8051 reset circuit

ted schrieb:

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

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.
http://www.autometer.de

I agree. The MAX1232 is my reset device of choice. I've used it in many designs with no problems at all.

Steve

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

Hi Ted,

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

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

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Hi Spehro,

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

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

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.

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

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Hi Spehro,

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.

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.

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.

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

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

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Hi Spehro,

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

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

National's versions of Microchip's parts.

Best regards, Spehro Pefhany

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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

Joop schrieb:

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.
http://www.autometer.de

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