Logic analyzer
A test instrument that records many digital signals at once and shows them as ones and zeros over time.
Where an oscilloscope shows the exact shape of the voltage on a couple of channels, a logic analyzer thresholds everything down to high or low — but across 8, 16 or hundreds of channels. You set a trigger ("when chip select goes low"), let it capture, and then scroll through what the bus actually did rather than what the code claimed.
The instruments appeared in the 1970s and long remained lab equipment with lab prices. What changed firmware work was small USB pods such as Saleae's and their countless clones, which push the heavy lifting onto the PC. Software like sigrok and PulseView then decodes SPI, I²C and UART into readable bytes. The most common discovery remains that clock and data are swapped.