11 — Pin No 9 10

On an Arduino Uno, pins 9, 10, and 11 are part of a specific group of pins with dual-purpose functionality:

These pins are critical for SPI communication, which is used to talk to SD card readers, RFID modules, and high-speed displays: Pin 10: SS (Slave Select) Pin 11: MOSI (Master Out Slave In) Pin No 9 10 11

These three pins (9, 10, and 11) are frequently used as the address lines for 4051/4052 analog multiplexers to expand the number of sensors you can read. On an Arduino Uno, pins 9, 10, and

MISO (Master In Slave Out) — often paired with 10 and 11 On an Arduino Uno