When interfacing 3.3V devices with a 5V Arduino, the logic level needs to be shifted. Most 3.3V devices will be damaged if operated with 5V signals, while others will simply operate unreliably. This is where the inexpensive 74LVC245 comes in. It is designed to operate within -0.5 and 6.5V, which makes it perfect for the Arduino. It has 16 pins (8 pairs) for conversion in a 20-pin dip package. Unidirectional shifting can occur in either direction so it’s perfect for SPI, Serial, Parallel and other similar logic interfaces.
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