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'[EE]: Watchdog IC'
2007\11\16@131141 by James Nick Sears

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I'm working on a one-off project that is controlling a pair of 90VDC  
motors through Minarik drives (thanks to the list for the earlier  
tip).  It's going to be entirely unsupervised, so just to ease my mind  
I'd like to have some sort of watchdog IC controlling the AC power to  
the motor controllers (through a relay), so that if the IC doesn't get  
the periodic signal from the PIC it will cut the power to the relays  
indefinitely until the microcontroller is brought back online by other  
means and the IC once again receives its watchdog pulse from the micro.

This last part seems to be the catch.  What I'm finding is no shortage  
of ICs that generate a reset pulse on a failure, but I want one that  
holds an output high (or low) until the failure is corrected.  I  
figure I'll use the PIC's internal watchdog to reset itself, but I'd  
like this extra measure of safety to make sure the moving parts don't  
destroy themselves in the event that the PIC should completely fail.

Any suggestions?

-n.

2007\11\16@134152 by Spehro Pefhany

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Quoting James Nick Sears <spam_OUTlistsTakeThisOuTspamjamesnsears.com>:

> I'm working on a one-off project that is controlling a pair of 90VDC
> motors through Minarik drives (thanks to the list for the earlier
> tip).  It's going to be entirely unsupervised, so just to ease my mind
> I'd like to have some sort of watchdog IC controlling the AC power to
> the motor controllers (through a relay), so that if the IC doesn't get
> the periodic signal from the PIC it will cut the power to the relays
> indefinitely until the microcontroller is brought back online by other
> means and the IC once again receives its watchdog pulse from the micro.
>
> This last part seems to be the catch.  What I'm finding is no shortage
> of ICs that generate a reset pulse on a failure, but I want one that
> holds an output high (or low) until the failure is corrected.  I
> figure I'll use the PIC's internal watchdog to reset itself, but I'd
> like this extra measure of safety to make sure the moving parts don't
> destroy themselves in the event that the PIC should completely fail.
>
> Any suggestions?

You could latch the pulse with a flip-flop.

Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
.....s...KILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


2007\11\16@140659 by alan smith

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Check out RAMTRON, they have a one-bit part that I think will provide the solution. It isnt a watchdog but more of...last state held...device.  So you would drive it from the watchdog

James Nick Sears <listsspamKILLspamjamesnsears.com> wrote:  I'm working on a one-off project that is controlling a pair of 90VDC
motors through Minarik drives (thanks to the list for the earlier
tip). It's going to be entirely unsupervised, so just to ease my mind
I'd like to have some sort of watchdog IC controlling the AC power to
the motor controllers (through a relay), so that if the IC doesn't get
the periodic signal from the PIC it will cut the power to the relays
indefinitely until the microcontroller is brought back online by other
means and the IC once again receives its watchdog pulse from the micro.

This last part seems to be the catch. What I'm finding is no shortage
of ICs that generate a reset pulse on a failure, but I want one that
holds an output high (or low) until the failure is corrected. I
figure I'll use the PIC's internal watchdog to reset itself, but I'd
like this extra measure of safety to make sure the moving parts don't
destroy themselves in the event that the PIC should completely fail.

Any suggestions?

-n.

2007\11\16@152853 by Harold Hallikainen

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flavicon
face
This seems fairly simple to do in hardware using a CMOS hex inverting
schmitt trigger. Have a series capacitor on the input, then a resistor to
Vcc driving an input of one section. The output of this will sit low
unless the left side of the capacitor is pulsed. It will go high when the
left side of the capacitor is pulsed low. Out of this first inverting
schmitt, have a diode to the input of another section (cathode towards the
input of the next section), resistor and capacitor to ground. When an
input pulse comes along, the second (timing) capacitor is charged through
the diode. Between pulses, it discharges (slowly) through the resistor. If
the voltage goes below the lower trip point, the output of the second
inverting schmitt goes high. If you are getting input pulses fast enough,
the second inverting schmitt stays high.

The input coupling capacitor is important, because, if you leave it out, a
crash with the input driven low would never time out.

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2007\11\16@185151 by James Nick Sears

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Thanks for the ideas.  I was hoping for a one part solution, but a  
typical watchdog chip and an SR flip flop seems like a winner.

Have a good weekend all.

-n.


On Nov 16, 2007, at 1:41 PM, Spehro Pefhany wrote:

{Quote hidden}

> --

2007\11\16@191755 by Jinx

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part 1 220 bytes content-type:text/plain; (decoded 7bit)

> Thanks for the ideas.  I was hoping for a one part solution

10F or 12F PIC ?

Possibly a 4001. Two gates as a re-triggerable monostable,
two gates for a flip-flop, and some glue


part 2 1421 bytes content-type:image/gif; (decode)


part 3 35 bytes content-type:text/plain; charset="us-ascii"
(decoded 7bit)

2007\11\16@194125 by Peter van Hoof

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Is it perhaps simpler to use a small pic chip for this.
A 6 or 8 pin device with internal rc is ideal for a simple
task like this.

If this chip would cut power without incomming pulses
but the motor commands come from the main pic it
would still be safe (except a multiple failure in for
example a power supply failure in which all bets are off
anyway)

Peter van Hoof

----- Original Message ----
From: James Nick Sears listsspamspam_OUTjamesnsears.com

Thanks for the ideas.  I was hoping for a one part solution, but a  
typical watchdog chip and an SR flip flop seems like a winner.

Have a good weekend all.

2007\11\16@200644 by Jinx

face picon face
> Possibly a 4001. Two gates as a re-triggerable monostable

Just thinking - you don't need a flip-flop to store state. If pulses
are lost, the output from a monostable (4001, 555,  whatever)
would go low and the relay will drop out. By that reasoning do
you need a WD chip anyway ? If you've got a periodic output
from the PIC, that could go to an RC that holds up the g/b of
a FET/transistor, keeping the relay on. Without a PIC I/O pin,
perhaps integrate OSC ?

2007\11\16@203443 by Spehro Pefhany

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At 06:51 PM 11/16/2007, you wrote:
>Thanks for the ideas.  I was hoping for a one part solution, but a
>typical watchdog chip and an SR flip flop seems like a winner.
>
>Have a good weekend all.
>
>-n

You could also use a charge pump (a couple of ceramic caps, a BAV99) and
a single gate ST inverter, plus a resistor. You have to toggle the
input often enough that the resistor doesn't bleed off the voltage to
the ST trigger point.

One smallest-possible-part-count circuit (1) is an ADM705, which has a
/WDO output and an extra comparator. I've used these parts in production
and found them very good. About half a dollar in volume. There's  also an
equivalent 3V part. They may also be offered by M*x*m, at least in theory.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
@spam@speffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2007\11\16@235002 by James Nick Sears

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I think the ADM705 is exactly what I'm looking for: "The Watchdog  
Output, WDO, goes low if the internal watchdog timer times out as a  
result of inactivity on the WDI input. It remains low until the  
watchdog timer is cleared".  Thanks for the recommendation!

In response to Jinx, I had thought about using an RC, but wanted to be  
sure to account for the case where the PIC died with the watchdog  
output held high.  I suppose I could use an extra series C in front,  
making essentially a bandpass filter, but I'm on a deadline for this  
(what else is new) and would like to keep things as plug and play as  
possible, b/c I need to get a PCB design turned within the next few  
days.

I thought also of using a baby PIC, but somehow the fact that it's a  
clocked micro running program code seems somehow less bulletproof than  
a purely logic based solution.  Not sure if this concern is truly  
valid though.

Many of the other ideas were very interesting, but for now it looks  
like the 705 is just what I need.

Thanks all.

-n.


On Nov 16, 2007, at 8:39 PM, Spehro Pefhany wrote:

{Quote hidden}

> --

2007\11\17@001403 by Jinx

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> I thought also of using a baby PIC, but somehow the fact that it's
> a clocked micro running program code seems somehow less
> bulletproof than a purely logic based solution.  Not sure if this
> concern is truly valid though

No reason a micro shouldn't be as reliable a solution as a dedicated
IC. There are pros and cons for either option (cost, simplicity, s/w,
availability etc)

My access to a single ADM705 would cost $20 (inc tax, courier)
whereas I have plenty of suitable PICs. That obviously need s/w,
so there's some time to spend on that

Other options I see at RS are

DS1811R-10, MAX812, MAX824

A fair number at Digikey too, and also at Microchip (which you can
free sample)

www.microchip.com/ParamChartSearch/chart.aspx?branchID=9008&mid=11&la
ng=en&pageId=79

Analog & Interface Products / CPU & System Supervisers

2007\11\17@050209 by Dario Greggio

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Jinx wrote:

> No reason a micro shouldn't be as reliable a solution as a dedicated
> IC. There are pros and cons for either option (cost, simplicity, s/w,
> availability etc)

Had some talk a couple of years ago with some people.
I agree on this.

A properly programmed and powered/configured uP (PIC among the others,
I'd say even more reliable) will do as better as std. logic.
Maybe just some debugging is needed (besides designing software , of course)

--
Ciao, Dario

2007\11\17@131927 by Bob Axtell

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Jinx wrote:
{Quote hidden}

A 555 can be arranged as a "missed pulse" detector. The time constant
can be VERY long. The CMOS
versions are more accurate, incidentally. Cheap, too.

--Bob A

2007\11\17@163952 by Jinx

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Some background reading, 85kB

Understanding and Using Supervisory Circuits

http://ww1.microchip.com/downloads/en/AppNotes/00686a.pdf



2007\11\18@073849 by Vasile Surducan

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On 11/16/07, Jinx <RemoveMEjoecolquittTakeThisOuTspamclear.net.nz> wrote:
> > I thought also of using a baby PIC, but somehow the fact that it's
> > a clocked micro running program code seems somehow less
> > bulletproof than a purely logic based solution.  Not sure if this
> > concern is truly valid though
>
> No reason a micro shouldn't be as reliable a solution as a dedicated
> IC. There are pros and cons for either option (cost, simplicity, s/w,
> availability etc)

There is one big reason why it should not be used. As long it needs
programming and is acting like a two penny 74SN7121 or 74SN7123 why
the hell to use a microcontroller or other $20 part ? There are sooo
many dedicated Ics (I think I can number around 20 without searching
at Digikey) that a programmed microcontroller (anyone would be that)
should be the last and the most complicated option.

2007\11\18@092459 by Peter Todd

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On Sun, Nov 18, 2007 at 04:38:47AM -0800, Vasile Surducan wrote:
{Quote hidden}

I take it in production too having a uC in a circuit can be a big pain
due to the need to either program it, or inventory and stock
pre-programmed units.

- --
http://petertodd.org
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2007\11\18@104225 by Dario Greggio

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Vasile Surducan wrote:

>  There is one big reason why it should not be used. As long it needs
> programming and is acting like a two penny 74SN7121 or 74SN7123 why
> the hell to use a microcontroller or other $20 part ? There are sooo
> many dedicated Ics (I think I can number around 20 without searching
> at Digikey) that a programmed microcontroller (anyone would be that)
> should be the last and the most complicated option.

Ok, if it does the same exactly-exactly.
But as soon as you get some advantage in using a uP, then the scenario
changes.
Other than that, I agree. I did agree more some years ago, though :)

--
Ciao, Dario

2007\11\18@112642 by Peter van Hoof

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Stated was this would be a one off project, and he/she was
looking for a low component count solution. a 8 pin pic has many advantages
no timing cap needed, no need for a cap in the input to avoid retriggering
by a continuous low or high signal and very good timing accuracy

Other things that mightcome up later can be implemented also such as
a power up delay.

Peter

{Original Message removed}

2007\11\18@121918 by Spehro Pefhany

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At 07:38 AM 11/18/2007, you wrote:
>On 11/16/07, Jinx <TakeThisOuTjoecolquittEraseMEspamspam_OUTclear.net.nz> wrote:
> > > I thought also of using a baby PIC, but somehow the fact that it's
> > > a clocked micro running program code seems somehow less
> > > bulletproof than a purely logic based solution.  Not sure if this
> > > concern is truly valid though
> >
> > No reason a micro shouldn't be as reliable a solution as a dedicated
> > IC. There are pros and cons for either option (cost, simplicity, s/w,
> > availability etc)
>
>  There is one big reason why it should not be used. As long it needs
>programming and is acting like a two penny 74SN7121 or 74SN7123 why
>the hell to use a microcontroller or other $20 part ? There are sooo
>many dedicated Ics (I think I can number around 20 without searching
>at Digikey) that a programmed microcontroller (anyone would be that)
>should be the last and the most complicated option.

It is very hard to guarantee the behavior of a complex device like a
microcontroller if you have to allow for "soft" errors, because there are
so many possible internal states. It's generally errant behavior of a device
like a micro that you're trying to guard against with a WDT. When there are
only two stable states it's pretty easy to analyze the behavior. When there
are 2^128 or more it becomes more difficult. Even if you take preventive
action such as only using a micro with hardware address and bad instruction
protection, and one that can do its own checksums and tests and use
completely bulletproof and precise supervisory and power subsystems, I think
doubt will still remain. Most low cost micros (including PICs) lack the
on-chip hardware to be really robust, although with clock monitors, the
ability to read checksums, bullet-proof supervisory circuitry (not there
yet) bad instruction and RAM protection (not on PICs), and better hardware
WDTimers (such as the 'windowed' variety, and those that require 'arming'
before they will accept a reset command we might get a lot
closer, at a reasonable price, some day. Depends on the engineering and
the consequences of a failure. People have died, and more will die, because
of such problems. Fortunately, usually it's just the user getting p*ssed
off because they have to cycle the power to get things to work properly
again. In many medical, aerospace and 24/7 industrial control applications
that may be considered unacceptable for some reason.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
RemoveMEspeffspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2007\11\18@162332 by Jinx

face picon face
> why the hell to use a microcontroller or other $20 part ?

Hi Vasile. Calm down, calm down ;-)

I did say there are pros and cons. If you need a WD right here
right now, there's no reason not to use a PIC

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