Suggested fix to SPI.begin()

There is a problem in SPI initialization as discovered in this thread:

http://arduino.cc/forum/index.php/topic,100671.0.html

The problem code is here from SPI.cpp:

void SPIClass::begin() {
  // Set direction register for SCK and MOSI pin.
  // MISO pin automatically overrides to INPUT.
  // When the SS pin is set as OUTPUT, it can be used as
  // a general purpose output port (it doesn't influence
  // SPI operations).

  pinMode(SCK, OUTPUT);
  pinMode(MOSI, OUTPUT);
  pinMode(SS, OUTPUT);
  
  digitalWrite(SCK, LOW);
  digitalWrite(MOSI, LOW);
  digitalWrite(SS, HIGH);

  // Warning: if the SS pin ever becomes a LOW INPUT then SPI 
  // automatically switches to Slave, so the data direction of 
  // the SS pin MUST be kept as OUTPUT.
  SPCR |= _BV(MSTR);
  SPCR |= _BV(SPE);
}

Bearing in mind that SS (slave select) is normally active low, what the above code does is briefly bring SS low, between these lines:

  pinMode(SS, OUTPUT);
...
  digitalWrite(SS, HIGH);

Depending on the clock polarity, it may appear to the device that it has been selected, and then a bit clocked. To fix this we simply need to bring SS high before making it an output, then the pin is not driven low and immediately high again, like this:

void SPIClass::begin() {
  // Set direction register for SCK and MOSI pin.
  // MISO pin automatically overrides to INPUT.
  // When the SS pin is set as OUTPUT, it can be used as
  // a general purpose output port (it doesn't influence
  // SPI operations).

  digitalWrite(SCK, LOW);
  digitalWrite(MOSI, LOW);
  digitalWrite(SS, HIGH);

  pinMode(SCK, OUTPUT);
  pinMode(MOSI, OUTPUT);
  pinMode(SS, OUTPUT);

  // Warning: if the SS pin ever becomes a LOW INPUT then SPI 
  // automatically switches to Slave, so the data direction of 
  // the SS pin MUST be kept as OUTPUT.
  SPCR |= _BV(MSTR);
  SPCR |= _BV(SPE);
}

That makes sense, maybe suggest in on the developer's mailing list or the bug reporting page (wherever that is).


Rob

maybe suggest on the bug reporting page (wherever that is).

http://code.google.com/p/arduino/issues/list

I know im stil sat in the stone age but this is just one more reason im still using IDE_18 & spi.h because it works, why SPI.h was included in later versions of IDE i dont know, when got new version nothing worked

[quote author=Coding Badly link=topic=101278.msg759948#msg759948 date=1334387083]

maybe suggest on the bug reporting page (wherever that is).

http://code.google.com/p/arduino/issues/list

[/quote]

Submitted as issue 888 (sort of like 666 but worse).