High(er) speed SPI on the Arduino without the SPI library

#include "pins_arduino.h"
#include "wiring_private.h" 

void spiwrite(uint8_t c) 
{
  int8_t i;
  // MSB first, clock low when inactive (CPOL 0), data valid on leading edge (CPHA 0) 
  // Make sure clock starts low

  // slow version - built in shiftOut function
  //shiftOut(_mosi, _clk, MSBFIRST, c); return;

  volatile uint8_t *clkportreg, *mosiportreg;
  uint8_t clkpin, mosipin;

  clkportreg =  portOutputRegister(digitalPinToPort(_clk));
  clkpin = digitalPinToBitMask(_clk);
  mosiportreg =  portOutputRegister(digitalPinToPort(_mosi));
  mosipin = digitalPinToBitMask(_mosi);

  for (i=7; i>=0; i--) {
    *clkportreg &= ~clkpin;
    if (c & (1<<i)) {
      *mosiportreg |= mosipin;
    } else {
      *mosiportreg &= ~mosipin;
    }    
    *clkportreg |= clkpin;
  }

  *clkportreg &= ~clkpin;
  // Make sure clock ends low
}

uint8_t spiread(void) 
{
  //return shiftIn(_miso, _clk, MSBFIRST);

  int8_t i, x;
  x = 0;

  volatile uint8_t *clkportreg, *misoportreg;
  uint8_t clkpin, misopin;

  clkportreg =  portOutputRegister(digitalPinToPort(_clk));
  clkpin = digitalPinToBitMask(_clk);
  misoportreg =  portInputRegister(digitalPinToPort(_miso));
  misopin = digitalPinToBitMask(_miso);


  // MSB first, clock low when inactive (CPOL 0), data valid on leading edge (CPHA 0) 
  // Make sure clock starts low

  for (i=7; i>=0; i--)  {
    *clkportreg &= ~clkpin;
    asm("nop; nop");
    if ((*misoportreg) & misopin)
      x |= (1<<i);
    
    *clkportreg |= clkpin;
  }
  // Make sure clock ends low
  *clkportreg &= ~clkpin;

  return x;
}

High(er) speed SPI on the Arduino, without the SPI library

void spiwrite(uint8_t c) 
{
  int8_t i;
  // MSB first, clock low when inactive (CPOL 0), data valid on leading edge (CPHA 0) 
  // Make sure clock starts low

  // slow version - built in shiftOut function
  //shiftOut(_mosi, _clk, MSBFIRST, c); return;

  volatile uint8_t *clkportreg, *mosiportreg;
  uint8_t clkpin, mosipin;

  clkportreg =  portOutputRegister(digitalPinToPort(_clk));
  clkpin = digitalPinToBitMask(_clk);
  mosiportreg =  portOutputRegister(digitalPinToPort(_mosi));
  mosipin = digitalPinToBitMask(_mosi);

  for (i=7; i>=0; i--) {
    *clkportreg &= ~clkpin;
    if (c & (1<<i)) {
      *mosiportreg |= mosipin;
    } else {
      *mosiportreg &= ~mosipin;
    }    
    *clkportreg |= clkpin;
  }

  *clkportreg &= ~clkpin;
  // Make sure clock ends low
}

uint8_t spiread(void) 
{
  //return shiftIn(_miso, _clk, MSBFIRST);

  int8_t i, x;
  x = 0;

  volatile uint8_t *clkportreg, *misoportreg;
  uint8_t clkpin, misopin;

  clkportreg =  portOutputRegister(digitalPinToPort(_clk));
  clkpin = digitalPinToBitMask(_clk);
  misoportreg =  portInputRegister(digitalPinToPort(_miso));
  misopin = digitalPinToBitMask(_miso);


  // MSB first, clock low when inactive (CPOL 0), data valid on leading edge (CPHA 0) 
  // Make sure clock starts low

  for (i=7; i>=0; i--)  {
    *clkportreg &= ~clkpin;
    asm("nop; nop");
    if ((*misoportreg) & misopin)
      x |= (1<<i);
    
    *clkportreg |= clkpin;
  }
  // Make sure clock ends low
  *clkportreg &= ~clkpin;

  return x;
}

include "pins_arduino.h"

include "wiring_private.h"

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