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20.1: Introduction

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  • In this chapter we shall investigate the basic digital input circuitry. This can be used to sense the state of external switches and other two-state devices. These devices can be active, that is, generating a high or low voltage, or they can be simple passive switches connecting to ground through the use of an optional internal pull-up resistor. A simplified diagram of the General Purpose Input-Output (GPIO) circuitry can be seen in Figure \(\PageIndex{1}\). As in the previous chapter, this is for a single bit. Once again, we shall remove sections that are not pertinent to the input function or for enhanced clarity.

    GPIO circuitry (from Atmel 2014).
    Figure \(\PageIndex{1}\): GPIO circuitry (from Atmel 2014).