#include <SPI.h>

ISR (SPI_STC_vect) {
    static uint16_t v = 0;

    uint8_t c = SPDR;

    // if bit 7 is set, then retrieve a value
    // from the adc and send its upper bits

    if (c & 128) {
        int ch = A0 + c;
        v = analogRead(ch);
        SPDR = v >> 8;
    }
    else {
        SPDR = v;
    }
}

void setup (void) {
    Serial.begin(115200);
    Serial.println(F("Init " __DATE__ " " __TIME__));

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

    // enable SPI slave mode
    SPCR |= _BV(SPE);

    // switch on interrupts
    SPCR |= _BV(SPIE);
}

void loop (void) {
}