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bourne
Joined: 07 May 2014
Posts: 10
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| PLL pic18f85J90 |
 Posted: Wed May 07, 2014 8:15 am |
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Hello,
I am trying to make a PLL with a PIC 18f85J90 with a high speed oscillator (quartz at 20MHZ), here is my code:
| Code: |
#include<18f85J90.h>
#device adc=16
#FUSES HS //high speed clock
#FUSES NOWDT
#use delay (osc=40000000) //I want to run the frequency at 40MHZ
void main (void){
setup_oscillator(OSC_PLL_ON);
while(1){
output_high(PIN_B0);
delay_ms(200);
output_low(PIN_B0);
output_high(PIN_B1);
}
}
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I read in the datasheet that the maximum speed frequency for this chip is 40 MHZ (which I wrote in the delay).
My problem is the following, when I decrease the value of the #use delay, my LED are blinking faster than when I let 40000000.
And I also saw that when I use a #use delay crystal at the place of #use delay clock, my LED are blinking faster. I would like to know if my frequency is good increasing (and if the PLL is working or not...)
Finally, I would like to know something. I saw that on many other chips the #fuses allow to activate the pll (with H4 for example) but on this chip, I am able to activate the pll bit but only for the internal clock (at 31 KHZ). Does the line "setup_oscillator (OSC_PLL_ON)" activate the PLL? and if it is not the case, how to activate that bit?
I could see that when I change that line nothing happen.
Finally, do you think my code is correct to make a PLL?
Thanks for your help |
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RF_Developer
Joined: 07 Feb 2011
Posts: 839
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| Posted: Wed May 07, 2014 8:43 am |
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There are a lot of variations of clock/oscillator configurations on PICs. The PIC18f85J90 PLL is quite limited. You can only use the PLL on external high speed crystal, and external oscillator - not with other clock sources, such as internal and low speed.
As on most non-USB PICs, the PLL is four times ONLY. To get the processor to run at 40MHz you have to use a 10MHz crystal or oscillator and enable the PLL by the fuses. You cannot get 40MHz processor clock from a 20MHz crystal. You can run the PIC with a 20MHz crystal, but not with the PLL, and even then not necessarily over the full power voltage and temperature range. |
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bourne
Joined: 07 May 2014
Posts: 10
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| Posted: Thu May 08, 2014 1:36 am |
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hello RF_Developer
Thank you for your response.
I would like to be sure, the PIC 18F85J90 can run at the 40MHZ maximum?or can I run it to 80MHZ (4x20MHZ) with a PLL?
Thank you |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19498
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| Posted: Thu May 08, 2014 2:08 am |
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Read the data sheet. Read the data sheet. Read the data sheet.
Quote (directly from it):
"The HSPLL and ECPLL modes provide the ability to
selectively run the device at 4 times the external oscillating
source to produce frequencies up to 40 MHz." (section 3.4.3).
Up to 40MHz.
4 times the external oscillating source
As RF developer has already told you you can't run the PLL with 20MHz in. |
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bourne
Joined: 07 May 2014
Posts: 10
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| Posted: Thu May 08, 2014 7:01 am |
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Thank you Ttelmah, you were right the datasheet explained everything.
But there is still something that I cannot explain and I need some help to understand:
When I write this code:
| Code: |
#FUSES HS //High speed oscillator
#FUSES H4_SW //PLL ON
#use delay (clock=40000000)
void main (void){
setup_oscillator(OSC_PLL_ON);
while(1){
output_high(PIN_B0); //mise à 1 du bit 0 du port B
delay_ms(200);
output_low(PIN_B0);
output_high(PIN_B1);
delay_ms(200);
output_low(PIN_B1);
output_high(PIN_B2);
delay_ms(200);
output_low(PIN_B2);
output_high(PIN_B3);
delay_ms(200);
output_low(PIN_B3);
}
}
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I can see my LED blinking faster than when i do not write the #fuses H4_SW, I would like to know why, and moreover are they blinking at the crystal's speed?
Thanks for your help |
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bkamen
Joined: 07 Jan 2004
Posts: 1615
Location: Central Illinois, USA
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| Posted: Mon May 12, 2014 5:16 pm |
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You can't run the PIC that fast.
Look at the MAX OPERATING frequency listed in the datasheet.
If you have a 20MHz crystal on the unit and then enable the PLL, what's 20x4?
(I'll let you do the math)
-Ben
_________________
Dazed and confused? I don't think so. Just "plain lost" will do. :D |
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