Lecture 11 - Serial Peripheral Interface (1)

Lecture 11 - Serial Peripheral Interface (1)

1. SPI Port Structure

   • Has 3 Wires:
     • SDI - Serial Data In
     • SDO - Serial Data Out
     • SCK - Shift Clock (O/P in Master Mode)

2. A Simple Serial Link

   • SPI port operates as Master or Slave
     • When operating in Master Mode
       • Generates SCK and sends it out to Slaves
     • When operating in Slave Mode
       • The devices uses the external clock generated by the Master
   • SPI is a Bi-Directional Synchronous Serial Interface

3. SPI Interconnections for Multiple Slaves

   • Only one active Slave at any time
     • Determined by which Slave Select line is activated by the Master
       • Sometimes Chip Select used for the same role

4. SPI Port Operation (Master Mode)

   • Loading Data into SPIBUF initiates an SPI Output
     • Data drops through into SPISR
       • Data is shifted serially out of SDO under control of SCK
       • Data is shifted serially into DSK
   • Interrupt Flag SPIIF goes high on completion of the 8/16-bit shift operation
   • Data can be 8-bit or 16-bit (selectable)

5. SPI Waveforms

   • Data written to SPIBUF
     • CLK Starts on transfer to SPISR
       • SDO updated on -VE edge of CLK (+VE also possible)
         • SDI samples on +VE edge of CLK (-VE also possible)
   • After 8/16 shifts SPIIF goes high

6. MOSI and MISO

   • May be alternatives terms for SDO and SDI
   • MOSI - Master Out, Slave IN
     • Pin connects to SDO in Master mode
     • Pin Connects to SDI in Slave Mode
   • MISO
     • Pin connects to SDI in Master mode
     • Pin connects to SDO in slave mode

7. SPI on MBED

   • MBED supports Master and Slave modes
     • uses MISO and MOSI
   • SPI
   • format
   • frequency
   • write

8. SPI Data Transmission Modes

   • Clock can idle, high, or low
   • Data can be O/P or I/P on +VE OR -VE edge of SCK

9. Input Via SPI Master

   • No direct way of inputting via SPI Master
   • Output dummy data, SPI Port reads in, while outputting dummy data
   • write() returns an integer
     • Which is the data input whilst outputting its operand