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'My PIC16C54 works unreliably'
1995\04\02@045838 by stephnss

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I my recent design, I have 16 bright leds and they need a lot of current. If I
switch them very fast (2kHz or so) the PIC resets. It happens after some time
and it seems to be a problem with the power supply to the PIC. If I insert a
capacitor in parallell, the problem goes away until I raise the supply current
even more to the leds (I use buffers to control the leds so the PIC doesn't need
that much current).



How much did you raise the LED current?
How are you raising the supply current to the LED's?
What device is the buffer for the LED's?
Describe the trace lengths/locations of the supply, PIC, buffer & LED's?
What are the V and I spec's for the supply, PIC, LED's & buffer?
How much did you increase the filter cap?

1995\04\03@113222 by Conny Andersson

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I wrote ...

> I my recent design, I have 16 bright leds and they need a lot of current. If I
> switch them very fast (2kHz or so) the PIC resets. It happens after some time
> and it seems to be a problem with the power supply to the PIC. If I insert a
> capacitor in parallell, the problem goes away until I raise the supply current
> even more to the leds (I use buffers to control the leds so the PIC doesn't
need
> that much current).

Scott Stephens wrote ...

> How much did you raise the LED current?
> How are you raising the supply current to the LED's?
> What device is the buffer for the LED's?
> Describe the trace lengths/locations of the supply, PIC, buffer & LED's?
> What are the V and I spec's for the supply, PIC, LED's & buffer?
> How much did you increase the filter cap?

The problem is that I don't want sixteen resistors on my board so I use
"built in" current limiting in two 74HC244:s. Yesterday I checked the
voltage drop over the PIC with my osc. and deltaV = 2V (too much I guess).
The simplest solution to this was to place a couple of diodes from
V+ to Vdd of the 74HC244 and now it works with deltaV = 0.2V (but with
a little less power output from the leds.)
-----------------------
Conny Andersson / LiTH

1995\04\04@012759 by Don Lekei

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It seems to be a common problem to not provide sufficient bypassing on PIC's
because they "use negligable current". There are two effects that happen when
you under-bypass a part like the PIC because they are CMOS (this is not new
btw., anyone remember the COSMAC 1802).

1) CMOS switches have low impedance, so when switching low impedance (or worse
yet capacitive) loads, if your PIC has insufficient bypass the power supply,
traces, etc. take a while (several nanoseconds) to catch up. The dip in voltage
can cause reset.

2) You should expect some very short pulses of high current when switching
capacitive or low impedance loads. This is because the output gates may turn
on/off asymetricly under adverse load conditions, causing a momentary current
path from VDD to VSS.

Proper bypasing with capacitors of various characteristics (and don't forget
proper termination of the reset line) is often a cure for this bizarre behavior.

As Scotty was so fond of saying:
"Ya canna change tha lawhz af phyzzicss, Caaptain"

- Don Lekei

PS. Of course I could be wrong, and this whole thing is a "not initializing all
registers" thing.

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