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noyz
Joined: 31 Jan 2009
Posts: 59
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| XT PLL issue ? |
 Posted: Sat Jun 18, 2011 11:04 am |
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Hi all
i have the following problem
i have PIC18F2550 with :
| Code: |
#include <18F2550.h>
#device *=16
#device adc=10
#FUSES NOWDT,XTPLL,FCMEN,NOPUT,NOBROWNOUT,MCLR
#use delay(clock=12 000 000,RESTART_WDT)
#use rs232(baud=115200,parity=N,xmit=PIN_C6,rcv=PIN_C7,bits=8)
#int_TIMER0
void TIMER0_isr (void) {
}
void main () {
setup_wdt ( WDT_OFF );
setup_timer_0 ( RTCC_INTERNAL | RTCC_DIV_64 );
setup_timer_1 ( T1_DISABLED );
setup_timer_2 ( T2_DISABLED, 0, 1 );
// enable_interrupts(INT_TIMER0);
// enable_interrupts(GLOBAL);
while (true)
printf ( "this is working !!! \n\r" );
delay_ms ( 10 );
}
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i have XT clock on board of 12Mhz,
i want to enable PLL to this so it will have 48Mhz, do i have to put
| Code: | | use_delay(48 000 000) |
or just leave it
| Code: | | use_delay(12 000 000) |
with that XTPLL FUSE
and another strange thing
i have my serial port from pc set to 9600 baud and it outputs ok taking into consideration that pic has baud=115200 .. how is this possible ?
i think that is from XT ? |
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noyz
Joined: 31 Jan 2009
Posts: 59
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| Posted: Sat Jun 18, 2011 11:42 am |
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it seems that the clock is actually 1 000 000
i've changed Xtal and the clock value is the same..
i think it is blocked on internal clock use.. only..
normaly when you touch the both pins of xtal with hand.. the mcu stops..
but it doesn't;
what do you suggest ? |
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Ttelmah
Joined: 11 Mar 2010
Posts: 19504
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| Posted: Sun Jun 19, 2011 2:22 am |
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First thing to understand. On the 2550, _the PLL is always enabled_.....
It is used for the USB. XT/HSPLL, says take the CPU clock from the USB PLL source, rather than from the direct crystal route.
Now if your external crystal is 12MHz, you _must_ use HSPLL, not XTPLL. The later enables the low power oscillator, designed for operation to 4MHz max. At 12MHz, you need the HS oscillator.
Then, the PLL, _must_ be fed with 4MHz. So you need PLL3, to say 'divide the incoming clock by 3' to give 4MHz. The PLL won't run if fed with frequencies much different from 4MHz.
You have 'FCMEN' enabled. This means 'if the primary oscillator fails, fall back to the internal oscillator'. Because you have the XTAL/PLL setup wrong, this is what is happening, and why you are seeing 1MHz, and putting your finger near the CPU crystal pins has no effect.
The clock statement, must be set to the frequency the CPU is actually clocked at, not the crystal frequency.
| Code: |
#include <18F2550.h>
//#device *=16 Only needed on PIC16 devices
#device adc=10
#FUSES NOWDT,HSPLL,PLL3,FCMEN,NOPUT,NOBROWNOUT,MCLR,CPUDIV1
#use delay(clock=48MHz,RESTART_WDT) //Much simpler to use MHz.
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Best Wishes |
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noyz
Joined: 31 Jan 2009
Posts: 59
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| Posted: Tue Jun 21, 2011 1:11 am |
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i don't understand where is the calculation for cpu = 48 mhz..
i understood that it is neccesary to have pll activated, and it must be fed with 4 mhz..
so 12 / 3 =4mhz
and CPU .. how it is calculated ?
12 /3 * 12 = 48 :-?
pll clock * hspll = cpu clock ? |
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