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Drain

Open-drain outputs have no upper PMOS transistor and are only capable of sinking current. The PCA9535C has no pull-up transistor, but the PCF8575C has a pull-up transistor which is turned on only during the low-to-high transition for ½ clock cycle to help pull the line high. To drive a static high, both devices rely on an external pull-up resistor. Additionally, the PCA9574, PCA9575, and PCA9698 which are 8-bit, 16-bit and 40-bit totem-pole GPIO Expanders respectively, can be configured to turn off the upper PMOS transistor and be used as open-drain devices. The open-drain devices have the additional benefit that they may be used in wired-AND connections. Further, they allow no current flow through parasitic diodes/LEDs when operated on different power supplies with one supply turned off.

PTM Published on: 2013-06-26