8051 reset circuit

Joop schrieb:

> 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.

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 Ma�.
http://www.autometer.de

Tilmann Reh <tilmannreh@despammed.com> wrote:

>Joop schrieb:
>
>> 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.
>
>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.

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