Setting 8Mhz clock speed on PIC16F684

[LEDave]

Can someone kindly help me please?

I am trying to make a PIC16F684 run at 8Mhz instead of the default 4Mhz.

Problem is, test led's just keeping flashing at 10 secs...

I have MPASM ticked as the Compiler/Assembler and using PBP 2.60

Simple code below:

@ __config _INTOSCIO & _WDT_OFF

DEFINE OSC 8

OSCCON = %01110000 ' Internal 8MHz osc
'OSCTUNE = %00000000 ' not in use
ANSEL = %00000000 'Disable analog select so ports work as digital i/o.
CMCON0 = %00000111 'Disable analog comparators.
TRISA = %00000000 'Set PORTA as OUTPUT.
PORTA = %00000000 'Set PORTA pins all low.


Start

main:

HIGH PORTA.2
PAUSE 10000
LOW PORTA.2
PAUSE 2000
HIGH PORTA.2
PAUSE 10000
LOW PORTA.2
PAUSE 2000

goto main

[richard]

Problem is, test led's just keeping flashing at 10 secs...

PAUSE 10000 is 10 seconds

if the osc is 8 mhz and you tell the compiler the osc is 8 mhz with DEFINE OSC 8
then the pause is 10 seconds as requested

what do you expect the result to be ?

[LEDave]

Hi Richard, thanks for the reply.

"what do you expect the result to be ?"

I thought (and was hoping) that double the OSC speed from 4 to 8Mhz would drive the led's twice as fast, 10secs would become five at 8Mhz if you see what I mean.

Dave

[LEDave]

Just to add, I based my last post on this article: http:// melabs.com/resources/pbpmanual/7_0.htm

But I may have misinterpreted it, in that I thought PAUSE 1000 with DEFINE OSC 4 internal OSC would give one second delay and pause 1000 with DEFINE OSC 8 internal OSC would give 0.5 second delay.....

So are we saying that the above PAUSE 1000 would both cause a one second delay?

Is it only external Xtals/resonators that throw the internal timing off?

Dave

[richard]

Is it only external Xtals/resonators that throw the internal timing off?

no not at all.

the only time it fails is when DEFINE OSC does not match the real osc frequency.

[LEDave]

Thanks again Richard.

So as long as I use DEFINE OSC X, PB knows and compensates for the change still giving the correct timings PAUSE etc?

David

[Dave]

Yes, That is correct. The PAUSE command is based on the OSC frequency. If you had an 4 Mhz. crystal connected and told PBP that the OSC was 8Mhz. the PAUSE command would give you 2 times the value entered into the PAUSE command.

[LEDave]

Dave & Richard, thank you both for taking the time to reply, much appreciated.

Dave