Configuring external oscillator

safwat · Joined: 08 Feb 2015 Posts: 20

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 · Joined: 08 Feb 2015 Posts: 20

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

Ttelmah · Joined: 11 Mar 2010 Posts: 19457

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 · Joined: 08 Feb 2015 Posts: 20

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 · Posted: Thu Feb 12, 2015 8:00 am

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 · Joined: 11 Mar 2010 Posts: 19457

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 · Joined: 08 Feb 2015 Posts: 20

yeah thanks both of you. table 8.3 solved the problem