External Crystal Oscillator

[AMSA84]

Hi,

I have a doubt here regarding the crystal oscillator for a PIC Micro-controller.

If the pic can work at, for example, an external oscillator of 48MHz, can I use a 20 MHz external oscillator instead? The PIC18F2250 works with internal 8 MHz oscillator or external 48 MHz oscillator. Instead using a 48 MHz oscillator, can I use the 20 MHz?

Best regards.

 

[cameo_2007]

The timing requirement section in the data sheet says that external Oscilaltor ratings can go from DC upto 48MHz. So that means 20MHz should work. Give a try.
I have not personally done.

.

 

[AMSA84]

Hi,

Thanks for your reply.

I have searched but couldn't find it. Maybe it passed along and I didn't notice. I will try then.

Best regards.

 

[imbichie]

Instead using a 48 MHz oscillator, can I use the 20 MHz?

Surely it will work...

I have done it in my project.

 

[wp100]

Hi,

Also think you are mixing up two different things.

You can attach a Crystal and caps to the Osc Pins of the 2550 and its internal circuitry will produce a matching frequency.
You can use any crystal from 4 to 25 mhz -see table 28.8 in the 2550 datasheet.

An External Oscillator is a complete circuit producing at square wave and probably using a crystal at its timebase.
Because its a ready to use signal the Pic chip can run on any freqency from Dc to 48mhz.

The Internal oscillators can produce varing times from 32k to 8mhz and will generally meet all but the most critical program timings which generally are on very high speed comms like usb. tcp etc.

Don't forget that many chips like the 2550 also have PLL where you can multipy the frequency to a max of 48mhz.

 

[AMSA84]

Yes, but the thing is that I have only a 20Mhz XTAL (Quartz) and the 18F2550 works at 48Mhz. So my doubt was about if I could use a 20Mhz instead of 48Mhz.

Best regards.

 

[wp100]

Yes, but the thing is that I have only a 20Mhz XTAL (Quartz) and the 18F2550 works at 48Mhz. So my doubt was about if I could use a 20Mhz instead of 48Mhz.

Best regards.

Hi,

Yes it will work at any crystal from 4 mhz to 25 mhz so there is no prblem with your 20mhz crystal and a couple of capactiors as shown in the data sheet.

48mhz is just the maximum speed , not the minimum.

Why use a crystal anyway ..? - with the internal oscillator you have speeds up to 8 mhz, 32mhz if used with the PLL.

I use the internal oscillator all the time on my projects, why use extra components ?