Re: Power-On Reset

From: Ken Smith (kensmith_at_green.rahul.net)
Date: 02/22/05 

Date: Tue, 22 Feb 2005 19:38:31 +0000 (UTC)

In article <r6um11pb6li1fk3q6fm7efi4c2t96pdt9h@4ax.com>,
Allan Herriman <allan.herriman.hates.spam@ctam.com.au.invalid> wrote:
>On Tue, 22 Feb 2005 18:20:23 +0000 (UTC), kensmith@green.rahul.net
>(Ken Smith) wrote:
>
[...]
>How would one go about making a 'crystal start' detector for an 8051?
>We don't want to load either of the xtal pins, and (IIRC) outputs like
>ALE won't be toggling while the reset is active.

I wasn't using the 8051's oscillator in the case where it was an issue.

I just did the RESET logic inside a CPLD. It worked ok. Normally, the
8051s RC reset works fine.

You can load the osc-out pin of the 8051 with a CMOS gate without causing
serious trouble. CMOS gates have less capacitance than you normally hook
to that node.

>>So there's another example of a reset that needed to be held and synced.
>>The fact that Intel provided the function on an external chip says
>>something, I'm not sure what, about doing it inside the micro.
>
>I think it says that in the days when 16000 transistors was a big
>chip, you should move as much stuff as possible off board. :)

OR: It says that the CPU designer screwed up and they didn't want to turn
the mask again.

The 8080 had an instruction that was an almost 16 bit subtract. It didn't
borrow correctly. I suspect that they changed the data *** rather than
fix a broken subtract. 

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