Calculate Cypress EZ-Serial BLE Firmware Checksum for a Hex String Input

cypress-checksum.py

##########################################
# File: cypress-checksum.py
#
# Calculates EZ Serial checksum from hex string input and appends
# - Hex string input does not require space separated hex bytes
# - Cut & paste output into serial terminal connected to CYBT-483056-EVAL
# - Default Baud 115,200, N, 8, 1
#
# See:
#   EZ-Serial BLE Firmware Platform User Guide
#       2.4.2.1 Binary Mode Protocol Characteristics
#
##########################################
import sys

CypressCheckSumSeed = 0x99
CypressHeaderLength = 4

# Binary Message Byte Indexes
ByteType = 0
ByteLength = 1
ByteGroup = 2
ByteId = 3

# Example command:
#   gap_get_device_name (GDN, ID=4/16) API command sent to get the configured device name
#       C0 00 04 10 6D
#   6D is the checksum


def getCheckSum(hexString):
    checkSumLsb = 0
    byteArray = bytearray.fromhex(hexString)
    byteCount = len(byteArray)
    if (byteCount < CypressHeaderLength):
        print("*** Error - Message header not found. Expected > %d bytes, found %d" %
              (CypressHeaderLength, byteCount))
        return checkSumLsb

    print("Type.........: 0x%02x" % (byteArray[ByteType]))
    print("Length.......: 0x%02x" % (byteArray[ByteLength]))
    print("Group........: 0x%02x" % (byteArray[ByteGroup]))
    print("Id...........: 0x%02x" % (byteArray[ByteId]))        

    dataLengthExpected = byteArray[ByteLength]
    dataLengthFound = byteCount - CypressHeaderLength
    if (dataLengthFound != dataLengthExpected):
        print("*** Error - Message length expected %d bytes, found %d, difference %d bytes" %
              (dataLengthExpected,
               dataLengthFound,
               dataLengthFound - dataLengthExpected
               ))
        return checkSumLsb

    checkSum = CypressCheckSumSeed
    for byteData in byteArray:
        checkSum = checkSum + byteData

    checkSumLsb = checkSum & 0x00ff
    byteArray.append(checkSumLsb)

    print("checkSumLsb..: 0x%02x" % (checkSumLsb))
    print("checkSum.....: 0x%04x" % (checkSum))
    print("byteArray[%02d]: %s" % (byteCount, byteArray.hex()))

    return checkSumLsb


def displayUsage(message):
    print(message)
    print("Usage:")
    print("python %s 'C0 00 04 10'" % (sys.argv[0]))
    print


if __name__ == '__main__':

    argCount = len(sys.argv)
    if argCount < 2:
        displayUsage("Missing Argument")
        sys.exit(0)

    hexString = sys.argv[1]
    getCheckSum(hexString)