Hi Tomas,
ICs typically have two max ratings for very fundamental things (like
temperature, voltage, current, etc.). Absolute max, and typical max.
Absolute max means "It may be permanently damaged if you go above this"
Typical max means "It may not work properly if you go above this, and
life may be shortened too"
Good engineering design practice is never to exceed the typical max,
since you need the device to operate correctly, not just remain
undamaged.
Sean
On Sun, Jul 13, 2008 at 8:23 PM, Tomás Ó hÉilidhe <
toe@...> wrote:
>
>
> Robert Young wrote:
>>> Do you reckon it's OK to pull a full 25 mA from a pin if you're doing
>>> display multiplexing?
>>>
>>
>> No it is not OK. There is a maximum power dissipation rating for the chip. This sets a limit to the total current that can be drawn from ALL pins.
>>
>
>
> The total limit -- if we understand each other -- is 90 mA for all pins
> combined. I've only got two pins sourcing current at a time, so I'm not
> going over the limit for "ALL pins" as you call it.
>
>
>>> I know you can probably draw 30 mA and get it way with it, (PIC's being
>>> bulletproof and all that), but I'm trying to think in terms of a
>>> developing a rock-solid product that has minimal faulty units, something
>>> that will sell in the millions all over the world.
>>>
>>
>> Then don't exceed ANY rating.
>
>
> I don't think anyone can be that pedantic about it, because after all
> the PIC datasheet itself doesn't give a maximum rating. It tells you "25
> mA" but then it kind of shies away and says "yeah but you probably don't
> wanna draw 25 mA from it". So then what's a safe current to draw? 24.9
> mA? 23 mA? 20 mA? We're left in the dark.
>
> Judging by the amount of abuse I've seen PIC chips being able to take, I
> get the feeling I'd be A-OK to draw 25 mA when multiplexing my display.
>
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