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.