gnupic: Re: [gnupic] more on weird resets
Subject:
Re: [gnupic] more on weird resets
From:
John Sheahan ####@####.####
Date:
1 Aug 2005 11:57:50 +0100
Message-Id: <42EE001A.2050201@optushome.com.au>
Peter wrote:
>
>> perhaps you could explain more why a connection from VDD to the reset
>> input is flawed?
>
>
> Because it precludes ICSP as well as deliberate resetting during
> debugging. I find that a reset button is the second most useful thing
> after the power supply on a prototype board ...
>
thats reasonable, If you add the reset or ICSP requirement.
In none of the following cases does your suggestion work better than a
wire connection to vdd, from what I read here. I like to make components
earn their keep.
restating where we might be at:
a well decoupled power supply is a good thing.
a high impedance (say just a pullup) to the reset pin can cause issues.
If you need an R, then decouple reset to VDD, not VSS.
If you don't need reset, then connect to VDD works.
is this reasonable?
regards, John
>> line pumping from spikes? positive spikes won't reset the chip for an
>> active low clr.
>
>
> Yes they do. The MCLR line is decoupled by its capacitor and stays at
> constant voltage while the VDD line *rises*. In theory the MCLR line is
> protected by a Schmitt circuit and probably by a monostable. In
> practice, the 220pF/100Ohm solution works many times when there are
> spike sources on the board.
your case (cap to vdd) decouples mclr to vdd via a cap , so no reset.
my (wire to vdd) case connects vdd to mclr, so no reset.
maybe you are worried about an external cap on mrst to gnd, and a
positive spike on vdd.
This could perhaps cause issues when you couple lead inductance (10's or
nh) in mclr to mutual coupling in the bond wires. mclr is often next to
a supply pin.
>
>> IO line switching ? This bounces either internal VSS up or VD down.
>> An external decoupler does not help here in either case.
>
>
> It helps to make MCLR follow VDD tighter. The zener curtails these
> excursions from going too high.
>
>> RFI? Reducing the coupling to the reset pin is the key here. depending
>> on where your suggested cap is connected to - it will work as well.
>> I don't see any obvious cases where it works better. I do assume the
>> wire I suggested is not longer than the RC individual elements.
>
>
> Often the RFI causes resets by being coupled into VDD or VSS, not MCLR.
> Older PICs (16C54) had a reputation for being extremely sensitive
> against positive spikes on VDD. Newer PICs have an internal filter on
> the MCLR line.
>
> Peter
>
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