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array in C/C   for AVR keeps appending
unsigned char responseFrame[300];

int main(void) {
   UART_init();
   while(1) {
       receive(responseFrame);
       myLog(responseFrame, sizeof(responseFrame));
   }
}

void receive(unsigned char *rcv_buff) {
    uint8_t recv_data;

    for (int i=0; i<300; i++){
            USART1_Flush();
        rcv_buff[i] = USART1_RX();
    }
}

// Logs this output to the serial port; used for debugging
void myLog(unsigned char *msg, int size) {
    for (int i=0; i<size; i++) {
        USART0_TX(msg[i]);
    }
}

0xFF 0xFF, 0x00, 0x00 ... 0x00

0xFF, 0xFF, 0x0A, 0x0A, 0x00, 0x00 ... 0x00

void USART0_TX(uint8_t myData) {
    // Wait if a byte is being transmitted
    while( !(UCSR0A & (1<<UDRE0)) );
    // Transmit data
    UDR0 = myData;
};
uint8_t USART1_RX(void) {
    // Wait until recv buffer is full
    while( !(UCSR1A & (1<<RXC1)) );
    // Return recvd data
    return UDR1;
};

//USART1 flush, clears USART1 buffer
void USART1_Flush( void )
{
    unsigned char dummy;
    while ( UCSR1A & (1<<RXC1) ) dummy = UDR1;
}