What this SPI expander brings to the board
The Microchip MCP23S08T-E/ML is an 8-bit SPI GPIO expander that adds eight push-pull I/O lines to any MCU with an SPI port. It runs at a 10 MHz clock rate over the SPI bus, which keeps the pin-count overhead low — three wires (SCK, SDI, SDO) plus a chip-select give you eight extra GPIOs without burning through the MCU's own pins. The supply range spans 1.8V to 5.5V, so it bridges 3.3V and 5V logic domains without extra level shifting. Each I/O can source or sink 25 mA, enough to drive LEDs directly or switch a small relay through a transistor. The part includes a Power-On Reset (POR) circuit that holds the outputs in a known state until the supply is stable, which avoids glitches on power-up. An interrupt output pin signals the host when an input changes, so you don't have to poll the expander continuously — useful for button matrices or sensor edge detection where the MCU needs to sleep between events.
The tape-and-reel variant (the 'T' suffix) is the standard production reel for automated pick-and-place — the cut-tape option exists for prototype runs and small-batch builds.