16 input-only GPIOs over a 5 MHz SPI bus
The PCA9703PW,118 is an SPI-to-GPIO expander from NXP that reads 16 digital inputs through a single serial interface. Each channel is a dedicated input-only pin — there is no output register, so the part is strictly a read-back port expander for monitoring switches, sensors, or logic levels. The SPI clock runs at 5 MHz. Reading all 16 bits in one transaction takes 3.2 µs plus the chip-select overhead, which keeps the bus utilisation low even when polling at a few kilohertz. The interrupt output (INT) flags a state change on any input without continuous polling — the host reads the shift register only when the line asserts.
24-TSSOP footprint and board integration
The fine pitch limits trace fan-out between the pins on a two-layer board — a 4-layer stack-up with a ground plane under the package simplifies routing and keeps the SPI signal integrity clean at 5 MHz. The output type is push-pull, so the INT line drives HIGH and LOW actively — no external pull-up resistor needed on that pin. The GPIO inputs are 5 V tolerant at the pin, matching the supply rail.