Too much F-IN for the CountTransitions() input?

[rpb]

Crud - posted this on the ZBasic language forum by mistake, probably should have rightly been here... squuuzzzz me ! anyway, here goes...

sampling RF from 1.8-30mhz with a gimmick capacitor, conditioned, and fed into a divide by 100 chain, tied to input port on zx24p.

On all bands thru 21mhz, frequency is accurately determined. Starting at 24mhz, and particularly on 28mhz, i dont get accurate frequency determination.

same circuit worked fine on Basic Stamp controller even at 28mhz. When using zx24p 28mhz comes out at about 22mhz. BasicStamp Count() only counts one edge, zx24 counts both. So i am sampling at half the rate to get equivalent.

Question - have i hit the limits of f-in for the zx24p hardware ? Or is there something i can to with the prescaler or other ideas to resolve? PCBs are built - dont want to muck with hardware if i can help it.

Also, dont want to stay in the CountTransitions() code long because it holds up my executive loop.

Any help out there??

-bob
BTW - ZBasic/zx24p is so much better than the BasicStamp i never want to look back.

[dkinzer]

[rpb]

tnx don.

yep, you hit it right on the head, a little over 21mHz is where it initially seems to start breaking down. I dont need great accuracy, being off a couple of kHz wouldnt matter much. But above this, its off too far to be useful. The project is a high power auto-tuning T-network antenna tuner twisting vacuum variables for C and L with steppers. I bust the bands up into segments, so being off a bit is no biggie.

Unfortunately, my hardware is setup to use pin 20 for freq detect, which isnt a valid pin for the Inputcapture method from what i see in the dox.

would have to modify the hw a bit - swapping pins. Any other recourse?

-bob

[dkinzer]

[dkinzer]

The code below is an (untested) implementation of the specialized version of the CountTransitions() function that I mentioned above. It is only useful on native mode devices and should be compatible with ZX devices based on the mega644P and mega328P. It could be modified fairly easily for ZX devices based on other AVR chips.

For the mega644P, the sampling loop compiles to code that requires a maximum of 12 cycles meaning that it will correctly detect transitions on an input signal with a low time or high time as small as about 81ns. For a waveform with 50% duty cycle, this corresponds to a frequency at the sampled pin of about 614KHz. With your divide-by-100 prescaler in place that would give coverage up to about 61.4MHz.