PIC18F4550 Frequency of Oscillation Question

Micro9900 · Nov 3, 2011

Hey guys, I looked over the data sheet and I found some sample code for what I believe is the proper configuration to have the microcontroller oscillate at 48MHz.

Hardware set up:

I have a 20MHz crystal oscillator connected across pin 14 (OC1) and 13 (OC2). Then, I have two ceramic 22pF capacitors going from each pin to ground.

**broken link removed**

The configuration using MPLAB C18 (for programming in C)

Code:

#pragma config PLLDIV   = 5         // 20Mhz external oscillator
#pragma config CPUDIV   = OSC1_PLL2 //determines frequency division  
#pragma config USBDIV   = 2         // Clock source from 96MHz PLL/2
#pragma config FOSC     = HSPLL_HS

Basically my question are as follows:
__________

1. Are the connections right?
2. Are the components (crystal and capacitor) correct?
3. If I have delays set for 1MHz FOSC, will I now have to adjust for 48MHz?
4. Do I need to further configure pins 14 (RA6/OC1) and pin 13 (RC0)?

The main reason for all of this is that I would like to increase the range of delays I can apply, since devices such as the DS18B20 temperature controller requires 1uS delays. Additionally, I may look into USB communication (I found a tutorial), and I would like to have this feature working. Any help in this matter is greatly appreciated. Thank you.

EDIT:
I have an LED connected and I am able to get it to blink when I use the following configuration:

Code:

#pragma config WDT=OFF, LVP=OFF, DEBUG=ON, MCLRE = ON
 
//Internal oscillator, port function on RA6, EC used by USB 
#pragma config FOSC = INTOSCIO_EC //1MHz oscillation

But, nothing happens (not even a flicker) when I run this code:

Code:

//turns watch dog timer off, turn low voltage programming off
#pragma config WDT=OFF, LVP=OFF, DEBUG=ON, MCLRE = ON
 
//oscillator configurations see pg. 25 /430 of pic18f4550 data sheet
#pragma config PLLDIV   = 5         // 20Mhz external oscillator
#pragma config CPUDIV   = OSC1_PLL2 //determines frequency division  
#pragma config USBDIV   = 2         // Clock source from 96MHz PLL/2
#pragma config FOSC     = HSPLL_HS

#include <p18f4550.h>
#include <delays.h>
 
//Power LED (PortA)
#define LEDPin LATAbits.LATA0 //Define LEDPin as PORT A Pin 1

void main()
{
	while(!OSCCONbits.IOFS);      //wait for osc stable
   	ADCON1 = 0x0F;                //make RA0 digital
 
        //data direction registers all 0's mean that all pins are set to output
	//all 1's means that all of the pins are set to operate as inputs
	TRISA = 0x00;  
	TRISB = 0x00;
	TRISC = 0x00; 
	TRISD = 0x00;
	TRISE = 0x00;

        while(1)
	{
			LEDPin = ~LEDPin;//Toggle LED Pin
			//Delay 250K cycles (1 second at 1MHz since each instruction takes 4 cycles) should blink faster now
			Delay10KTCYx(25);

         }//end main while loop        
}

atferrari · Nov 4, 2011

Look into the specs

Look into the specs for the PLL .

There you should find the value (max - min) of the crystal that you could use to be multiplied *4 to finally get what you want. Most common 4 to 10 MHz.

If I recall right, that limit of 48MHz was valid within a limited temperature range.

Micro9900 · Nov 4, 2011

The tutorial here recommends a 20MHz crystal: https://www.waitingforfriday.com/index.php/Building_a_PIC18F_USB_device

The calculations, I believe are correct. Hmm, I'm not too sure what the problem could be... The only difference is that I am using a 22pF capacitor instead of a 15pF capacitor (I hope that's not the problem).

From data sheet:

...It is designed to produce a
fixed 96 MHz reference clock from a fixed 4 MHz input.
The output can then be divided and used for both the
USB and the microcontroller core clock.

So, using 20MHz and a PLLDIV=5 results in 4MHz (20/5)

Here is a better description of the configuration:

Code:

FOSC = HSPLL_HS                 //HS with PLL enabled 
PLLDIV = 5	                            //PLL prescaler 20/5=4 MHz (96MHz PLL needs 4 MHz from prescaler)
config CPUDIV = OSC1_PLL2  // PLL divided by 2 (96/2=48MHz) 
config USBDIV = 2	           //if full speed (FSEN=1) USB is fed by PLL

Also, found some info on the capacitor:

...Higher capacitance increases the stability
of oscillator but also increases the
start-up time.

EDIT:

I tried this:

Code:

#pragma config WDT=OFF, LVP=OFF, DEBUG=OFF, MCLRE = ON
#pragma config FOSC = INTOSCIO_EC 
void main()
{
...
	OSCCONbits.IRCF2 = 1;
	OSCCONbits.IRCF1 = 1;	
	OSCCONbits.IRCF0 = 1;
...
}

With the above snippet of code, I am able to have the internal oscillator at 8MHz. But, I am still a bit confused as to why the internal oscillator did not change to 4MHz and instead seemed to have not worked at all when I was using the previous bit of code. Either way with the OSCCON bits specified doesn't affect the other configuration, which sets FOSC = HSPLL_HS. So, the previous bit of code still doesn't work even when configuring OSCCON bits.

atferrari · Nov 5, 2011

Maybe here...

Would this help you?

https://www.mikroe.com/forum/viewtopic.php?t=10646

Post the outcome, please.

Micro9900 · Nov 5, 2011

Thank you for the response, but I'm not too sure what I should change from the link posted above. I'm using MPLAB, so I know the code that I posted above is acceptable in terms of syntax and use of variables. I'm not sure if I need to set the OSCCON bits to match the 4MHz as well? What if I wanted 8MHz instead?

EDIT:

PLL Prescaler (see Datasheet->Figure 2-1) will then divide 20MHz by 5 and produce 4MHz (needed on 96MHz PLL input).

If you choose HSPLL oscillator (check _FOSC_HSPLL_HS_1H flag, uncheck _FOSC_INTOSC_HS_1H and _FOSC_HS_1H if checked)

In this case CPU will use 96MHz HSPLL divided by PLL Postscaller, set CPUDIV flags to choose the divider.
_CPUDIV_OSC1_PLL2_1L will divide 96MHz by 2 and CPU will get 48MHz [you need to set Clock field in EditProject to 48MHz]
...
_CPUDIV_OSC4_PLL6_1L will divide 96MHz by 6 and CPU will get 16MHz [you need to set Clock field in EditProject to 16MHz]
Note: PLLx part of the flag is relevant, ignore the OSCx part of the flag

Unfortunately, this what I have already posted:

Code:

FOSC = HSPLL_HS                 //HS with PLL enabled 
PLLDIV = 5	                            //PLL prescaler 20/5=4 MHz (96MHz PLL needs 4 MHz from prescaler)
config CPUDIV = OSC1_PLL2  // PLL divided by 2 (96/2=48MHz) 
config USBDIV = 2	           //if full speed (FSEN=1) USB is fed by PLL

EDIT: It could be the capacitors I am using I try to order some 15pF caps and see if it affects the circuit. I'll post back if it works or fails. But again, thank you for the responses.

PIC18F4550 Frequency of Oscillation Question

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