SPI Master-Slave Operation between Two ATmega328 under ArduinoUNO

[GolamMostafa]

With reference to the drawing of the attached file and the codes stated below, I am experiencing difficulty like:

The Slave receives 75 from the Master correctly and shows it on the 7-segmment display unit (DP0-DP1); but, the Master does not receive 23 from the Slave (LCD shows FF). Why?

//-- Master Initialization

#include <SPI.h>
#include <LiquidCrystal.h>

// APin PC4 PC5 PC0 PC1 PC2 PC3
LiquidCrystal lcd(A4, A5, A0, A1, A2, A3); //<--- APin
// LCD Signal D-I/ E D4 D5 D6 D7
// LCD Pin-----> 4 6 11 12 13 14 R-W/(5) = tied to LL

unsigned char x1;

void setup()
{

pinMode(8, INPUT_PULLUP);

lcd.begin(16, 2);
lcd.setCursor(0,0);
lcd.print("SPI Interface");

pinMode(10, OUTPUT);
digitalWrite(10, HIGH);

SPI.begin(); //SPI bus is enabled, SPI interrupt disable, SPI bus enabled, Master SPI
// Ouputs: SCK, MOSI, SS/; LL-->SCK, MOSI; LH---> SS/
//what's about MISO(PB4)? No need to do anything
// SPSR = 0x00;
//PB2(16):SS/---DPin-10 ----->
// PB3(17): MOSI--DPin-11 ----->
// PB4(18): MISO---DPin-12 <---
// PB5(19):SCK ----DPin-13 ---->
/------------------------------------------------------------/

SPI.setBitOrder(MSBFIRST); // MSB-bit will be transmitted first 2.
SPI.setClockDivider(SPI_CLOCK_DIV128); //TX Rate = 16 MHz/128 = 125 KBit 3.
SPI.setDataMode(SPI_MODE1); //Clock Polarity & Phase (must match with peripheral) 4.

/-------------------------------------------------------/
digitalWrite(10, LOW); //activate slave
delay(10);

while(digitalRead(8) !=0) // Check that K1 is pressed down
;
x1 = SPI.transfer(0x75);

lcd.setCursor(0, 1);
lcd.print(x1, 16);

SPI.end(); // SPI bus is disabled

}

void loop()
{

}

//--- Slave Initialization

#include <SPI.h>

unsigned char lupTable[] = {
0x3F, 0x06, 0x5B, 0x4F, 0x66, 0x6D,
0x7D, 0x07, 0x7F, 0x6F, 0x77, 0x7C,
0x39, 0x5E, 0x79, 0x71
};

unsigned int x1;

void setup()
{
DDRD = 0xFF; // PORTD is output
pinMode(8, OUTPUT);
pinMode(9, OUTPUT);

pinMode(10, INPUT_PULLUP); //SS/-pin Slave SPI is enabled

SPI.begin(); //int disabled, SPI enable
bitClear(SPCR, 4); //SPI slave selection; must be here
SPI.setBitOrder(MSBFIRST); // MSB-bit to be transmitted first
SPI.setDataMode(SPI_MODE1); //Clock Pol& Phase Mode1
SPI.setClockDivider(SPI_CLOCK_DIV128); //TX Rate = 16 MHz/128 = 125 KBit

SPDR = 0x23;

while (bitRead(SPSR, 7) !=1)
;
x1 = SPDR; // x1 = 75; it has come from Master

}

void loop()
{

x2 = x1;
x2 = x2>>4;
PORTD = lupTable[x2];
bitWrite(PORTB, 0,0); // = 0b11111110;
bitWrite(PORTB,1,1);

delay (5);
//------------
x2 = x1;
x2 = x2 & 0b00001111;
PORTD = lupTable[x2];
PORTB = 0b11111101;
delay (5);
//;--------------

}