8051 reset circuit

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I m not much of an expert on this side of things, and there are
thousands of reset and power management chips from people like Maxim
out there. It would take me ages to go through all that lot...but as
there are some experts here, I might as well ask!

Can anybody recommend a chip number, or pointer to a discrete circuit
for a reset management chip for an 8051 that will:

a) produce a clean reset pulse on power supply going up and down (I
know they all do this..!)

b) respond to a regular watchdog pulse input from the micro and
produce a reset signal if the pulse is missing for a given time (I
know the 8051 has an internal WDT circuit, but this is in addition as
the reset signal is needed elsewhere).

TIA...

ted

Re: 8051 reset circuit
ted schrieb:

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The MAX/DS/TC 1232 is a fairly common chip that has these two functions.
It also has a manual reset (pushbutton) input.

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No. The 8051 does *not* have an internal WD.
Only few (most recent) parts have one.

--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
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Re: 8051 reset circuit
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I agree. The MAX1232 is my reset device of choice. I've used it in many
designs with no problems at all.

Steve
http://www.fivetrees.com



Re: 8051 reset circuit
Hi Ted,

Whatever you use make sure it also features a good brown-out handling,
when the voltage doesn't drop to zero but briefly dips under the limit
needed for stuff to function normally. You can test that on the bench.
Personally I always roll my own reset with a transistor.

A few newer uC have brown-out features on board, for example some of the
MSP430.

Regards, Joerg

http://www.analogconsultants.com

Re: 8051 reset circuit
Hi Ted,

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You can do this with a CD4060. Configure it as an oscillator, tap off at
the desired divider for time-out and have the uC reset it at prescribed
intervals. That way you can create a really long timer without low
leakage caps and all that. The CD4060 comes in the tiny TSSOP package if
space is an issue.

Regards, Joerg

http://www.analogconsultants.com

Re: 8051 reset circuit
On 26 Oct 2004 01:45:20 -0700, the renowned snipped-for-privacy@yahoo.co.uk

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The ADM805 is a good WDT/reset/power fail supervisory circuit with
active high output (as required for many 8051-type parts). Maxim and
AD make it (and maybe others).



Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: 8051 reset circuit
Hi Spehro,

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Somehow AD always comes to the rescue. My only gripe with this kind of
stuff is the price, more than $2 for the ADM805. That would be about the
same as the cost for a middle class uC.

I'd still do it with a home grown reset and something like a 4060 for
the WD timer. Until some company comes along with a chip that does it
reliably for about 40 cents or so including any peripheral discretes.

Regards, Joerg

http://www.analogconsultants.com

Re: 8051 reset circuit

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Exactly!  I've used an HC132 based design for the 80C32's I've used in the past,
built on a tiny smt piggyback for the 8032, which included both power-on and
watchdog features.

Jon

Re: 8051 reset circuit
On Tue, 26 Oct 2004 20:48:35 GMT, the renowned Joerg

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We paid about 90 cents in 1K or 2K for it, IIRC. That was about 5-6
years ago. Maxim never gave quotes that low, so maybe they got some
design-ins and jacked the price up.

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Yeah, it really depends what's required. It would be nice if the
microcontroller on-chip peripherals were both present and good enough
to depend on. If you're not running the chip flat-out speed-wise,
modern MCUs have enough voltage range that a sloppy & cheap voltage
detection circuit will be good enough. The original
pseudo-bidirectional 8051 ports are nasty anyway- reset the chip and
they become outputs and source enough current to turn on an NPN
darlington, so you have to drive active low or play other tricks. Some
later versions of the 8051 are quite nice hardware-wise (and I like
the programming model) but the original is a nasty bit of hardware,
IMHO.


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: 8051 reset circuit
Hi Spehro,

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That could be although AD usually doesn't play that kind of game. The
chip lists as non-stock at Digikey so maybe it isn't so popular anymore.

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Actually with an LMV431 you can make a really accurate yet cheap voltage
detector. This little part is under 20 cents. I really don't know why
brown-out/reset chips are so expensive. So I am just not using them in
my designs until that changes.

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and they become outputs and source enough current to turn on an NPN darlington,
so you have to drive active low or play other tricks. Some later versions of the
8051 are quite nice hardware-wise (and I like the programming model) but the
original is a nasty bit of hardware, IMHO.
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The 89C51 was a tad better but I remember the drill, boot it and the
first thing to do is set the ports right. We tried our best to avoid
connecting anything that could smoke out if it would hang for some
reason. It did have a very nice sleep mode feature though from which you
could wake it up by wiggling the RS232 port. I guess the MSP series
leads here these days since you can keep the realtime clock running,
something we could not do with the C51 in the uA range.

Regards, Joerg

http://www.analogconsultants.com

Re: 8051 reset circuit
On Tue, 26 Oct 2004 22:57:38 GMT, the renowned Joerg
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They're not anymore. Now they are available for 25 cents or so, one
reel quantity, with tightly guaranteed trigger voltage plus a
reasonable time delay. I think that's a no-brainer if you need such a
device.

Adding the WDT pushes the price up, but most chips have a WDT in them
these days. If it's really a safety-critical application I prefer to
completely bypass the micro and drive critical outputs to a safe
position directly with an external WDT of some kind.


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: 8051 reset circuit
Hi Spehro,

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25 cents sounds like a good deal. Which one is that?

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That is the way to go. I also prefer that and this is where things such
as the long CMOS binary counter chips come in handy. Sometimes just a
Schmitt and some discretes will do.

Regards, Joerg

http://www.analogconsultants.com

Re: 8051 reset circuit
On Wed, 27 Oct 2004 00:42:14 GMT, the renowned Joerg

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National's versions of Microchip's parts.


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: 8051 reset circuit

...
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..
Hmm,
1) your 8051 has an internal WDT
2) you reserved an output pin (to hush the WD)

Why not shake this pin by the micro right after a reset? Essentially
making it function as the circuit you asked for.

Joop  

Re: 8051 reset circuit
Joop schrieb:

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Hmm,

you don't know how the internal WD (of those few 8051 which have one)
works.
(It is triggered by a sequence of accesses to internal registers, not
by an externally applied signal.)
Additionally, the OP asked for power-up/down reset, too.

--
Dipl.-Ing. Tilmann Reh
Autometer GmbH Siegen - Elektronik nach Maß.
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Re: 8051 reset circuit

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Yes, but the OP did not mention time critital requirements.
When the 8051 resets (either at power-on or WDT intervention) it is
possible to execute the necessary instructions to pull the output pin
low or high for some time.
Just put them in the startup code, perhaps even before setting the SP
and other usual startup code.

The external circuitry gets reset a little later than the 8051 itself,
but the OP did not give an indication this might be a problem.

As for hushing the WDT, the OP also did not mention it should be
hushed by external circuitry. Only that the external stuff should be
reset.
Of course if external devices do need this, than things would be
different as you seem to be assuming.
This might still be done by connecting the external 'hush-signal' to
an additional 8051 (interrupt) pin and put some WDT code behind it.

Joop


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