struct StatusStruct {
    volatile unsigned char RXbusy;
    volatile unsigned char TXbusy;
    unsigned char RXbuffer[15];
    unsigned char TXbuffer[15];
    unsigned char DataForSend[15];
    unsigned char RXdataSize;
    unsigned char TXdataSize;
    unsigned char RXcurrentByte;
    unsigned char TXcurrentByte;

    unsigned char ADCrawData[20];

    unsigned char ConversionChannel;
    unsigned char ConversionType; // 1 - All; 2 - One

    unsigned char DataForSendSize;
    unsigned char DataCurrentSendByte;

} Status;

......
.....

ISR(USART_RXC_vect){

    unsigned char Rbyte, ByteCount;

    volatile unsigned char  a;

    Rbyte = UDR;
    
    if (Status.RXcurrentByte < 15){
        Status.RXbuffer[Status.RXcurrentByte] = Rbyte;
        Status.RXcurrentByte++;

        a = Status.RXbuffer[Status.RXcurrentByte-2];

        if ((Rbyte == 0xBC) && (a==0xBB)){
            ByteCount = Status.RXcurrentByte-1;
            Status.RXcurrentByte = 0;
            DecodeFrame(ByteCount);
        }
    } else {
        Status.RXcurrentByte = 0;
    }
    
}

Кто "он"?

if(){
   bla_bla_bla();
   foo();
}

if()
{
   bla_bla_bla();
   foo();
}