Jim wrote:

>  BODY { font-family:Arial, Helvetica, sans-serif;font-size:12px; } =20
>
>	Hi guys,=20
>
>	I am using a pic18F4553 in a project and I am having problems with
>the oscillator.=20
>
>	I am trying to use the internal 8MHz clock PLL, prescaler etc (which
>works nicely clocking at 32MHz), but when it comes to working with the
>USB I am failing miserably.=20
>
>	All of the Microchip documentation seems to contradict itself in
>each section when discussing the USB clock.=20
>
>	My question is this:- =20
>
>	Can i use the 8MHz internal oscillator to drive the 48MHz/6MHz
>needed for the on-chip USB peripheral, and if so, what settings would
>you use?=20
>
>	Here is what I am using (I have documented the config settings) =20
>
>	(MPLab 8.83, Hi-tech C 9.65)=20
>
>	 // 8Mhz internal oscillator producing 48MHz USB clock and 48MHz CPU
>clock
>      // clk src from 96MHz PLL/2, =20
>      // Internal External Switch Over Mode disabled
>     // Fail-Safe Clock Monitor Disabled
>    // INTOSC: INTOSC+CLK0{RA6}, USB-EC=20
>    // CPU No divide
>    // OSC Select div by 2 {8MHz internal input}=20
> __CONFIG(1, USBPLL & IESODIS & FCMDIS & INTCLKO & CPUDIV1 &
>PLLDIV2); =20
>	       // USB Voltage Regulator Enabled
>        // Power Up Timer Disabled
>        // Brown Out Detect Disabled (2.0v)
>        // Watchdog Timer Disabled (1:32768)
> __CONFIG(2, VREGEN & PWRTDIS & BORDIS & BORV20 & WDTDIS & WDTPS32K);
>	      // PortB A/D disabled (digital)
>       // Low Power Timer1 Osc Disabled
>      // Master Clear Enable
> __CONFIG(3, PBDIGITAL & LPT1DIS & MCLREN);
>	     // Disable extended instruction set (Legacy mode)=20
>      // Stack Overflow Reset Disabled
>      // Low Voltage Program Disabled
>      // Dedicated In-Circuit Port {ICD/ICSP} Disabled
>      // Background Debug Enabled
> __CONFIG(4, XINSTDIS & STVRDIS & LVPDIS & ICPORTDIS & DEBUGEN);
>	      // Protect bits Unprotected
> __CONFIG(5, UNPROTECT);
> __CONFIG(6, UNPROTECT);
> __CONFIG(7, UNPROTECT);
>	Many Thanks,=20
>
>	Jim
> =20
>
Jim,
I just glanced at the datasheet for the 4550 and found this:

2.2.5 INTERNAL OSCILLATOR BLOCK
The PIC18F2455/2550/4455/4550 devices include an
internal oscillator block which generates two different
clock signals; either can be used as the microcontroller=92s
clock source. _If the USB peripheral is not used_, the
internal oscillator may eliminate the need for external
oscillator circuits on the OSC1 and/or OSC2 pins.

And looking at figure 2.1 on page 24, you see that the source of the USB=20
clock can't be fed from the Internal Clock. It can only be derived from=20
the Primary Osc.

Bill


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