PLL never locks - HC908AP64 - clueless...help ?

[JSA]

Hi,

I have a problem with a PLL not locking, never finishing and I cannot find the problem. I need your help please.

I'm using a MC908AP64 (HC08) with an external crystal 32768Hz according to suggested build from cpu-pdf-manual.

The board is custom built. We have designed OSC1 and OSC2 connected to R1 10M parallell, and X1 32768Hz, and 2 x C1 (12p) to ground, like the design example in the manual.

I attached the schematics for you to see.

I created a CPU setup with initialization tool... and have attached the project as well.

Noticed that it never seems to lock. 

I designed so that after this cpu-setup is done.. it goes into loop of blinking a led. It works, in debugger (clocked from Cyclone Pro), but then I'm skipping the cpu_setup since it's been clocked from Cyclone Pro instead then by 9.8304MHz I get a LED-toggle frequency of about 240KHz..which seems a bit slow too. But that's is not why I'm asking so, never mind that

/Johan Sandgren

Svep Design Center AB

Lund, Sweden

+46 701 73 41 52

[JSA]

Found the error, was missing the optional resistor Rs, which made it work.

Strange..optional shouldn't mean it breaks the oscillator....

[rocco]

Hi Johan,

It really is optional, depending on your crystal. Some crystals may require it, others may not. And it isn't always obvious from looking at the crystal's specification.

My philosophy is to always include a place for the "optional" components on the PCB, and then decide at build-time whether or not to include it. In this case, I would have put a trace between the two pads of Rs. Then, if I needed to include Rs, I would cut the trace.

I learned the hard way (I never make the same mistake more than ten or eleven times).

[bigmac]

Hello Johan,

Further to Rocco's comments, the purpose of the resistor Rs is to limit the drive power (dissipation) within the crystal itself.  The low frequency crystal type that you are using is much more fragile than higher frequency types of say, a few MHz.  Therefore, the low frequency type almost always needs a series resistor, typically 30-100k, to prevent over-drive, and damage to the crystal.  However, should the resistor value be too high for a particular crystal, it will either not start to oscillate, or starting will be erratic.  It is best to observe the crystal manufacturerer's recommendations, if available.

On the other hand, crystals of >1Mz (that use the quartz AT-cut) almost never need the series resistor, i.e. Rs = 0.

Regards,

Mac