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, it's 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 F3 board we are using has three motion sensors in it: an accelerometer, a magnetometer and gyroscope. The accelerometer and magnetometer are packaged in 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 "master" / "slave" model where the "master" is the device that starts and drives the communication with a "slave" device. Several devices, both "masters" and "slaves", can be connected to the same bus at the same time. A master device can communicate with a specific slave device by first broadcasting its address to the bus. These address can be 7 bits or 10 bits long. Once a master has started a communication with a slave, no other device can make use of the bus until the master 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).
LSM303DLHC specific protocol
I2C operation - Section 5.1.1 - Page 20 - LSM303DLHC
I2C registers - Section 28.7 - Page 868 - Reference Manual
Read a single register. Each register is one byte.
(gdb) next # several times (gdb) p/t byte $1 = 1001000
Receive several bytes.