I²C LCD alternative

Free GPIO pins by moving from parallel LiquidCrystal to an I²C backpack module when the Ethernet shield consumes the pin budget.

Parallel mode uses six GPIO lines plus backlight drive—tight on an Uno stacked with W5100. I²C backpacks (typically PCF8574) need SDA/SCL only, freeing pins for extra probes or backlight PWM. The tradeoff is another library, address jumpers, and shared bus timing with any future I²C sensors.

Parallel LCD versus I²C backpack pin count comparison on Arduino Uno
I²C backpacks reduce GPIO use to two data lines plus power—helpful when SPI pins are reserved.

Pin budget math

Review reserved SPI pins and parallel map: two DHT22 lines plus six LCD lines leaves little headroom. I²C reclaims four to six pins depending on wiring.

Migration notes

  1. Install LiquidCrystal I²C library and set backpack address (often 0x27).
  2. Rewire SDA/SCL—Uno A4/A5 on classic boards.
  3. Update display strings in LCD format—character width unchanged.

When to stay parallel

If your shield stack already works and pins suffice, parallel mode remains simpler—DHT22 and LCD overview. Consider I²C when adding a third probe or PWM backlight on a crowded pin map.