Oscillator noise on squarewave output pin

ken261

I am trying to develop a fixed data output source for a test set. As part of that source, I need a 10 kHz (approx) square wave CLK signal coming out. This was simple enough to create with interrupts, but I am getting oscillator noise on the square wave output.
I am using a 4 MHz Murata on the SX Tech dev board. There is no other circuitry added to the board, and I am reading the output on RA0. I get a steady 4 MHz signal (about 600mV) riding on the 5V output.

I have tried all the DEVICE OSC settings and it happens with the SX-Key as well. I also tried adding the 1Mohm resistor across the resonator, but that didn't change anything. I thought perhaps the chip was bad, but two other chips show the same thing. Unfortunately, I don't have another Tech board. For grins I changed everything to a 50 MHz resonator I had and ended up with a 50 MHz signal on the square wave. That at least tells me it's not coming from the SX28 internal clock.

I may try to breadboard the whole thing from scratch to see how that works. I've included the code just in case it's a simple setting error.

Thanks for any help.

Ken


'

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' Device Settings
'

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DEVICE SX28, OSCXT2, OPTIONX, STACKX, TURBO, BOR42
FREQ 4_000_000

'

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' IO Pins
'

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CLK PIN RA.0 OUTPUT ' square wave output

'

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INTERRUPT NOPRESERVE 20_000
'

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ISR_Start:
CLK = ~CLK ' toggle CLK around 10 kHz

RETURNINT

' =========================================================================
PROGRAM Start
' =========================================================================

Start:

LOW CLK ' assure startup state

Main:
pause 50
GOTO Main

PJAllen

If it's really real OSC riding on the output, and it's a 10kc output, that should be easily filterable (LPF).

Are you using the ground clip and thereby "coupling" noise in or are you using good 'scope method - a stub from the probe ground ring to the ground plane of your board?
When you get picky, you have to be picky. I don't know your background, that's why I'm asking questions.

wass

There's a 10uf capacitor across the input of the voltage regulator and a 1.0uf capacitor after across the output of the voltage regulator (and a 0.1uf bypass capacitor across the SX chip socket). My guess is that one of these is bad, probably the 1.0 uf capacitor. This would cause the 5 volt supply to move up and down with the load from the SX chip (which, of course, is running a 4MHz).

If you don't have a spare capacitor, try running the SX directly (not going thru the voltage regulator) using 3 AA (or AAA) cells in series at 4.5 volts.

-Katie

ken261

Well, I've rebuilt the circuit on a breadboard and still get the noise on the output, so I guess it's not the Tech board. I've tried a couple grounding methods for scope measurements, but it's always there.

I've thought about the LP filter, but I'm still curious why I'm seeing this in the first place. I had built a PWM circuit a year ago with this same Tech board and I don't think I saw the problem then.

Thanks for your suggestions.

wass

What are you using for a power supply?