Why do we need Open Drain kind of connectivity for devices? What are the advantages? (Day#1 Embedded Hardware Challenge)

 

Open drain in one of the common interfaces used in Embedded applications. The topology of open drain in such a way that DRAIN terminal of the MOSFET is left open, SOURCE is grounded and the GATE is applied with the data we need to output over DRAIN. As the DRAIN in kept open, the open drain configuration cannot be used directly without using a pull-up on drain pin. Keeping the DRAIN open helps to:

• sink the current and the amount of current sinking base don external connectivity like the pull-up resistor 
• tie it to pull-up with any voltage value based on the requirement. 
• helps to tie the output signals together there by helping in OR kind of connectivity
• multiple signals can be connected together which meant interfaces like I2C which use open drain can have several devices connected over the same bus
• the devices connected over that bus can communicate with each other over the same line (bi-directional)

If you heard or know about push-pull then compare with open-drain this way. Open-drain is also a form of push-pull except that open-drain has only pull-down option in the form of BJT or MOSFET internal to the device and a fixed pull-up in the form of resistor connected to a supply desired.