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dspic33f PLL & frequency tuning...

 
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mohammad3d



Joined: 28 Mar 2009
Posts: 17

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dspic33f PLL & frequency tuning...
PostPosted: Sat Apr 11, 2009 2:26 am     Reply with quote
Hi All,

I found this result when working on dspic33f micro with CCS C compiler.
I try to get 40 MIPS performane with dspic33fj256GP710.
But, when you use XT or HS oscillator with PLL (#fuses HS, PR_PLL),
CCS consider default value of PIC micro for PLL setting.
This means in follow equation(see EQUATION 8-2: FOSC CALCULATION
in dsPIC33FJXXXGPX06/X08/X10 Data Sheet, page 136):

FOSC = FIN* (M/N1*N2)
M=50,N1=2,N2=4
and if you using 10Mhz crustal for FIN :
FOSC=10000000*(50/2*4)=62500000 Hz
and in FCY = FOSC/2 equation:
FCY=62500000/2=31250000(~31 MIPS)

In this status you get only 31 MIPS of micro .
For get maximum performance of dspic33f I modify oscillator setting
manually as follow:
First define special register in CCS as follow(for dspic33fj256GP710):
Code:

#word OSCCON   = 0X0742 //OSCILLATOR CONTROL REGISTER
#word CLKDIV   = 0X0744 //CLOCK DIVISOR REGISTER
#word PLLFBD   = 0X0746//PLL FEEDBACK DIVISOR REGISTER

#bit CLKDIV0=CLKDIV.0//‘N1’, PLL prescaler ->N1=2
#bit CLKDIV1=CLKDIV.1
#bit CLKDIV2=CLKDIV.2
#bit CLKDIV3=CLKDIV.3
#bit CLKDIV6=CLKDIV.6//‘N2’, PLL prescaler ->N2=2
#bit CLKDIV7=CLKDIV.7
#bit CLKDIV11=CLKDIV.11//DOZE Mode Enable bit
#bit CLKDIV12=CLKDIV.12//Processor Clock Reduction Select bits->1:1
#bit CLKDIV13=CLKDIV.13
#bit CLKDIV14=CLKDIV.14

#bit OSCCON8=OSCCON.8
#bit OSCCON9=OSCCON.9
#bit OSCCON10=OSCCON.10

*************************************************************
Second Modify this registers to custom value in main():
*************************************************************
CLKDIV0=0;//‘N1’, PLL prescaler ->N1=2
CLKDIV1=0;
CLKDIV2=0;
CLKDIV3=0;
CLKDIV6=0;//‘N2’, PLL prescaler ->N2=2
CLKDIV7=0;
CLKDIV11=1;//DOZE Mode Enable bit
CLKDIV12=0;//Processor Clock Reduction Select bits->1:1
CLKDIV13=0;
CLKDIV14=0;
OSCCON8=1;//Primary oscillator (XT, HS, EC) with PLL
OSCCON9=1;
OSCCON10=0;

PLLFBD=0b000011110;//‘M’, PLL multiplier->M=32

with this setting you have:
FOSC=10000000*(32/2*2)=80000000Hz
FCY=80000000/2=40000000(40 MIPS)

I test it and this setting work very well.
In next step I try to over clocking dspic33f to get higher performance.
I increase "PLLFBD" register to 0b000100111(M=41) and micro work well but
I have some problem with "#use delay(clock=X)" that just work
until "Clock=98Mhz". You can test this over clocking with different M value
and "#use delay(clock=X)". Then check result with square wave output and see time of execution on oscilloscope.

Best regards,
Mohammad Kouchekzadeh.
Guest
PostPosted: Sat Apr 11, 2009 8:33 am     Reply with quote
it will do the calulations

setup_oscillator(OSC_INTERNAL,80000000); = 40 mips

or

setup_oscillator( OSC_CRYSTAL, 2000000, 8000000); = 40 mips
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