>>>> Almost anything can drive a low-corrent type LED (about 1-2 mA required)
>>>> A lot of PICs have some especially strong outputs, like the PIC14000 I'm
>>>> using.
>>>> Select one you think you can use and RTFM.
>>>I am trying to understand the Port C capability of "Direct LED drive" 
>>>in the PIC 14K manual. It seems to be rated the same source/sink as 
>>>the other ports. What exactly do they mean?
>>>
>>Good question. In the detaild port description (DS40122B-page 30) it 
>>says Port C 5:0 can drive LEDs. No other Port description tells that.
>>But in the electrical characteristics section they only specify different 
>>(lower) current for RC6, RC7, RD0, RD1, OSC2. 
>>All other output pins specified to 0,6 V drop when output high @8,5mA
4,5V Vdd.
>>So the same document tells two different things! 
>>(I also found other things in >>this manual strange, 
>>undefined or double defined, but it is still preliminary, so...)
>>Other manufacturers specifies LED capability when they can drive 20mA.
>>So, are the LED outputs that strong (20mA), and just forgotten in the
electrical 
>>characteristics section?
>>And are they assymetric like all other, (sink much stronger than drive)?
>>/Morgan

>I was hopeful that they had some type or current limit that would let 
>you drive LEDs with no resistor.
>
They don«t.
A while ago, however, I used CMOS hex inverter 4069U to directly drive
LEDs. Theu are weak (deliver 3-5mA I think) but suitable for small LED
indicator. I beleive it was even within the manufacturers spec to to so,
but I don«t remember which.

Maybe the PIC don«t risk get damaged either? (This of course depends
strongly on supply voltage) PIC:s are much weaker in pullup than sink, so
maybe using them to pullup? 

Under maximum ratings they specify (looking at C84) max sink on a pin to
25mA, and source 20mA. (OBS there are also max limitations for a ports
total current)

At 4,5V supply volt the C84 have a max loss of 0,7V@3mA=233ohms.
If the LED need 2V, the loss of the pin = 2,5V
2,5V/233ohms=11mA.  Nice.
BUT BEWARE:
1)      0,7V@3mA is minimum drive of the pin. They don«t specify how good a PI
C
can be at best. In this case a strong low-loss output will maybe make to
much current, so it will get damaged!
2)      The resistance is lower at higher voltages so the current rises quickly.

For LED drive I recommend using PIC to sink, and series resistor.

However, when using the A/D cirquitry in PIC14000 Vss current causes 
measurement ofset, so then use PIC to source instead!
/Morgan
Morgan Olsson, MORGANS REGLERTEKNIK, Sweden, ph: +46 (0)414 70741; fax 70331
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