[sdiy] PSU rise-time and uP reset

[Magnus Danielson]

From: "Trevor Page" <t at introspectiv.eclipse.co.uk>
Subject: [sdiy] PSU rise-time and uP reset
Date: Mon, 14 Jul 2003 22:01:22 +0100

> Hi list,

Hi Trevor,

> This is going to be a bit of a 'How long is a piece of string' type
> question, but here goes.
> 
> Say we have a microcontroller that contains an internal POR (Power-on
> Reset) circuit. The datasheet for this device specifies that the +Vdd
> rise rate, to ensure good POR operation, should be at minimum 0.05 V/ms.
> If this cannot be guaranteed, then the user is advised to use an
> external reset circuit. 
> 
> I know a reset circuit is only a negligable amount of components, yet I
> am still curious as to whether I could get away with using the internal
> reset circuit or not. The design in question is a MIDI to CV converter
> (yawn!) that will be used by more people than myself and I have no idea
> what sort of power supplies are going to be in use. 
> 
> Would it be unsafe to make the assumption that the rise time of the 5V
> from my on-board 5V regulator, which takes its power from the modular
> +12/+15V PSU, is going to take longer than, say, 100ms to fully rise?
> 
> This is a really stupid question, isn't it? I reckon I should just stick
> to using a slow RC reset.. 

If I where you I _would_ stick with the RC-network stuff. Why? Not that it is
the neatest solution, but it is a way for you to avoid getting a bunch of
questions from customers using the brand X PSUs and not the flashy brand Y PSU
which you used for all the testing. That is, you don't have control over how
fast the rampup really is, so why not make yourself sufficiently independent of
it to make this issue virtually irrelevant in your relations to all those other
people having it.

But I agree, it would be neat to do without it. Minimalism is an artform in its
own right, but you should cut the right corners. If you where suppling the PSU
to go along with the MIDI-2-CV then you could rely on it's rise-time to make
it work. But that would be more hassle for you I guess...

Cheers,
Magnus - DIYed a new gate for the summerhouse today - a hammer & nail job!