When using the I2C bus, if the internal chip does not integrate pull-up resistors, two external pull-up resistors need to be added.
The reason is that the I2C bus generally has an open-drain (OD) structure, where OD stands for Open Drain, meaning it is a leakage circuit, with Drain referring to the drain of the MOS transistor.
As shown in the I2C bus structure diagram below, the pull-up resistors marked in red and the internal MOS transistors.
The following diagram shows a typical OD gate circuit built with two MOS transistors.
Working Principle:
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When INPUT=0, Q1 is off, Q2 is on, which is equivalent to the switch being closed, OUTPUT=0V;
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When INPUT=1, Q1 is on, Q2 is off, which is equivalent to the switch being open, OUTPUT is in a high-impedance state;
The above OD gate circuit cannot output a high level, so pull-up resistors are needed.
The I2C bus can have one master and multiple slaves (1 master, multiple slaves), because the high-impedance state of the OD gate allows multiple OD gates to be connected together, forming a “wired logic”, as shown in the diagram below. This is the reason why I2C can have one master and multiple slaves.
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