SV: RE: [sdiy] Semi-OT: PIC trouble

[Karl Ekdahl]

Yes, the circuit did reset all the time which was
quite annoying. I didn't know about the brown-out, but
i did locate the problem to be the HDLG's internal
osciallator putting *huge* interference down the
powerlines. I solved it with some caps sprinkled
around the circuit. Also my serial interface to the
HDLG was so slow (somehow, i ran it directly from the
MCU without pauses = 48Mhz (?)) that the ^CLR pin was
low for long enough to reset inbetween every call so
i've tied it to VDD for now.

Karl

--- Fredrik Carlqvist <Fredrik.Carlqvist at iar.se>
skrev:

> 
> 
> I noticed the display pulls a lot of current
> (>100mA), very much
> depending on the number of pixels lit. Does the Vcc
> on the PIC change
> when you pull the HDLG? 
> 
> If the PIC has brown-out reset enabled, it could
> reset as soon as the
> voltage falls below for example 4.5V (voltage set by
> configuration
> bits). Remember that a 100ns glitch could be enough
> (although the data
> sheet states 200us). This could make the LED blink
> irregularly when the
> display is in-circuit. 
> 
> Even if brown-out reset is not enabled, power line
> problems could
> influence your RC-oscillator. Maybe a lower voltage
> will push the
> frequency above the max freq for the chip, since you
> are normally on
> 48MHz?
> 
> Since the display unit has no outputs and the inputs
> are CMOS, I see no
> way for it to influence the PIC on a
> logical/interface level. The only
> possible communication from display to PIC is via
> the power line. 
>  
> Separate the power lines all the way to the PSU and
> add capacitors at
> both chip Vcc/GND until the problem goes away :-) If
> that does not help,
> try a crystal oscillator.
> 
> Some PIC18 derivatives have a problem with the
> interrupt consistency,
> the shadow registers does not behave well.
> PIC18F2455 does not seem to
> be one of them though. 
> 
> 
> Fredrik C
> 
> 
> 
> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf
> Of Karl Ekdahl
> Sent: den 31 oktober 2005 22:56
> To: synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Semi-OT: PIC trouble
> 
> Hi list, i'm having trouble interfacing a HDLG-2416
>
(http://www.datasheetcatalog.com/datasheets_pdf/H/D/L/G/HDLG-2416.shtml)
> with a PIC18F2455. I seem to get some feedback from
> the HDLG back to the
> pic that makes the oscillator unstable, i have a
> eternal loop flashing a
> LED and it's *not* flashing in a perfectly
> sequencial manner.
> I have a 2.4A PSU and the HDLG is connected via two
> 4094's so i don't
> really understand what's going on. I have verified
> that it's really the
> HDLG by simply pulling it out of its socket and
> observing the LED.
> Though since the pic is fast, i do run it on 48Mhz
> (oscillator, not
> crystal/resonator) which i guess might be a problem
> while not using 1st
> grade components....
> 
> hrmm.. now when i come to think about it, haven't
> people here been
> talking about resonance/noise in LEDs?
> 
> Ideas (or directions to a good pic forum) highly
> appreciated!
> 
> Karl
> 
>