Correct usage of SPI with ISR

Question

My Arduino needs to do:

1. Run ISR as many times as possible/ as fast as possible.
2. Occasionally receive data over I2C.
3. Output data over SPI in ISR.

My question is regarding SPI and ISR. The documentation about SPI I found, was quite poor.

SPI.beginTransaction(SPISettings(16000000, MSBFIRST, SPI_MODE0));
SPI.usingInterrupt(255);
SPI.begin();

void isr_function()
        SPI.transfer(data);
        pulse_latch();
};

Is this the correct use of SPI in my case? Can it be faster?

Answer 1

It is actually fairly easy just to use the SPI peripheral without using the arduino libraries. The attached functions could be used to send a number of bytes through the SPI with one call: (Assuming ATMEGA328)

#include <avr/io.h>
#include <avr/interrupt.h>

static unsigned char spiCnt, *userBufr;

/* Call this function during setup */
void SpiInit (void)
    {
    PPR &= ~0x04;   // Be sure the SPI has power
    DDRB |= 4;      // Be sure SS signal on PORTB bit 2 is output
    SPCR = 0x50;    // Turn SPI on, maximum CLK rate
    }

/* Call this function to initiate the multi-byte send */
void SpiWrite (void *data, unsigned char bytCnt)
    {
    /* Save the the caller's messsage information */
    userBufr = data;
    spiCnt = bytCnt;

    /* Enable the SPI device, send first byte */
    SPCR |= 0x40;           // Enable SPI
    SPDR = *userBufr++;     // Send the first byte
    SPCR |= 0x80;           // Enable interrupts
    }

/* Call this one to be sure the message is finished */
int SpiIsDone (void)
    {
    /* Non-zero when operation is finished */
    return ( ! (SPCR & 0x80));
    }

/* Stay out of this one */
ISR (SPI_STC_vect)
    {
    /* Check the remaining number of bytes to send */
    if (--spiCnt)
        SPDR = *userBufr++;     // Send the next byte

    /* Operation is finished, shut down SPI */
    else
        SPCR &= ~0x80;  // Disable interrupts for SPI
    }