8. Inter-Integrated Circuit#
Similar to UART protocol, that we discussed in last chapter, I2C (Inter-Integrated Circuit) is also a communication protocol. It also uses two wires, but one for a clock signal and the other for the data transmission in this case. Thus, I2C is a synchronous protocol since all the devices communicating through I2C use the same reference clock signal to operate. Moreover, I2C allows more than 2 devices to share a set of wires for communication, which was the biggest drawback in the case of UART.
The direction of information flow in I2C communication is controlled by a single device at a given time. This device is termed as a Controller. Other devices that take instructions from the controller are termed as Targets. The I2C protocol allows for multiple controllers and multiple targets connected to a set of clock and data lines, wires carrying clock and data signals. Following figure shows a general wiring configuration for I2C communication,
The two wires, clock and data lines, are called SCL (Serial Clock) and SDA (Serial Data) respectively. Note that the two lines are connected to Vdd, 3.3V in case of RP2040, through a resistor. This represents the Pull-Up configuration that we discussed in Chapter 4. In the idle state, i.e. when no communication is happening, pins of all devices connected SCL and SDA are in Input configuration. Thus, the pull-up resistors keep the voltage in SCL and SDA lines high. Moreover, these pins change the configuration between Input and Output based on whether a device is supposed to control the line voltage or not at a given moment. This process is discussed in further detail in the next section.
The RP2040 has two independent I2C instances, i2c0 and i2c1. Each instance can be connected to multiple GPIOs with a specific function, SCL or SDA, as defined in the GPIO Functions table. The external physical pull-up resistors are not needed here since the internal pull-up configuration for the μC can be set up through software. However, if no device communicating through I2C have pins with internal pull-up configuration then the physical pull-up resistors are necessary. In any case, the wiring for I2C communication with the μC can be done as shown below,
