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From: Steve H on 31 May 2010 06:20
In message <zmd2qyrog4q5.mhhj91xxk0az$.dlg(a)40tude.net>, Frank Buss
<fb(a)frank-buss.de> writes
>Steve H wrote:
>
>> The first problem I faced was that the PIC would frequently lock up when
>> either the case or any part of the 0v rail was touched. I realised this
>> was due to a mild static charge building up when I got up off a chair
>> and being discharged through whichever point was touched. The PIC would
>> crash and would draw then excessive current from the 5V supply.
>> Investigations found the solution to this particular type of crash was
>> to pull the MCLR up to Vdd via a 4k7 resistor and not have it connected
>> directly to the 5v (as the Microchip datasheet suggests is acceptable).
>
>The datasheet doesn't suggest this:
>
>http://ww1.microchip.com/downloads/en/DeviceDoc/39582b.pdf
>
>Take a look at figure 14-5, recommended MCLR circuit.
>
>I think it could be a problem, that the MCLR pin is used for HV
>programming, too, so maybe add an additional BAV99 before R2 to limit the
>voltage to about 0V..Vdd and place the reset components as close as
>possible to the MCLR pin.
>

Ah yes, I was mistakenly looking at the 877 datasheet rather than the
877A, and the MCLR pin is a key difference, hence "Microchip recommends
that the MCLR pin no longer be tied directly to Vdd". Clearly my circuit
demonstrated what may happen if it is. Thanks for pointing me to the
correct datasheet!

--
Steve H
From: Steve H on 31 May 2010 06:48
In message <htvop7$566$1(a)speranza.aioe.org>, Jeroen Belleman
<jeroen(a)nospam.please> writes

>As the circuit previously suffered from latch-up (the increased current
>after a crash), some of the controller pins must have seen voltages
>well outside their permitted range.
>
>As a rule, any wire that leaves or enters the circuit should be
>filtered, using e.g., RC low-pass sections, transformers or common-
>mode chokes, opto-couplers, feed-through capacitors, and/or clipping
>diodes. The choice depends on the kind of signal that you need to
>pass, of course.
>
>Then there are a number of lay-out rules, valid for both PCB and
>cable dressing, that will minimize trouble:
>
>First, keep inductive coupling small by systematically using a
>return wire with every 'hot' wire, and routing them along the same
>path. Minimize the open area between them by twisting them together,
>if you can. Use wide GND tracks, or a full GND plane, if you can.
>Also, keep circuits carrying large, fast changing currents
>well away from low-level circuitry. Static discharge is a fast
>changing current.
>
>Then, minimize common-impedance coupling, don't share wires between
>different circuits, especially not if one is a low-level signal and
>the other is a large signal. Keep in mind that for fast changing
>currents, even a short piece of wire can have a sizable (inductive)
>impedance, which will therefore have a sizable voltage between its
>ends if the current rises fast enough. A useful rule of thumb for
>straight wire is ~10nH/cm.
>
>Finally, minimize capacitive coupling by keeping conductors with
>rapidly changing voltages away from nodes with high impedances.
>You can also reduce the coupling by reducing the rate of change
>of agressor nodes, by reducing the impedance of victim nodes,
>or by putting grounded or guarded shields between them.
>
>How far you need to go in all this, and what sins you can get away
>with, is a matter of judgment and experience.
>

Thank for this. As noted in a previous post, I'd originally made an
error by referring to the 877 datasheet rather than the 877A, with the
correct datasheet suggesting that connecting MCLR directly to Vdd isn't
a good idea. I intend to alter the arrangement I currently have to what
the datasheet suggests and see if it improves, i.e. the decoupling
capacitor refitted and a 1k between MCLR and the junction of the cap and
4k7 pull-up.

I've attempted with the board layout to have the ground track layout as
well designed as I can make it, with respect to keeping current paths
away from each other, together with nice wide tracks. I know from past
experience it's easy to fall into a trap even so, with what may be
considered an adequately wide track having enough inductance to block a
transient current. I'll look over it again to see if I can see anything
I've missed, and bear in mind your other suggestions.

--
Steve H
From: Royston Vasey on 31 May 2010 06:51

"Frank Buss" <fb(a)frank-buss.de> wrote in message
news:83zi89fgg1hk.1d924b2cylddb.dlg(a)40tude.net...
> Jan Panteltje wrote:
>
>> I have had similar effects with driving steppers, with a different
>> processor.
>> In my case ESM made it via the wires to the steppers.
>> Use transzorbs on each outgoing and ingoing connection,.
>> Those transzorbs will have to be grounded at the right points,
>> probably the metal casing.
>> Layout of wires is also very important as Vladimir points out.
>
> For digital signals, and if you don't need it lightning fast, you can use
> a
> resistor and a BAV99:
>
> http://www.mikrocontroller.net/attachment/74930/Schutz.png
>
> --
> Frank Buss, fb(a)frank-buss.de
> http://www.frank-buss.de, http://www.it4-systems.de


I've used that circuit as well.

How much better do you think it is to add a resistor, say 180R, between the
junction of the two diodes and the CPU pin to further limit the drag on the
input pin?


From: Jamie on 31 May 2010 11:33
Frank Buss wrote:

> Steve H wrote:
>
>
>>The first problem I faced was that the PIC would frequently lock up when
>>either the case or any part of the 0v rail was touched. I realised this
>>was due to a mild static charge building up when I got up off a chair
>>and being discharged through whichever point was touched. The PIC would
>>crash and would draw then excessive current from the 5V supply.
>>Investigations found the solution to this particular type of crash was
>>to pull the MCLR up to Vdd via a 4k7 resistor and not have it connected
>>directly to the 5v (as the Microchip datasheet suggests is acceptable).
>
>
> The datasheet doesn't suggest this:
>
> http://ww1.microchip.com/downloads/en/DeviceDoc/39582b.pdf
>
> Take a look at figure 14-5, recommended MCLR circuit.
>
> I think it could be a problem, that the MCLR pin is used for HV
> programming, too, so maybe add an additional BAV99 before R2 to limit the
> voltage to about 0V..Vdd and place the reset components as close as
> possible to the MCLR pin.
>
I've been using TVS diodes since I experimented with one a few years
ago.. I love them. They come in perfect sizes in the low voltage arena.



From: Paul Keinanen on 31 May 2010 16:48
On Sun, 30 May 2010 19:42:18 +0200, Frank Buss <fb(a)frank-buss.de>
wrote:

>Jan Panteltje wrote:
>
>> I have had similar effects with driving steppers, with a different
>> processor.
>> In my case ESM made it via the wires to the steppers.
>> Use transzorbs on each outgoing and ingoing connection,.
>> Those transzorbs will have to be grounded at the right points,
>> probably the metal casing.
>> Layout of wires is also very important as Vladimir points out.
>
>For digital signals, and if you don't need it lightning fast, you can use a
>resistor and a BAV99:
>
>http://www.mikrocontroller.net/attachment/74930/Schutz.png

While this circuit will keep the input voltage between -0.7 V and +5.7
V _provided_ that the +5 V remains stable.

You may have to add a capacitance and a 5.5 V zener between the 5 V
rail and ground close to this protection circuit.

Assuming +1 kV peak at input, the 1 kohm resistor will pass 1 A into
the +5 V rail. Unless the 5 V load is greater than 1 A, the +5 V line
potential will be elevated, possibly killing other chips on the +5 V
line.

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