Consider a GPIO as a pin whose behavior will be controlled by the software. The raspberry pi board has a 26-pin expansion header containing 17 GPIO pins that can be used to control external devices (lights, LCD display, ...) or to read information from the outside (temperature sensor, ADC output, etc ...).
As shown on the picture, some of the GPIOs are already used (depending on your kernel) for communication lines such as I2C, SPI or UART. It doesn't mean that we can't use them for other purposes, their usage can be customized.
GPIO voltage level
1 : High-Level : 3,3V0 : Low-Level : 0V
Current is configurable from 2mA up to 16mA (for the whole block, no just only one pin)
Important note : there is no over-voltage protection circuit on the board. Be careful when plugging your active devices.
GPIO control
Two choices here :- usage of the kernel drivers in arch/arm/mach-bcm2708 (not included in vanilla kernel sources)
- usage of a dedicated library installed in user space
Note : the raspberry pi is usually referred as bcm2835 but some of the drivers are located in bcm2708 directory. Technically bcm2708 is the chip family and bcm2835 is one of its implementation, the only one officially supported by linux.
Senior software engineer with 12 years of experience in both embedded systems and C# .NET
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