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Configuring external oscillator

 
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safwat



Joined: 08 Feb 2015
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Configuring external oscillator
PostPosted: Thu Feb 12, 2015 12:14 am     Reply with quote

How can i configure an external oscillator (crystal) in pic 16F877A? what is the highest frequency that can be achieved with a crystal oscillator?
safwat



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PostPosted: Thu Feb 12, 2015 12:23 am     Reply with quote

sorry for being trivial but do i have to add anything in the code to initialize the oscillator ?
Ttelmah



Joined: 11 Mar 2010
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PostPosted: Thu Feb 12, 2015 3:02 am     Reply with quote

Just select the HS fuse as the oscillator (make sure you remove all other oscillator fuses - the number of people posting settings with two or more oscillators selected is 'major'). Maximum speed, read the data sheet - typically 20MHz, on these older chips, increasing to 25MHz on newer models.
safwat



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PostPosted: Thu Feb 12, 2015 7:31 am     Reply with quote

so i can get a maximum PWM frequency of maximum 5000 Hz right? what if i want to increase the PWM frequency much higher than that?
temtronic



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PostPosted: Thu Feb 12, 2015 8:00 am     Reply with quote

You really should read the datasheet section about the CCP module. There lies specific information that you need to know as well as charts and figures.
The obsolete 877 has a max of 20MHz for the xtal.

jay
Ttelmah



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PostPosted: Thu Feb 12, 2015 8:54 am     Reply with quote

As Temtronic says, 'read the data sheet'. You can run the PWM, over 20KHz, on a 4MHz oscillator. What you lose is resolution.

On 20MHz, at full resolution, the PWM can go to just under 20KHz. You have a factor of 4 involved somewhere that doesn't exist. Look at table 8-3 in the data sheet.

Remember also, that this is a very old chip. Most newer ones allow faster oscillators, and often PLL's as well. There are PIC16's that go to 48MHz. These offer full PWM resolution up to 46.875KHz.
safwat



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PostPosted: Thu Feb 12, 2015 10:26 am     Reply with quote

yeah thanks both of you. table 8.3 solved the problem
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