Here’s something random for a Thursday, the following is some simple C++ code for checking the checksum of a u-blox binary GPS packet, for some reason we get quite a few packet data errors, so it turns out that it is important to check the checksum!<\/p>\n
There must be an unwritten (or written?) software engineering rule which states that you should always check a checksum if one’s provided??!?<\/p>\n
Note: Make sure to pass only complete packets to this function, it assumes it has everything to work with!<\/p>\n
\r\n\/\/\r\n\/\/\r\nbool is_checksum_ok(unsigned char* buf) {\r\n\r\n\t\/\/ Packet format:\r\n\t\/\/ SYNC<1> SYNC<1> CLASS<1> ID<1> LENGTH<2> PAYLOAD CHKA<1> CHKB<1>\r\n\t\/\/ --------------- 4 ----------->|\r\n\t\/\/ --------------- 6 --------------------->|\r\n\t\/\/ --------------- 6 + length ---------------------------->|\r\n\r\n\t\/\/ Payload length field is little-endian (bless)\r\n\tunsigned short length = buf[5] << 8 | buf[4];\r\n\r\n\tunsigned char a = 0;\r\n\tunsigned char b = 0;\r\n\r\n\t\/\/ We calculate the checksum over the entire packet _except_\r\n\t\/\/ for the 2 bytes at the beginning and the 2 checksum bytes at\r\n\t\/\/ the end.\r\n\tfor (unsigned short i = 2; i < 6 + length; ++i) {\r\n\t\ta += buf[i];\r\n\t\tb += a;\r\n\t}\r\n\r\n\t\/\/ Pull out checksum bytes\r\n\tunsigned char ra = buf[6 + length];\r\n\tunsigned char rb = buf[6 + 1 + length];\r\n\r\n\t\/\/ and compare\r\n\treturn a = ra && b == rb;\r\n}\r\n\/\/\r\n\/\/\r\n<\/pre>\n","protected":false},"excerpt":{"rendered":"Here’s something random for a Thursday, the following is some simple C++ code for checking the checksum of a u-blox binary GPS packet, for some reason we get quite a few packet data errors, so it turns out that it is important to check the checksum! There must be an unwritten (or written?) software engineering […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[365,182,295,364],"class_list":["post-2176","post","type-post","status-publish","format-standard","hentry","category-uncategorized","tag-checksum","tag-code","tag-gps","tag-ublox"],"aioseo_notices":[],"aioseo_head":"\n\t\t\n\t\n\t\n\t\n\t\n\t\n\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t\n\t\t