Re: [Gumstix-users] Getting started

[Dave H. <dhy...@gm...>]

Hi Drew,

> I'm wanting to maybe link a few basic circuits up to my gumstix system for
> such things as temperature sensing, status LEDs etc.
>
> I think the simplest idea would be to get an LED working that I can control
> from the gumstix for such things as "new mail" etc.

You need to find a GPIO line that isn't being used and connect the
LED, along with a current limiting resistor to that pin.

A likely set of pins would be the AC97 pins (normally used for
connecting up an audio codec - like the one used on the audiostix).

Lets suppose you use pin 2 (pin1 is the square pad - pin 2 is next to
pin 1, but right next to the edge of the board). This is the X_BIT_CLK
signal which connects to pin 58 on the hirose connector, which is GPIO
28.
This page shows this as well:
<http://docswiki.gumstix.com/index.php/Verdex_60pin_connector_chart>,
although it's called just BITCLK.

You can then do this from the command line:

modprobe proc_gpio
echo "GPIO out set" > /proc/gpio/GPIO28
echo "GPIO out clear" > /proc/gpio/GPIO28

to toggle between set (3.3v) and clear (0v) on that pin.

The pin can only drive a few mA, so you need to size your current
limiting resistor appropriately. For a typical red LED, which drops
1.7v, the voltage through the current limiting resistor will be (3.3 -
1.7 = 1.6v)
V=IR, so R=V/I. Picking a resistor of 1600 ohms or higher will
restrict the current to 1mA.

-- 
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/