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theteaman
Joined: 04 Aug 2006
Posts: 98
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| PLL 40MHz |
 Posted: Thu Nov 02, 2006 6:37 am |
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Hello
Currently I'm using an 18F2525 with a 20MHz crystal (20/4=5MHz internal clock).
I want to make the PIC as fast as it can go. If I use a 10MHz crystal and enable PLL, I can get 40MHz correct? And is this equal to a 10MHz internal PIC clock because 40/4=10?
If so, I am wondering how to do this. I set the H4 fuse and enabled the PLL bit in the OSCTUNE byte, but my PIC ends up behaving really slow... obviously there is something wrong - I don't know how to setup the PLL mode properly. Can someone help?
Thanks |
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Ttelmah
Guest
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| Posted: Thu Nov 02, 2006 7:26 am |
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In a sense, yes, but I disagree with your terminology. 
The internal clock, is 40Mhz. The _instruction clock_ is then 10MHz. This is important, since some things like the PLL, can access the 'raw' internal clock.
You do not need to touch OSCTUNE. When using the HSPLL (H4), the PLL is enabled in hardware. PLLEN in OSCTUNE, is only used, when working with the _internal_ oscillator. If you look at the oscillator block diagram (Figure 2-8 in the data sheet), it shows how this works. Turning this bit 'on', with HSPLL selected, is not legitimate, and is probably resulting in an unexpected clock behaviour.
Best Wishes |
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jma_1
Joined: 08 Feb 2005
Posts: 147
Location: Wisconsin
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| Posted: Thu Nov 02, 2006 7:36 am |
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Greetings,
I would set up the oscillator as listed below, but I would check that the configuration register is setup correctly according to the HSPLL mode of the datasheet. CCS does not use this wording in their #fuses.
For a 40 MHz frequency using a 10 MHz crystal:
#use delay(clock=40000000)
#fuses H4
If you view the fuses with the windows IDE version, it lists 'H4' as the 4X pll option.
Hopefully this helps. If not, double check that your oscillator is really switching at the right frequency. High frequency oscillators and bread boards do not always play nice together.
Cheers,
JMA |
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sjbaxter
Joined: 26 Jan 2006
Posts: 141
Location: Cheshire, UK
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| Posted: Thu Nov 02, 2006 7:58 am |
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Remember .... the H4 fuse won't actually any effect until you program the PIC then CYCLE the PIC's power !! so don't worry if you don't see any speed improvements straight away.
_________________
Regards,
Simon. |
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theteaman
Joined: 04 Aug 2006
Posts: 98
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| Posted: Thu Nov 02, 2006 3:44 pm |
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Wow you guys are really helpful, thanks for the replies!  Just one more question (probably gonna sound stupid now)... what do you mean by cycling the power of the PIC?
Last edited by theteaman on Thu Nov 02, 2006 3:47 pm; edited 2 times in total |
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newguy
Joined: 24 Jun 2004
Posts: 1909
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| Posted: Thu Nov 02, 2006 3:46 pm |
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| Turn it off & back on. |
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theteaman
Joined: 04 Aug 2006
Posts: 98
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| Posted: Thu Nov 02, 2006 3:47 pm |
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| thanks, i had a feeling it was that |
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theteaman
Joined: 04 Aug 2006
Posts: 98
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| Posted: Wed Nov 08, 2006 5:06 am |
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| Code: |
#use delay(clock=40000000)
#fuses NOWDT, WDT128, H4, NOPROTECT, NOIESO, NOBROWNOUT, BORV21, PUT, NOCPD, NOSTVREN, NODEBUG, NOLVP, NOWRT, NOWRTD, NOEBTR, NOCPB, NOEBTRB, NOWRTC, NOWRTB, NOFCMEN, NOXINST, NOPBADEN, NOLPT1OSC, NOMCLR |
Hi again. When I use the above code with a 10MHz crystal, I get a really slow operation, ie. the delay_ms function delays alot longer than it should..... could there be any conflicting fuses with the H4?
Thanks |
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btklister
Joined: 16 Jul 2009
Posts: 14
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| Posted: Thu Mar 01, 2012 5:04 pm |
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It also happens to me, anyone knows where is the problem? It only happens if it was used delay funcions, in normal code like
| Code: |
while(1)
{
output_high(pin_b1);
output_low(pin_b1);
}
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no problems are detected.
My configuration is:
| Code: |
#include <18F46K20.h>
#device adc=16
#FUSES NOWDT //No Watch Dog Timer
#FUSES WDT128 //Watch Dog Timer uses 1:128 Postscale
#FUSES H4 //High speed osc with HW enabled 4X PLL
#FUSES PUT //Power Up Timer
#FUSES NOBROWNOUT //No brownout reset
#FUSES NOPBADEN //PORTB pins are configured as digital I/O on RESET
#FUSES NOLPT1OSC //Timer1 configured for higher power operation
#FUSES NOHFOFST //High Frequency INTRC waits until stable before clocking CPU
#FUSES NOLVP //No low voltage prgming, B3(PIC16) or B5(PIC18) used for I/O
#FUSES NOXINST //Extended set extension and Indexed Addressing mode disabled (Legacy mode)
#use delay(oscillator=10Mhz, clock=40Mhz) |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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