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jaime
Joined: 25 Nov 2005
Posts: 56
Location: Porto - Portugal
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| PIC18LF2550 Oscillator configuration |
 Posted: Tue Sep 01, 2009 6:33 pm |
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Hello.
I'm working with PIC18LF2550 and I have some questions that I have difficult to understand.
1-At 5V, Can I use a 16MHZ crystal and put the pic to work at 48MHz or 20MHZ? What instructions must I give?
2-What is the maximum frequency that I can use at 3,3V. Datasheet have figure 28-2 but no formula.
Thanks |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Tue Sep 01, 2009 6:49 pm |
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| Quote: | | At 5V, Can I use a 16MHZ crystal and put the pic to work at 48MHz |
The formula is really very simple. The PLL must have an input frequency
of 4 MHz. So, just use the PLLx fuse to divide your crystal frequency
so that you get 4 MHz. For example, you have a 16 MHz crystal. If you
divide it by 4, then you get the required 4 MHz. So use the PLL4 fuse
to do this. If you have a 20 MHz crystal, then use PLL5. If you have an
8 MHz crystal then use PLL2. It's easy.
Example for running at 48 MHz with a 16 MHz crystal:
| Code: |
#include <18F4550.h>
#fuses HSPLL,NOWDT,NOPROTECT,NOLVP,USBDIV,PLL4,CPUDIV1
#use delay(clock=48000000)
//==========================
void main()
{
while(1)
{
output_high(PIN_B0);
delay_ms(500);
output_low(PIN_B0);
delay_ms(500);
}
} |
| Quote: | | 2-What is the maximum frequency that I can use at 3,3V. Datasheet have figure 28-2 but no formula. |
It can't run at 48 MHz. Interpolating the diagram, it looks like it will
run at about 18 MHz maximum. |
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jaime
Joined: 25 Nov 2005
Posts: 56
Location: Porto - Portugal
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| Posted: Wed Sep 02, 2009 4:53 am |
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| Thanks for your help |
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