joe
Posts: 29
Joined: Tue Oct 18, 2011 2:40 pm

Re: GPIO level

Tue Oct 18, 2011 3:49 pm

It might be an already-asked question, so sorry, but would the GPIO pins be happy to drive TTL? Would people recommend putting them through a TTL buffer before even think about connecting them to anything else? I imagine they're current-rated for driving logic rather than, say, LEDs?

NoSuchNick
Posts: 42
Joined: Tue Sep 20, 2011 1:38 pm

Re: GPIO level

Wed Oct 19, 2011 1:05 am

According to the wiki, the GPIOs are 3.3V
( http://elinux.org/RaspberryPiB.....Components )
Broadcom seems to be somewhat stingy with their product info, but i wouldn't source more than 20mA from any pin, just to be safe, i would try to be lower.
I didn't find any info on whether the pins are 5V tolerant, so i wouldn't risk it.

LEDs are a wide range, i usually source mine with 2-350mA, so the answer is maybe ;)
If you just want an indicator light, many modern LEDs do pretty well at 5mA, that should work directly. But you are probably going to have a problem driving blue and white LEDs from 3V3.

n1ywb
Posts: 26
Joined: Mon Oct 10, 2011 6:54 pm
Contact: Website

Re: GPIO level

Wed Oct 19, 2011 6:29 pm

Yeah depends what your driving. It won't drive a real 7400 series TTL chip, but many chips nowadays have wider input voltages to accomodate lower voltage logic.

Most logic can't source current for squat but some chips, notably microcontrollers like PICs or AVRs, can sink a reasonable amount, certainly enough to light an LED. I don't know about this chip in particular. Since it's more of an embedded CPU than a microcontroller I would assume that it lacks the open collector driver circuit that makes this possible. Anyway an LED driver circuit consists of a generic NPN switching transistor and a current limiting resistor, both of which you can get at RadioShack.

Here's an example http://www.sciencetronics.com/.....driver.png

n1ywb
Posts: 26
Joined: Mon Oct 10, 2011 6:54 pm
Contact: Website

Re: GPIO level

Wed Oct 19, 2011 6:33 pm

Also blue and white LEDs have a Vf drop of like 3.5-ish volts, so you aren't going to drive one at all directly from 3.3v.

http://www.oksolar.com/led/led....._chart.htm

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