def convert_to_signed_int_8_bit(hex_str):
    """
    Utility function to convert a hex string into a 8 bit signed hex integer value
    :param hex_str: hex String
    :return: signed 8 bit integer
    """
    val = int(hex_str, 16)
    if val > 0x7F:
        val = ((val+0x80) & 0xFF) - 0x80
    return val