bias the gates into their linear regions so they work as amplifiers rather than switches. I don't know about this circuit though as I would have thought they would be built-in to the IC.