What is the significance of these pins MOSI MISO and SS? I am looking at SparkFun DeadOn RTC Breakout - DS3234

  • If you haven't already purchased one I recommend a module using DS3231 which uses I²C. These are easier to connect to the Pi, which has inbuilt drivers (and probably cheaper).
    – Milliways
    Commented Mar 3, 2017 at 22:58

2 Answers 2


They are the SPI (Serial Peripheral Interface) interface pins. SPI is an interface bus used to exchange data between microntrollers and peripherals. It uses four pins MOSI which stands for master out slave in, MISO is Master in slave out, SS is slave select and a clock line (CLK).

You can read more on the SPI protocol on wikipedia and from sparkfun

  • The DS3234 operate as slave device on serial bus. If I want to work with 3 difference sensors real-time does that mean I need to buy 3 of these clocks?
    – tam
    Commented Mar 3, 2017 at 20:00
  • 2
    @tam Why would you need three real time clocks for a single Pi? Commented Mar 3, 2017 at 20:08
  • @tam this should make connecting multiple devices clearer corelis.com/education/… Commented Mar 3, 2017 at 20:21
  • @SteveRobillard I'm trying to track the speed of three robot cars. 3 Lidars are placed +x, -x, and y. Three cars are coming from those directions where the lidar is at the origin and need it to output the speeds of those three cars.
    – tam
    Commented Mar 3, 2017 at 20:22
  • @SteveRobillard I'm picturing it wrong, but I'm reading the manual on page 11 and says the clock is a slave and the master is the Pi so that's why I was confused of needing three clocks
    – tam
    Commented Mar 3, 2017 at 20:24

A MOSI, MISO, CS and CLK are pins of SPI (Serial Peripheral interface protocol)protocol. SPI uses those separate pins to communicate with the target device. The fullform of those pins are serial clock (CLK), Master Input Slave Output (MISO), Master Output Slave Input (MOSI) and Chip Select (CS).

A CLK pin which provides timing information. A MOSI pin which the Pi uses to send information to the device. A MISO pin which the Pi uses to receive information from the device. A Chip select pin which the Pi uses to indicate which device it’s talking to.

This chip select pin is necessary because multiple devices can share the clock, MOSI, and MISO pins, but only one device can be connected to each select pin.

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