RE: [stella] Current for Port Output

Subject: RE: [stella] Current for Port Output
From: "patrick lichty" <voyd@xxxxxxxx>
Date: Wed, 29 Sep 2004 17:41:58 -0400
All right!
Now here's a subject where I'm not lagging the crowd.  

OK< here we go.

First, there are technical concerns, and pragmatic ones.

In the 70's most circuits were operating on what's called TTL logic.
That's where you get the whole 5v thing.  What I see coming out of the
port is a logic level (5v) that's enough to drive something like a pin
set up as a serial port on a Stamp or a PIC processor.

However, in being able to sink 1.6 mA, I don't recommend driving an LED
with it.  

In 1985, I networked 2 Atari 800's though their serial ports, and it
worked fine, but to drive anything other than a logic level, I needed to
throw it through a transistor driver, a 7406 buffer, or something that
could drive more than 5-6 mA.

And, considering that the new white LEDs have forward voltages of up to
3V @ 20 MA, that's a hell of a lot of power.  (Currentxvoltage=power,
therefore the white LED's pulling 60mW, while a red LED[1.7 Forward
Voltage, 12ma current would draw 20mW)

The Riot's port can only handle about 9 mA.  You can probably find an
LED to run from there, but you'll probably bake it eventually.

Pragmatically, we're talking about 20+year-old electronics!  Why stress
them out?  Slap a buffer on there, and give them a break!  It's not
tough.  Just make sure you have the right resistor on the LED to limit
the current.

Patrick Lichty
Intelligent Agent Magazine
1556 Clough Street, #28
Bowling Green, OH 43402
225 288 5813
"It is better to die on your feet 
than to live on your knees." 

-----Original Message-----
From: Paul Slocum [mailto:paul@xxxxxxxxxxxx] 
Sent: Wednesday, September 29, 2004 10:29 AM
To: stella@xxxxxxxxxxxxxxxxxx
Subject: Re: [stella] Current for Port Output

> From the RIOT datasheet:
>Output High Current (Sourcing):    -100 uA
>Output Low Current (Sinking):    1.6 mA
>Is that what you mean?

I don't think so, but not totally sure what those values mean.  My EE 
knowledge is pretty limited.  I'm wondering how much current it can 
drive.  I know they were thinking of using it to operate an LED, so I'm 
assuming it can drive at least 20mA or so?


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