Sometimes in digital electronics you need to figure out how things communicate. Maybe you need to hack LCD, or capture data packets from unknown device. This is some sort of reverse engineering. To do this you definitely need a logic analyzer. As a hobbyists you might skip the buying option especially when you need it rarely. Joonas have been debugging his PS/2 keyboard project and decided to build a simple logic analyzer to ease the work.
As base he’s chosen Attiny2313 microcontroller clocked at 20MHz. FT232RL USB to serial converter takes care of sending data to PC. Using fast serial mode it is possible to achieve 1Mbps baud. Attiny2313 has 8 port B pins that are used for 8 channels. For sake of speed microcontroller software simply sends port B values to USART using interrupt routine. On PC using terminal software like Putty the data can be captured and logged that are then plotted for analysis. Analyzer is capable to capture rates up to 100kHz. This is not very high but for slower signals might do the business.