Discussion of APA102 and the SPI interface

If you cast your mind back to Chapter 5, Connecting Your Raspberry Pi to the Physical World, where we discussed Serial Peripheral Interface (SPI), you may remember that we mentioned it uses four wires for data transfer. However, if you consider our circuit in Figure 8.6, we're only using two wires (DI and CI), not four. What's going on?

Here is the SPI mapping for the APA102:

• Master-Out-Slave-In (MOSI) on your Raspberry Pi connects to Data In (DI) on the APA102. Here, your Raspberry Pi is the master sending data to the slave APA102 LEDs on the strip.
• Master-In-Slave-Out (MISO) is not connected because the APA102 does not need to send data back to the Raspberry Pi.
• SCLK on your Raspberry Pi connect to the Clock In (CI) on the APA102.
•  Client Enable/Slave Select (CE/SS) is not connected.

The last line CE/SS of importance and worthy of further discussion. A CE/SS channel is used by a master device to tell a specific...