[PIC] CCP1 Glitches in noisy environment (RC Tx)

Hi All,

I'm having problems tracking down the reason of some glitches occurring on
the CCP1 output and it is driving me a bit crazy.

I'm putting some smarts into a basic RC transmitter.  I've replaced the
current chip producing the pulse train to the RF section with a daughter
board and added a cell phone LCD, some switches and Serial port for input
and output.  The serial port and LCD (spi) work fine and they don't have any
glitches.  The 5 ADC readings for the joysticks (4) and battery voltage also
work well with only 1 bit of noise on a 10 bit reading (referenced to the
power and gnd lines).

The PIC that I am using is the 16F886.  Its supply is a 78L05 on the main
board and I have a 10uf Tant and three 0.1uF on the power lines on the
daughter board.  The daughter board is single sided with a ground plane pour
under the PIC and every else where I could.  Because I don't have ADC
problems or serial or spi errors that noise must be reasonably constrained,
I think.

I'm using CCP1 (pin C2) to produce the PPM that goes to the RF section.  It
is the standard PPM protocol (each channel pulse is 1 to 2 ms wide including
the following 0.3 ms pulse, repeated every 20ms).

Timer1 runs freely.  With a CCP1 interrupt (my only interrupt) I update
CCPR1 with the duration until the next transition high or low and the
CCP1CON bits to set the next transition in the correct direction.  My main
program clears a flag and that is all.

Most of the time it works perfectly and looks like:
http://www.cyberus.ca/~g_will/TxGlitch/NoGlitch1.jpg

or detail

http://www.cyberus.ca/~g_will/TxGlitch/NoGlitch.jpg

The receiver pulses for the first 4 channes are shown in the logic section
below.

Every 20 to 100 frames on average (sometimes they are consectutive frames) I
get a glitch:

http://www.cyberus.ca/~g_will/TxGlitch/Glitch1.jpg

or

http://www.cyberus.ca/~g_will/TxGlitch/Glitch2.jpg

The green lines indicate where the signal should have been.

There is a fair bit of noise - about 1.8 volts p-p because the scope ground
was attached to the battery neg lead.  I don't think that my probes are very
well shielded either.  The noise is from the 72 Mhz RF.

In each case the low output flips high.  My question is WHY?

Timer 1 keeps on rolling as there is no problem with frame spacing or the
width or placement of the next good pulse.

CCPR1 is not upset or incorrectly set for the same reasons - because the
next good pulse is in the proper location relative to the last good pulse.

The serial and LCD have been comment out.  I'm reading 2 ADC on port A and
3ADC on port B.  All other pins on ports A,B,C are inputs and some are
floating (could this be a problem?)

Is there anything else on the chip that may be miss-configured that may be
giving me a problem?

The chip docs say:
Note: Clearing the CCP1CON register will force
the CCPx compare output latch to the
default low level. This is not the PORT I/O
data latch.

But I don't think that this applies to me.  The only time that I do anything
with the CCP1CON is during the long bit in the frame between the pulse
series and the glitch doesn't always happen there.

It is not read-mod-write ... I'm not writing anything to the port.

I read the errata - nothing there indicated a problem.

Any ideas what I should be looking at/ trying next??

- I guess I should look at the 5 v power going to the chip to see that it is
truly quiet and doesn't have a spike.  Brown out is disabled.

- Should set the un-used outputs to either high or low.  - Good right?

Funny that it is only affecting one pin that I can see.

Looking for suggestions...

Regards,

Gordon Williams

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