No doubt this has been done before, but I didn't immediately find example code. So, here's what I came up with when I needed to read some SPI data coming from an external Master-mode SPI device. I had to read a set of two bytes as an unsigned short, which were framed by the SS/ line (Slave_Select). I found out that you cannot reliably read the SS line (Digital_10) as a digital input line while the SPI module is active, at least with the settings I used. So I turn SPI on, read two bytes, turn SPI off again immediately, and wait for the next SS falling edge. That way I ensure correct byte synchronization (in my case).
// ====================================================================
// Arduino code example for SPI Slave Mode
// Read unsigned short (two bytes) from SPI, send word to serial port
// On 16 MHz Arduino, can work at > 500 words per second
// J.Beale July 19 2011
// ====================================================================
#define SCK_PIN 13 // D13 = pin19 = PortB.5
#define MISO_PIN 12 // D12 = pin18 = PortB.4
#define MOSI_PIN 11 // D11 = pin17 = PortB.3
#define SS_PIN 10 // D10 = pin16 = PortB.2
#define UL unsigned long
#define US unsigned short
void SlaveInit(void) {
// Set MISO output, all others input
pinMode(SCK_PIN, INPUT);
pinMode(MOSI_PIN, INPUT);
pinMode(MISO_PIN, OUTPUT); // (only if bidirectional mode needed)
pinMode(SS_PIN, INPUT);
/* Setup SPI control register SPCR
SPIE - Enables the SPI interrupt when 1
SPE - Enables the SPI when 1
DORD - Sends data least Significant Bit First when 1, most Significant Bit first when 0
MSTR - Sets the Arduino in master mode when 1, slave mode when 0
CPOL - Sets the data clock to be idle when high if set to 1, idle when low if set to 0
CPHA - Samples data on the trailing edge of the data clock when 1, leading edge when 0
SPR1 and SPR0 - Sets the SPI speed, 00 is fastest (4MHz) 11 is slowest (250KHz) */
// enable SPI subsystem and set correct SPI mode
// SPCR = (1<<SPE)|(0<<DORD)|(0<<MSTR)|(0<<CPOL)|(0<<CPHA)|(0<<SPR1)|(1<<SPR0);
}
// SPI status register: SPSR
// SPI data register: SPDR
// ================================================================
// read in short as two bytes, with high-order byte coming in first
// ================================================================
unsigned short Read2Bytes(void) {
union {
unsigned short svar;
byte c[2];
} w; // allow access to 2-byte word, or separate bytes
while(!(SPSR & (1<<SPIF))) ; // SPIF bit set when 8 bits received
w.c[1] = SPDR; // store high-order byte
while(!(SPSR & (1<<SPIF))) ; // SPIF bit set when 8 bits received
w.c[0] = SPDR; // store low-order byte
return (w.svar); // send back unsigned short value
}
void setup() {
Serial.begin(115200);
SlaveInit(); // set up SPI slave mode
delay(10);
Serial.println("SPI port reader v0.1");
}
// ============================================================
// main loop: read in short word (2 bytes) from external SPI master
// and send value out via serial port
// On 16 MHz Arduino, works at > 500 words per second
// ============================================================
void loop() {
unsigned short word1;
byte flag1;
// SS_PIN = Digital_10 = ATmega328 Pin 16 = PORTB.2
// Note: digitalRead() takes 4.1 microseconds
// NOTE: SS_PIN cannot be properly read this way while SPI module is active!
while (digitalRead(SS_PIN)==1) {} // wait until SlaveSelect goes low (active)
SPCR = (1<<SPE)|(0<<DORD)|(0<<MSTR)|(0<<CPOL)|(0<<CPHA)|(0<<SPR1)|(1<<SPR0); // SPI on
word1 = Read2Bytes(); // read unsigned short value
SPCR = (0<<SPE)|(0<<DORD)|(0<<MSTR)|(0<<CPOL)|(0<<CPHA)|(0<<SPR1)|(1<<SPR0); // SPI off
// float seconds = millis()/1000.0; // time stamp takes more serial time, of course
// Serial.print(seconds,3);
// Serial.print(",");
Serial.print(word1);
Serial.println();
} // end loop()
