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[[Category:Raspberry Pi]]{{Chris Tyler Draft}}
{{Admon/note|Model B - Revision 1|Most of these notes apply specifically to the Raspberry Pi Model B Revision 1 board. They are not applicable to the Beta boards, and there are some small differences between the Revision 1 and Revision 2 boards.}}
For custom-designed devices, the GPIO pin headers (and sometimes the audio output and/or USB connections) are usually used.
== PinMux ==
The Broadcom BCM2835 system-on-a-chip used in the Raspberry Pi, like other SOCs, uses pin multiplexing (pinmux) to select which signals will be present on the pins/balls of the chip. This is done because the number of available signals exceeds the number of available pins.
== Precautions ==
The BCM2835 is directly connected to signals present on the GPIO connector. Therefore, when you connect to the GPIO pin header, you are directly attaching to the SOC, and an overvoltage or overcurrent condition can seriously and irreversibly damage or destroy the SOC and render the Raspberry Pi useless.
These conditions cause particular problems:
# Grounding a pin which is expected to be an input when it is configured as an output, either through the GPIO configuration or the PinMux configuration. This will cause as much current to flow as is available from the power supply,