Lpc2000

rtstofer wrote:

>--- In lpc2000@yahoogroups.com, "Gus" <gus_is_working@...> wrote:
>  
>
>>I don't know what Leon is trying to do here but no you can't do 
>>that. If you know electronics well you will know why but if you are 
>>hobbyist then just follow the many examples and schematics out there.
>>
>>I agree you should read the data sheet but to newbies, data sheets 
>>are nightmares LOL
>>
>>Gus
>>
>>    
>>
>
>Leon is correct!  For a short period of time...
>
>According to the LPC2141_42_44_46_48 Datasheet, Iohs (HIGH-state
>short-circuit current is -45 mA and note 8 applies.  Note 8 says "Only
>allowed for a short time period."  So, it can be done.
>
>It is a poor idea and I wouldn't do it even if I could limit the time
>period.  But the correct answer to the question is yes, it can be done.
>
>I would have to dig deeper to resolve the LED issue.  I have seen it
>done with multiplexed displays and I have done it myself.  It works
>ok.  But, I don't have to guarantee my work.
>
>  
>
That depends on the driver used, some of those multiplexed dot matrix 
driver chips have internal current limiters and can withstand a dead 
short.  This is unlike a CPU port pin, the short-circuit current spec is 
typically for ATE testing purposes.  Understanding that spec can assist 
you in debugging a logic problem, watch the signal on a 'scope and then 
tap it with a voltage lead to see if it moves type of thing (e.g. "is 
this shorted to ground?").

Having said that, you can eliminate current limiting resistors if you 
"pop" (pulse) current into the LEDs.  I'm sure that a steady state drive 
would fry the LEDs, though you can "hit" them with a pulse of current 
designed to make them "ignite" and remove the current before the 
internal junction temp gets too high.

It does mean that you have to read the datasheet and understand the 
parameters of the parts.

TomW

-- 
Tom Walsh - WN3L - Embedded Systems Consultant
http://openhardware.net, http://cyberiansoftware.com
"Windows? No thanks, I have work to do..."
----------------------------------------------------