SPI can communicate with multiple slaves using different connection methods:

1. Multidrop Configuration (Most Common)

• Shared lines: MISO, MOSI, and SCLK are shared by all slaves.
• Individual SS lines: Each slave has its own SS (slave select).
• Tri-state MISO:
  • Slaves must tri-state their MISO when not selected to avoid bus contention.
  • If a slave does not support tri-state → use an external tri-state buffer controlled by SS.
    Important:
• All SS lines should start HIGH before communication begins.
• If SS is controlled by GPIOs, configure them as outputs set HIGH before any initialization code runs.
• Optional: Add pull-up resistors to ensure SS defaults HIGH on power-up.
  

2. Daisy Chain Configuration

• Connection:
  • Master MOSI → Slave 1 input
  • Slave 1 output → Slave 2 input
  • … until final slave output → Master MISO
• Effect: Combines all slave shift registers into one long shift register.
• Advantage:
  • Only one SS line needed for all slaves.
  • Ideal for devices that support daisy-chaining (e.g., shift registers).
    Use Cases:
• Parallel-in/serial-out shift registers (e.g. 74xx165).
• Serial-in/parallel-out shift registers (e.g. 74xx595).
• Can integrate with protocols like SGPIO, JTAG, I²C for special applications.
  

3. Expander Configuration

• Uses SPI-controlled addressing units (e.g., binary decoders, demux, or shift registers) to generate multiple chip selects.
• Example:
  • One SS selects a demultiplexer.
  • Master sends an index to choose which slave to activate.
  • Second SS is routed through the demultiplexer to enable that specific slave.