20MHZ External Oscillator Settings

[mackrackit]

I am not sure if measuring the ADC time is the best test. There are other things that effect the ADC speed.

You might try setting up an LED to blink at 1 Hz then test.

The settings are for the MCU speed, not just USB.

[Charles Linquis]

I guess that no one reads my posts. I have mentioned numerous time that you can do a superb job of measuring your PICS actual speed by
using a routine such as:

DEFINE OSC X

LEDON

For X = 1 to 60
PAUSE 60000
Next X

LED OFF

Of course, you don't have to time for a full hour, but the accuracy gets better the longer you wait.
If have

If you use DEFINE OSC 20 and the LED stays on for 31 minutes, you are actually running at

(60/31) * 20MHz = 38.7096Mhz.

Just make sure you don't have interrupts running when you do the timing.

[cncmachineguy]

charles I read them with great intrest!!

[ShaneMichael]

Thanks for the speed test, I'll give it a try and post my results.

[ShaneMichael]

I tried the speed test to check code execution time at 24Mhz and 48Mhz. I left the DEFINE OSC 20 for both tests. I used pause loops like Charles suggested, and used 10 minute long tests. I know longer would be better, but I think 10 minutes should show any difference. Both times the LED was on for 10 minutes, the best I could measure with a clock second hand.


I only changed the _CONFIG1L setting "_CPUDIV_OSC1_PLL2_1L" and _CPUDIV_OSC3_PLL4_1L between the tests. So I'm still scratching my head, seems if the CPU was running at 48Mhz but my define is at 20Mhz, it should have taken 4.1 minutes. So it seems the clock speed for code execution is derived directly from the external XTAL?

My A/D sampling frequency is now fast enough for my needs, as long as I keep the rest of my code in check I should be OK. I'd still like to understand why this is happening (or not happening)???
Thanks
Shane

Here is the code I used:

                   '_CPUDIV_OSC1_PLL2_1L '48 Mhz
                    '_CPUDIV_OSC3_PLL4_1L '24 Mhz
                    
DEFINE LOADER_USED 1
 asm
          __CONFIG  _CONFIG1L, _PLLDIV_5_1L & _CPUDIV_OSC1_PLL2_1L & _USBDIV_2_1L
          __CONFIG  _CONFIG1H, _FOSC_HSPLL_HS_1H
          __CONFIG  _CONFIG2L ,_BOR_ON_2L & _BORV_2_2L
     
        
    endasm
    
 DEFINE OSC 20
 
X VAR  Byte    
  
TRISC.0 = 0          
          
Portc.0 = 0
 
For X = 1 to 10   'Turn LED on for 10 min
 PAUSE 60000
 Next X
 
 Start:
 Portc.0 = 1        'Start Toggling LED
 For X = 1 to 10
 PAUSE 100
 Next X
 Portc.0 = 0
 For X=1 to 10
Pause 100
Next X
Goto START

[mackrackit]

http://www.picbasic.co.uk/forum/showthread.php?t=5806&p=68359#post68359