SPI pins reserved for Ethernet

Which Arduino pins the W5100 shield uses and which GPIO remain available for LCD and DHT22 lines.

The W5100 Ethernet shield uses hardware SPI on the Uno: MOSI, MISO, SCK, plus chip select on pin 10. Pins 10–13 and SPI lines must not be repurposed for LCD or sensors in standard layouts.

Reserved lines

  • D10: W5100 chip select (SS).
  • D11–D13: SPI MOSI, MISO, SCK on Uno.
  • Interrupt pin: Shield-specific; follow vendor silkscreen.

Planning GPIO budget

LCD 4-bit mode consumes six GPIOs; two DHT22 lines consume two more—plan before adding features. Stacking details: Ethernet shield stacking.

Alternative stacks

If pins run out, consider I²C LCD modules or offloading HTTPS to Python feeds while keeping JSON on HTTP locally.