We just saw the serial communication protocol. It's a widely used protocol because it's very simple and this simplicity makes it easy to implement on top of other protocols like Bluetooth and USB.
However, its simplicity is also a downside. More elaborated data exchanges, like reading a digital sensor, would require the sensor vendor to come up with another protocol on top of it.
(Un)Luckily for us, there are plenty of other communication protocols in the embedded space. Some of them are widely used in digital sensors.
The micro:bit board we are using has two motion sensors in it: an accelerometer and a magnetometer. Both of these sensors are packaged into a single component and can be accessed via an I2C bus.
I2C stands for Inter-Integrated Circuit and is a synchronous serial communication protocol. It uses two lines to exchange data: a data line (SDA) and a clock line (SCL). Because a clock line is used to synchronize the communication, this is a synchronous protocol.
This protocol uses a controller target model where the controller is the device that starts and drives the communication with a target device. Several devices, both controllers and targets, can be connected to the same bus at the same time. A controller device can communicate with a specific target device by first broadcasting its address to the bus. This address can be 7 bits or 10 bits long. Once a controller has started a communication with a target, no other device can make use of the bus until the controller stops the communication.
The clock line determines how fast data can be exchanged and it usually operates at a frequency of 100 kHz (standard mode) or 400 kHz (fast mode).