Jan-Erik,

  "Very high" could be any value.  Most likely greater than 1 volt at 
triple the rated current.  If I assume a Vol of 0.2v at 0mA then the Rs 
is about 47 ohms at 8.5mA.  Thus at 21mA the Vol would be ~1.2v 
(neglecting other effects.)

  Why do I care?  In this case the LEDs forward drop is rated at 3.5v @ 
7mA.  That only leave me with about 1.5 volts to play with.  If the 
output is 0.5v then I need a 140 Ohm resistor but if it is 1v I need a 
70 Ohm resistor.  If I choose one end or the other I could have a 
two-to-one variance in LED current and thus about 25% change in 
luminosity.  Maybe choosing a "middle" value is the best solution.

Phil

Jan-Erik Soderholm wrote:

>Phil Keller wrote :
>
>  
>
>>The way I am interpreting this is that the device will not 
>>be damaged at 25mA but the Vol could be very high and
>>would/could be different from one device to the next.
>>    
>>
>
>Define "very high" !
>
>  
>
>>To guarantee a logic low (Vol <= 0.8) I need to keep the 
>>current down to 8.5mA.
>>    
>>
>
>Why does it matter ?
>Ae you going to use the pin as a LED-drive and as a logic
>output pin at the same time ?
>
>Note also that modern high-efficiency LEDs are
>perfectly visible down to a few mA's.
>
>Regards,
>Jan-Erik.
>
>
>
>  
>

-- 
===========================================
PK Consulting
Phil Keller
Fremont, Ca

Phil@PKConsulting.net



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