Lecture 12 - Serial Peripheral Interface (2)

Lecture 12 - Serial Peripheral Interface (2)

1. SPI on MBED

   • SPISlave
   • format
   • frequency
   • receive
   • read
   • reply

2. I/O Port Expansion

   • The MBED has 26 general purpose I/O lines (p5-p30)
   • Many I/O lines have multiple roles
     • Not all I/O lines are available
   • Extra I/O resources are sometimes required
     • e.g. where a large number of indicators are required
     • e.g. external memory, accelerometer, temperature sensors...
   • The MBED incorporates an SPI for this type of I/O expansion

3. Use of SPI to Realise Extra 8-bit Output Port

   • Extra 8-Bit Parallel Output Port
     • The 74HC595 is an 8-bit serial load register with output latch, outputs remain unaffected while new data is shifted in
     • Data shifted in by ShiftCLK, O/P updated by LatchClock
     • Transfer to 74HC595 O/P register is achieved by writing to SPIBUF then pulsing LatchClock
     • HC595 inputs on +VE clock edge
     • Therefore SPI Port set up to Output on -VE clock edge

4. Use of SPI to Realise Extra 16-Bit Output Port

   • In this case two 74HC595s are wired as a 16-bit shift register
   • Can be used as 16-bit O/P port or two 8-bit ports
   • This approach can be extended

5. Reading Input Port

   • Data shifts on +VE edge of SCK
     • Data Sampled by MISO on -VE edge of SCK so data is stable when sampled
   • To read I/P Port:
     • Load 74HC165, by pulsing LOAD
       • Clear SPIIF
         • Write dummy data to SPI port to start shifting process
           • Wait for SPIIF to go high (Data shift complete)
             • Read SPI Port

6. Interfacing to Several Devices using SPI

   • SPI Pins connect to all devices
   • Separate CS Lines
     • P10 selects EEPROM
     • P11 selects RTC
     • P12 selects RS232 interface chip

7. SPI Assessment

   • The electronic hardware is simple, data can be transferred rapidly
   • Negatives:
     • Each Slave requires its own CS Line
       • Expensive in pinout for large system
     • There is no acknowledgement from the receiver
     • Three is no error-checking
     • Overall SPI is simple, convenient and low-cost, but not suitable for complex/high reliability systems