MEGA328P SPI - Issue - please help me understand

[Anobium]

I justed posted the code below.

My question Why must I set portb.2 as an output? This took me hours to figure out. The demos use portb.2 as the CS line but the code below did not - I used a shield and the shield exposes portd.4.

I had to set portb.2/Digital 10 as an output - if I did not hardware SPI failed - why?

Ideas? Comments?

Sample Code Commentry

This shows how the MEGA328p can support hardware and software SPI. To use hardware make sure the #define SPI_HardwareSPI is not commented out.
To use software comment out #define SPI_HardwareSPI.

To use the hardware SPI the Data Out, Data In and Clock (DO/DI and SCK) cannot be moved but the optional Data Command, Chip Select and Reset are all moveable.

To use the hardware SPI the Data Out, Data In and Clock (DO/DI and SCK), Data Command, Chip Select and Reset are all moveable.

I have called InitSPIMode to call SPIMode if needed, and set the lines.

I have also define a sub called SendByteviaSPI to handle whether to call the Hardware or use Software (bit-banging) SPI.

'Chip Settings.
    #chip mega328p, 16
    #option explicit
    #include <UNO_mega328p.h >

    #define SPI_HardwareSPI  'comment this out to make into Software SPI but, you may have to change clock lines

    'Pin mappings for SPI - this SPI driver supports Hardware SPI
    #define SPI_DC       DIGITAL_8         ' Data command line
    #define SPI_CS       DIGITAL_4          ' Chip select line
    #define SPI_RESET    DIGITAL_9          ' Reset line

    'mandated for hardware
    #define SPI_DI       DIGITAL_12          ' Data in | MISO 
    #define SPI_DO       DIGITAL_11          ' Data out | MOSI
    #define SPI_SCK      DIGITAL_13          ' Clock Line

    dir SPI_DC    out
    dir SPI_CS    out
    dir SPI_RESET out
    dir SPI_DO    Out
    dir SPI_DI    In
    dir SPI_SCK   Out

    dir DIGITAL_10 Out    'needed and I am not sure why!

    DIM byte1 As byte
    DIM byte2 As byte
    DIM byte3 As byte
    DIM tt as byte
    DIM SPISendByte as byte

    byte1 = 100 ' temp values (will come from potentiometer later)
    byte2 = 150
    byte3 = 200

    InitSPIMode



    do forever
        set SPI_CS OFF;
        set SPI_DC OFF;
        SendByteviaSPI (byte1)
        set SPI_CS ON;
        set SPI_DC ON

        set SPI_CS OFF;
        set SPI_DC OFF;
        SendByteviaSPI (byte2)
        set SPI_CS ON;
        set SPI_DC ON

        set SPI_CS OFF;
        set SPI_DC OFF;
        SendByteviaSPI (byte3)
        set SPI_CS ON;
        set SPI_DC ON

        wait 100 ms
    loop



sub InitSPIMode

      #ifdef SPI_HardwareSPI
          SPIMode ( MasterFast, SPI_CPOL_0 + SPI_CPHA_0 )
      #endif

      set SPI_DO OFF;
      set SPI_CS ON;   therefore CPOL=0
      set SPI_DC ON;   deselect

End sub

sub  SendByteviaSPI( in SPISendByte )

  set SPI_CS OFF
  set SPI_DC OFF;

  #ifdef SPI_HardwareSPI
     FastHWSPITransfer  SPISendByte
     set SPI_CS ON;
     exit sub
  #endif

  #ifndef SPI_HardwareSPI
  repeat 8

    if SPISendByte.7 = ON  then
      set SPI_DO ON;
    else
      set SPI_DO OFF;
    end if
    SET SPI_SCK On;           ; therefore CPOL=0 ==ON, and, where CPOL=1==ON
    rotate SPISendByte left
    set SPI_SCK Off;          ; therefore CPOL=0  =OFF, and, where CPOL=1==OFF

  end repeat
  set SPI_CS ON;
  set SPI_DO OFF;
  #endif

end Sub

~~~

[Anobium]

Read the datasheet!!

*Master Mode
When the SPI is configured as a Master the user can determine the direction of the SS pin (this is DIGITAL10). If SS is configured as an output, the pin is a general output pin which does not affect the SPI system.
Typically, the pin will be driving the SS pin of the SPI Slave.
If SS is configured as an input, it must be held high to ensure Master SPI operation. If the SS pin is driven low by peripheral circuitry when the SPI is configured as a Master with the SS pin defined as an input, the SPI system interprets this as another master selecting the SPI as a slave and starting to send data to it.
*

So, when we initialise the Mega328p it is an input. The input left floating will be interpreted as another master selecting the SPI and all communications cease. Therefore, if I do not use Digital 10 as SS/Chip Select either set as an output or pull high.

Case Closed - RTFM