HS 25Mhz vs. H4 10Mhz

cfernandez · Joined: 18 Oct 2003 Posts: 145

Hi,

I test in my design with a PIC18F8720 the time elapsed for my application with the follow configurations:

1) HS with 25Mhz Crystall
2) H4 with 10Mhz Crystall (40Mhz)

and the results are that the opcion 2) is more fast!!!.

My question is: Is really more fast H4 10Mhz (40Mhz internall) that HS 25Mhz??

What you think????

Best Regards,

newguy · Joined: 24 Jun 2004 Posts: 1907

The 10 MHz external crystal with the x4 pll enabled means that the processor runs at 40 MHz, which is faster than the 25 MHz without the x4 pll as you observed.

Darren Rook · Joined: 06 Sep 2003 Posts: 287 Location: Milwaukee, WI

Yes but the 18F8720 has a bug and doesn't work properly over 25Mhz.

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

The early silicon had problems running over 4MHz. I haven't heard of a bug running over 25MHz? The emulator will not run over 25MHz though.

Trampas · Joined: 04 Sep 2004 Posts: 89 Location: NC

Did you all hear that? That was the light going on....

When the 18F8720 first came out I had bug where the GIE bit would magically get cleared. I called Microchip for weeks trying to figure out why, they kept telling me it was my problem. I cursed them up one side and down the other, then switched to a TI processor.

Now it makes since since this was I think one of the first releases of the chip and I was runing at 10Mhz x 4. I guess they may have found their problem.

Trampas

Mark · Joined: 07 Sep 2003 Posts: 2838 Location: Atlanta, GA

Basically it was a core problem that certain code sequences would cause strange results. It was totally unpredictable. Not all processors were affected. As a result, we have a hundred or so lighting controllers out there that have failed. Mad

Haplo · Joined: 06 Sep 2003 Posts: 659 Location: Sydney, Australia

I too learned it the hard way: Never use the first couple of revisions of anything that comes out of Microchip.