Setting up Internal Oscillator

[Guest]

Hi

How do I set up the internal oscillator to the desired frequency?
I am using PIC 16F819. As per the data sheet internal osc freq are
31K,250K,500K,1M & 2M,4M,8M

But when I use the Command #use delay(clock= ) , I found that , clock is not changing for the different values that I had used

Is it that When Internal Clock is selected , PIC will be at only one freq?

Any relevant info would be of great help

Thanks & Best regards,
Raghu PV

[Haplo]

From the device datasheet:

[Guest]

Hi Ali

Thanks for the info on the fuses
I will try it out
Btw , say to set up 1MHz, is it correct to use the command
#use delay(clock=1000000) ?
If not , what is the function to be used for setting up the above internal clock

Thanks again & Best regards,
Raghu

[Haplo]

You have to use the INTIO2 fuse, and for a 1MHz operation you need to write the value of 0b01000000 (0x40) to the OSCCON register. I'm not sure if CCS has any special function for this purpose but the following code will do the job:

#byte OSCCON=0x8F
OSCCON=0x40;

I recommend you to take a look at sections 4.5.3 and 4.5.4 of the datasheet.

By the way, the '#use delay' preprocessor does NOT perform any frequency setup on the PIC, it just tells the compiler how fast your crystal is, for delay (and some peripheral timings) generation purposes. You still have to indicate a #use delay(clock=1000000), but it is not enough. The correct fuse and OSCCON values need to be chosen too.

[SteveS]

--oops haplo posted just before me - sorry for some repeat....

The 'delay' command is used to tell the compiler what clock you intend to use so it can generate correct code to set bauds, run delays, etc. It doesn't directly affect the oscillator settings.

Here's what I used in the 18F1320 to get an internal 8Mhz clock:

#include <18F1320.h>
#use delay(clock=8000000) // tell compiler what we intend to use
#fuses INTRC, FCMEN, BROWNOUT, BORV20, STVREN, NOLVP, NOIESO, MCLR, NOWDT
#BYTE OSCCON = 0xFd3
// NOTE: I'm sure the register adr is different for you

and in main:

OSCCON = 0x72; // boost up to 8Mhz
// The bit pattern may be different too...

Newer versions of the compiler have the SETUP_OSCILLATOR function (see the 'readme' file) that takes care of this more transparently.

- SteveS

[Guest]

Thank you very much for all the reply

I will try it & post the findings tomorrow

Thanks a lot again

Thanks & Best regards,
Raghu

[Guest]

Yes my internal clock is working fine now after setting the fuses & OSCCON

Ali & Steve, Thanks a lot for your help

Thanks & best regards,
Raghu

[ThomasC]

is there any command to use so we can display the real clock speed out of oscon to make sure its correct?

[Ttelmah]

OSCON, does not hold a 'real clock speed', it holds a 'trim' factor, applied to adjust the RC osillator.
The best ways of finding out if your timing is right, are simple, and depend on the chip involved. Simplest, just setup a program to output a level change, and wait for (say) 100 seconds, then change the level again, and time this with a stopwatch. Alternatively, if the chip has a CCP, and you have a frequency meter, output a frequency with the PWM, and measure this. The value that is written into the OSCON register, is available in the top memory location of the chip, and can be read by your programmer. Some programmers (Mach-X for example), have the ability to test the oscillator frequency, and reprogram this value, but the accuracy is limited since the frequency actually developed changes with the supply (different when the chip is in circuit), and with temperature...
Because of this, no factor will every be 'right', they may just be slightly 'better' for you.

Best Wishes

[ThomasC]

Is there a way to change it on the fly

for example:

[Ttelmah]

Do a search on "Load Frequency Oscillation from EEPROM". PCM Programmer posts an example of how to do this. See in particular the comment about how #use delay works.

Best Wishes