🚀 go-pugleaf

OK, I know I'm setting myself up for a fall here, but I'm going to
assert that most of the "answers" about hooking an LED to an 8051
port pin have been only partially correct, and some have been completely
wrong.

  The MCS-51 port pins (with some exceptions) are what Intel
refers to as "quasi-bidirectional". What this means, is that they
can operate as both an input and output. The way it works, a port
pin is essentially an open-drain (open-collector for TTL fans) output
with a weak (5k) pull-up resistor to the +5V supply. This is internal
to the chip. When the pin is outputting a '1', it is capable of sourcing
very little current, and can be used as an input. When the output is set
to '0', it is capable of sinking something like 2mA (I don't have the data
in front of me).

  There are some gotchas, however. Port 0 is normally open drain, with
no internal pullups. When using external memory with the processor, Port 0
and Port 2 provide the external multiplexed address/data bus, and will use
strong internal pullups when driving address/data information.


 So, where does this leave the gent trying to interface an LED to the
pin?

  Well, if you connect the LED from the pin to ground, it won't light
very brightly since the LED current is maybe 750 uA. If you connect it
from the +5V supply to the pin (observing correct polarity), it will
light with reasonable intensity since the LED current is probably
5-10mA, but, if you connect several LEDs to the processor this way,
you could potentially exceed the chip power dissipation rating since
each output pin would be good for about 40 mW of dissipation.

  A good choice is to use a common emitter NPN transistor buffer for
the LED, as previously suggested. However, you do not need a
resistor in series with the base, since the 8051 will drive the
base through a 5k resistor. Also, I'd suggest using a 330 ohm resistor
to limit the LED current to 10 mA. If you prefer a brighter LED, you
could use a 150 ohm resistor giving about 20 mA of current. The transistor
could be a 2N2222, but any general purpose plastic part will do (2N3904
or MPQ3904, for example).

Good luck.

-- 
 * Dana H. Myers KK6JQ 		| Views expressed here are	*
 * (213) 337-5136 		| mine and do not necessarily	*
 * dana@locus.com  DoD #466 	| reflect those of my employer	*
 * "Dammit Bones, spare me the lecture and give me the shot!"   *