Board design for 18f452

Robotan · Joined: 12 Jul 2008 Posts: 18 Location: Canada

Hello,

I am currently designing a board for the 18f452 pic and I have a couple questions. I would like to to run the chip at 40 Mhz. I understand this can be done in two ways: use a 10Mhz crystal and PLL to multiply the clock internally by four. Or, I could just use a 40Mhz crystal. Which method is better? Is one more reliable than the other?
And my next question is about grounding. I have read elsewhere on this board that it is good design practice to use ground planes with analog sensors to minimize noise interference from the digital ground. I have done this, but I am wondering if connecting the two grounds with a ferrite bead or small resistor would help reduce the noise transmitted to the analog ground even further. I have some fairly sensitive analog sensors, so the less noise on the analog ground, the better. Any suggestions?

Rob

SherpaDoug · Joined: 07 Sep 2003 Posts: 1640 Location: Cape Cod Mass USA

I don't think PICs will support a 40MHz crystal. I think the internal PLL or a full external clock are the only options to run at 40MHz.

The analog and digital ground planes should meet at the A/D or D/A chip ground pin, or at the power supply if there is no A/D or D/A.
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asmallri · Joined: 12 Aug 2004 Posts: 1634 Location: Perth, Australia

dyeatman · Joined: 06 Sep 2003 Posts: 1923 Location: Norman, OK

I have a number of designs using the 18F452 and the PLL. The only problem I have ever encountered is known bug where the PLL won't kick in after programming. I have to power the chip off and back on to get it to run at 40MHZ. Beyond that it has always worked with no problems.

As was stated before, the 452 is obsolete. I use the 18F45xx, 46xx for all my designs now.

Dave

Ttelmah · Guest

One thing to do, is to leave provision on the board for a series resistor, between the crystal, and the PIC.
The latest 18F452's are pretty good, but in common wth quite a few of the early PICs with PLL's, _some_ can exhibit problems with the PLL, if overdriven. On these, adding a series resistor, to reduce the overall loop gain, solves the problem. On the newer chip releases, they will usually work if overdriven, but the accuracy of the oscillator, does decline.
I would agree though, use the 4520, or 4620 in preference. These are generally cheaper now, then the 452, and give you the extra flexibility of potentially an extra I/O pin (MCLR), and slightly lower power consumption.

Best Wishes

Robotan · Joined: 12 Jul 2008 Posts: 18 Location: Canada

Thanks for the tips. I will definitely look into using an 18F4620. I do have a place on the board for a resistor between the crystal and the pic, so I'll make use of that.
I'm still curious about the best way to connect the analog ground to the main ground...

Rob