This is what we'll be using. I hope your laptop has one!
Nah, don't worry. This connector, the DE-9, when out of fashion on PCs quite some time ago; it got replaced by the Universal Serial Bus (USB). We won't be dealing with the DE-9 connector itself but with the communication protocol that this cable is/was usually used for.
So what's this "serial" communication? It's an asynchronous communication protocol where two devices exchange data "serially", one bit at a time, using two data lines (plus a common ground). The protocol is asynchronous in the sense that neither of the data lines carries a clock signal. Instead both parties must agree on how fast data will be sent along the wire before the communication occurs. This protocol allows "duplex" communication as data can be sent from A to B and from B to A simultaneously.
We'll be using this protocol to exchange data between the microcontroller and your laptop. In contrast with the ITM protocol we have used before, with the serial communication protocol we can send data from your laptop to the microcontroller.
The next practical question you probably want to ask is: How fast can we send data through this protocol?
This protocol works with frames. Each frame has one "start" bit, 5 to 9 bits of payload and 0 to 2 "stop"" bits. The speed of the protocol is known as baud rate and it's quoted in bits per second (bps). Common baud rates are: 9600, 19200, 38400, 57600 and 115200 bps.
To actually answer the question: With a common configuration of 1 start bit, 8 bits of payload, zero stop bits and a baud rate of 115200 bps one can, in theory, send 12,800 frames of 9 bits per second. Since each one carries a byte of information that results in a data rate of 12.8 KB/s. In practice, the data rate will be lower because of processing times on the slower side of the communication (the microcontroller).
Today's laptops/PCs don't support the serial communication protocol. So we can't directly connect your laptop to the microcontroller. But that's where the serial module comes in. This module will sit between the two and expose a "serial" interface to the microcontroller and an USB interface to your laptop. The microcontroller will see your laptop as another "serial" device and your laptop will see the microcontroller as a virtual serial device.
Now, let's get familiar with the serial module and the serial communication tools that your OS offers. Pick a route: