How to convert 4 bytes to an integer ?

[GiggsB]

Can you show the code on the sending and receiving side that transfers the data?

This is on slave side (just adding snippet related to SPI, because its a very long code):

//data transfer variables
uint32_t data_adc;
uint8_t b1,b2,b3,b4;

// Configure MOSI pin as an input
  GPIO_PinModeSet(US0MOSI_PORT, US0MOSI_PIN, gpioModeInput, 0);

  // Configure MISO pin as an output
  GPIO_PinModeSet(US0MISO_PORT, US0MISO_PIN, gpioModePushPull, 0);

  // Configure CLK pin as an input
  GPIO_PinModeSet(US0CLK_PORT, US0CLK_PIN, gpioModeInput, 0);

  // Configure CS pin as an input pulled high
  GPIO_PinModeSet(US0CS_PORT, US0CS_PIN, gpioModeInput, 1);

void initUSART0(void){
//not adding code snippet but these are the setting:
1. set as slave
2. speed: 4000000
3. MSB transferred first
4. 8 data bits
5. clock mode 0
}
while(1){
.
.
USART_SpiTransfer(USART0, 'S'); /* this function performs one 8-bit frame SPI transfer. 
This function will stall if the transmit buffer is full. When a transmit buffer becomes available, data is written and the function will wait until data is fully transmitted. The SPI return value is then read out and
 returned. */

      for(uint32_t i = 0; i < 98; i++){
          data_adc = *((uint32_t *)StartPage+i); //data read from flash memory

          b1 = (unsigned int)(data_adc & 0xff);
          b2 = (unsigned int)(data_adc >> 8) & 0xff;
          b3 = (unsigned int)(data_adc >> 16) & 0xff;
          b4 = (unsigned int)(data_adc >> 24);

          int testarray[4]={b4,b3,b2,b1};

          for (int i = 0; i < 4; i++){
                        USART_SpiTransfer(USART0, testarray[i]); //no new line character??
                    }
      }
      USART_SpiTransfer(USART0, 'D'); //'D' will be sent as an integer!! To tell RP, all data has been transferred.
.
.
}

Below is code on master side (Raspberry Pi):

# spi.py
import time
import spidev
import array as arr
import struct

# We only have SPI bus 0 available to us on the Pi
bus = 0

#Device is the chip select pin. Set to 0 or 1, depending on the connections
device = 0

# Enable SPI
spi = spidev.SpiDev()

# Open a connection to a specific bus and device (chip select pin)
spi.open(bus, device)

# Set SPI speed and mode
spi.max_speed_hz = 4000000
spi.mode = 0

print("Enter '1' to start the process")
a=input()
if a:
    print("SM1 Process started...")
    spi.xfer2([0x01])
    
    while True:
        #spi.xfer2([0,0,0,0])
        b=spi.readbytes(4)
        value=struct.unpack("<I", bytearray(b))[0]
        print (value)
     
else: 
    print("Wrong input.")

One thing, I didn't add in the slave code is that: when the slave is powered on, master sends a command, '1' to slave and slave checks if it receives '1'. If it receives '1' then this whole above process is started. So that much communication is happening correctly.