1台のarduinno nanoで割込みとSPIの同時使用

/**
 *  Arduinoで発生した割り込み回数を、SPIで送信する。
 *  ただし、Arduinoはスレーブとして動作する。
 *  マスターは、RaspberryPi3
 */
#include "Arduino.h"
#include <SPI.h>

#define INTERRUPT_PIN   (2)
#define SPI_BUFF_SIZE   (4)

int interruptCount;
int interruptCountPrev;
int spiCount;
int spiCountPrev;
byte spiBuffer[SPI_BUFF_SIZE];
int spiBufferIndex;
int isDataChanged;

/**
 * @brief   セットアップを実施する。
 */
void setup()
{
    //割込み設定
    pinMode(INTERRUPT_PIN, INPUT_PULLUP);
    attachInterrupt(digitalPinToInterrupt(INTERRUPT_PIN), interrupHandle, RISING);

    //SPI設定：ピン
    pinMode((unsigned char)MISO, OUTPUT);
    pinMode((unsigned char)MOSI, INPUT);
    pinMode((unsigned char)SCK, INPUT);
    pinMode((unsigned char)SS, INPUT);

    //SPI設定：モード
    SPI.setBitOrder(MSBFIRST);
    SPI.setDataMode(SPI_MODE0);
    SPI.setClockDivider(SPI_CLOCK_DIV128);  //125kHz
    SPI.begin();

    //SPIレジスタ
    SPCR &= ~_BV(MSTR);
    SPCR |= _BV(SPE);
    SPI.attachInterrupt();

    //シリアル設定(デバッグ用)
    Serial.begin(9600);

    //変数の初期化
    spiCount = 0;
    spiCountPrev = 0;
    interruptCount = 0;
    interruptCountPrev = 0;
    SPDR = interruptCount;
    isDataChanged = 0;

    for (int index = 0;index < SPI_BUFF_SIZE;index++) {
        spiBuffer[index] = 0;
    }
}

/*
 * @brief   SPIの割込みハンドラ
 */
ISR(SPI_STC_vect) {
    SPDR = spiBuffer[spiBufferIndex];
    spiBufferIndex++;
    if (SPI_BUFF_SIZE <= spiBufferIndex) {
        //送信データのインデックスをクリアする
        spiBufferIndex = 0;
    }

    spiCount++;
}

/**
 * @brief   メインループ処理
 */
void loop()
{
    int interruptCountLatch = interruptCount;
    int spiCountLatch = spiCount;
    spiBuffer[0] = (byte)(interruptCountLatch & 0x00FF);
    spiBuffer[1] = (byte)((interruptCountLatch & 0xFF00) >> 8);
    spiBuffer[2] = (byte)(spiCountLatch & 0x00FF);
    spiBuffer[3] = (byte)((spiCountLatch & 0xFF00) >> 8);

    SPDR = spiBuffer[0];
    spiBufferIndex = 1;

    Serial.print("interruptCount = ");
    Serial.print(interruptCountLatch);
    Serial.print(", interruptCountPrev = ");
    Serial.print(interruptCountPrev);
    Serial.print(",  spiCount = ");
    Serial.print(spiCountLatch);
    Serial.print(",  spiCountPrev = ");
    Serial.print(spiCountPrev);
    Serial.print(",  isDataChanged = ");
    Serial.println(isDataChanged);

    interruptCountPrev = interruptCount;
    spiCountPrev = spiCount;

    delay(10);  //10msec待つ
}

/**
 * @brief   割込みハンドラ
 */
void interrupHandle()
{
    interruptCount++;
}

    SPDR = spiBuffer[0];
    spiBufferIndex = 1;