The Serial Peripheral Interface bus (SPI) is a synchronous serial communication interface specification used for short distance communication, primarily in embedded systems. SPI devices communicate in full duplex mode using a master-slave architecture with a single master. The master device originates the frame for reading and writing. Multiple slave devices are supported through selection with individual slave select (SSEL) lines.
All pin variants provide an SPI interface which has a single slave select pin (SSEL).
If you need more slave selelects, the UX2-GPIO or UX2-ALT variants provide Gx (GPIO) pins that can be utilised as additional slave selectors.
Max 5 slaves; UX2-GPIO – 4 x GPIO, 1 x UART, 1 x I2C, 1 x SPI:
Max 3 slaves; UX2-ALT – 2 x GPIO, 1 x UART, 2 x I2C, 1 x SPI:
Max 1 slave; UX2-1IS – 1 x 1-WIRE, 1 x INTERRUPT, 1 x UART, 1 x I2C, 1 x SPI, 1 x SOUND-WIRE:
Max 1 slave; UX2-I2C – 3 x I2C, 1 x UART, 1 x SPI:
The SPI interface uses at least 6 wires (MISO, MOSI, SCK, SSEL, 3V3 and GND); GPIO (Gx) pins can be used as additional slave selects if more than one slave is required (alternatively, use the daisychain topology; see later).
| UX2 | SPI |
|---|---|
 |
MISO (master in, slave out) |
 |
MOSI (master out, slave in) |
 |
SCK (slave clock) |
 |
SS (slave select) |
 |
SS (use as exta SSEL) |
 |
VCC |
 |
GND |
SPI provides two network topologies...
Linear:
This is the most common topology; it uses one slave select (SSEL or Gx pin) per slave.
This allows direct communication with each specific slave, but only one slave can be used at a time.
Daisychain:
An alternate topology which arranges all slaves in a circular circuit, requiring only a single SSEL pin.
This topology is generally only used with combinations of devices that are designed for this mode of operation; specifically where it is beneficial to daisychain all the shift registers togheter.