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

You have a huge noise there, try to see why. Almost 2Vpp of noise is a
real mess.
It's possible that sometime the PWM dutycycle be longer that PWM period?
Only then the CCP output remains high. Maybe a compiler problem ?

Wesley-Basil-Vasile

On 5/12/08, Gordon Williams <g_will <at> cyberus.ca> wrote:
> 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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