teensylcSPIslave

#include <SPI.h>
int ss=10;
 SPISettings settingsA(250000, MSBFIRST, SPI_MODE1);
void setup() {
  Serial.begin(9600);
  pinMode(ss, INPUT); // CS set to input for slave
  SPI.begin(); // wake up the SPI bus.
  SPI.setBitOrder(MSBFIRST);
  attachInterrupt(ss,isrSpi,FALLING); // interrupt on cs down
}

void isrSpi(){
  SPI.beginTransaction(settingsA);
  byte q = SPI.transfer(0);

  /*for (int i = 7; i > -1; i--){ // check binary
    Serial.print((q>>i)&1);
  }
  Serial.print(' ');*/
  Serial.print(q);
  Serial.println();
  SPI.endTransaction;
}

void loop()
{

}