Checksum Calculation for Intel HEX File Format

What is an Intel HEX File?

• An Intel HEX file is an ASCII text-based file format used for representing binary data.  
• It's commonly used to transfer program code and data to microcontrollers and other embedded systems.

History

The Intel HEX file format was introduced by Intel in the 1970s. It has since become a standard in the industry for firmware and microcontroller programming.

Structure of an Intel HEX File

SYNTAX of each record

• Each record follows a specific syntax = :llaaaatt[dd...]cc
• It has 5 fields as denoted above.
• Each letter denotes a hexadecimal digit.
• Each field is made up of a minimum of 2 hexadecimal digits. 
• Hence, denotes 1 byte each.
  

Record Types - ‘tt’ field

There are six main types of records in an Intel HEX file:

• Type 00 - Data Record
• Type 01 - End of File Record
• Type 02 - Extended Segment Address Record
• Type 03 - Start Segment Address Record
• Type 04 - Extended Linear Address Record
• Type 05 - Start Linear Address Record

Calculation of Checksum

• Considering two hex digits at a time, add all hex pairs until the ‘dd’ field.
• Convert the hex sum to decimal. If the Hex Sum contains 3 digits, ignore the first digit (MSB), and convert the remaining 2 digits to decimal.
• Subtract the above decimal sum from 256.
• Convert the result back to hex. Hence, this hex output will be the required Checksum.

EXAMPLE 1: 

Considering the Intel HEX record :03000000020C1F

Sum of all Hex Pairs: 03H + 00H + 00H + 00H + 02H + 0CH + 1FH = 30H

Converted Decimal Output: 48

256 - Decimal Output: 208

Checksum: D0H

Complete Intel HEX record with checksum :03000000020C1FD0

EXAMPLE 2: 

Considering the Intel HEX record 

:10080000E517240BF8E60517227808300702780B

Sum of all Hex Pairs:

10H + 08H + 00H + 00H + E5H + 17H + 24H + 0BH +F8H + E6H + 05H + 17H + 

22H + 78H + 08H + 30H + 07H + 02H + 78H + 0B = 49BH 

(Ignoring 4 (MSB) for step 2)

Converted Decimal Output: 155

256 - Decimal Output: 101

Checksum: 65H

Complete Intel HEX record with checksum

:10080000E517240BF8E60517227808300702780B65

For a detailed walkthrough, including practical demonstrations and more in-depth analysis, do watch our YouTube video on Checksum Calculation for Intel HEX File Format

Video Link: Checksum Calculation for Intel HEX File Format

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