ERROR SPI on ATmega328P of Arduino - arduino

Question

I use SPI on ATmega328 with raw code, data is transferred correctly, but the value return just correct in the second time. I don't know why. I think the data buffer register doesn't update immediately, when I push the button (this is video left is yellow LED, right is red LED: https://drive.google.com/open?id=1bf2XlxRBGCaUo0j2ezB2XFxfSItUmChN and code below)
I use Arduino Uno is a master to send data and get response. Arduino Nano is a slave to receive data and response to master.
Arduino Uno (master) code:
#define DDR_SPI DDRB
#define PORT_SPI PORTB
#define MOSI 3
#define SCK 5
#define SS 2
#define red 2
#define cbi(port, bits) (port) &= ~(1 << (bits))
#define sbi(port, bits) (port) |= (1 << (bits))
volatile uint8_t Data1;
volatile uint8_t Data2;
void setup()
{
Serial.begin(9600); //debug by UART
SPCR = 0; // Reset SPCR = 0
DDR_SPI |= (1<<MOSI)|(1<<SCK)|(1<<SS);// MOSI,SCK,SS - OUTPUT , MISO - INPUT
SPCR |= (1<<SPE)|(1<<MSTR)|(1<<SPR0); // Enable spi , Set master , div/16
pinMode(red,OUTPUT); // led output at pin 2
pinMode(6,INPUT_PULLUP); // internal res input at pin6
pinMode(7,INPUT_PULLUP); // internal res input at pin7
sbi(PORT_SPI,SS); // disable slave
}
uint8_t SPI_MasterTransmit(uint8_t cData)
{
SPDR = cData; // Start transmission
delay(50);
while(bit_is_clear(SPSR,SPIF)); //Wait for transmission complete
return SPDR;
}
void loop()
{
if(bit_is_clear(PIND,6)) // if i push button at pin 6
{
while(bit_is_clear(PIND,6)); // if i still push button at pin 6
cbi(PORTB,SS); // enable slave
Data1 = SPI_MasterTransmit(1); // send value 1 to slave
Serial.print(Data1); // debug
delay(50);
if(Data1 == 2) //if slave response value = 2
digitalWrite(red,LOW); //turnOFF led at pin 2
sbi(PORT_SPI,SS); // disable slave
}
else if(bit_is_clear(PIND,7)) // if i push button at pin 7
{
while(bit_is_clear(PIND,7)); // if i still push button at pin 7
cbi(PORT_SPI,SS); // disable slave
Data2 = SPI_MasterTransmit(3); // send value 3 to slave
Serial.print(Data2); // debug
delay(50);
if(Data2==4) //if slave response value = 4
digitalWrite(red,HIGH); //turnON led at pin 2
sbi(PORT_SPI,SS); // disable slave
}
}
Arduino Nano (slave) code:
#define MISO 4
#define yellow 2
#define cbi(port, bits) (port) &= ~(1 << (bits))
#define sbi(port, bits) (port) |= (1 << (bits))
volatile uint8_t Data;
void setup()
{
SPCR = 0; // reset SPCR = 0
DDRB |= (1<<MISO); // MISO - OUTPUT , SCK,MOSI,SS - INPUT
SPCR |= (1<<SPE)|(1<<SPIE); // enable spi , spi_interrupt
pinMode(yellow ,OUTPUT); // Led output at pin2
}
void SPI_Response(uint8_t cData)
{
SPDR = cData; //Start transmission
while(bit_is_clear(SPSR,SPIF)); //Wait for transmission complete
}
void loop()
{
}
ISR(SPI_STC_vect) // Vector interrupt spi
{
Data = SPDR; // volatile Data = data receive
if(Data == 1)
{
digitalWrite(yellow ,HIGH); // i turnOn led if i receive data = 5
SPI_Response(2); // and response to master a value = 7
delay(10);
}
else if(Data == 3)
{
digitalWrite(yellow,LOW); // i turnOFF led if i receive data = 6
SPI_Response(4); //and response to master a value = 8
delay(10);
}
}

Answer 1

You know the four wire SPI is full duplex, right? The master and the slave are sending the data simultaneously.
The common way how to do what you want is sending control byte and then dummy byte (or bytes) to receive the data.