CHECKSUM CALCULATION, Please Help!

[wasilus]

Hello guys,

I use a pic 16f88 to capture data from the rs232 PC port, the data comes in packets,

for example I get (ASCII characters): P1234 1A , where 1A is the checksum of P1234

ASCII - HEX

P - 50
1 - 31
2 - 32
3 - 33
4 - 34

1 - 31
A - 41

The software that sends the data packets calculates the checksum as follow:

50(P) + 31(1) + 32(2) + 33(3) + 34(4) = 11A and sends only 1A as checksum,
BUT it sends the 1A checksum as ASCII characters, in HEX 31 and 41.

So my question is, how can I calculate the checksum? If I add P1234 I will get 11A in HEX but I could not find a way to compare the 11A with
the 2 bytes checksum I received 31 and 41.

Can anyone please help me to solve this problem?
I am programming tha pic in assembler.
Thak you.

[Charlie]

Given you only have 16 possibilities, you could use a lookup table. Calculate the checksum, take the low nibble, look up the ascii in the table, compare with what you received, shift the checksum left (or simply take the high nibble), and look up the ascii in the table, and compare with what you received.

[scrwld]

Hello
, i use this:

you call this sub, length contains the number of characters in array
'********************* GENERAR CHECKSUM ********************************
crc__16: ' Funcion calculo checksum
Generator var word
Temp var word
CRC var word
j var byte
BitVal var bit
CRC = 65535
Generator = 40961
For i = 1 To Length ' this contains the number of characters in buffer
Temp = buffer[i]' data AIBDCF10
CRC = CRC ^ Temp
For j = 1 To 8
BitVal = CRC.0
CRC = CRC >> 1
If BitVal Then
CRC = CRC ^ Generator
EndIf
Next j
Next i
crc16[1] = (CRC // 256) ' CRC16 low byte
crc16[2] = (CRC / 256) ' CRC16 high byte
Return

RESULT A1-BD-CF-10-crc16[1]-crc16[2]
************************************************** *************

or use this from VisualBasic 6 use lookup table, you needed adapt to PICBASIC PRO

Public CRCTable(0 To 511) As Byte
Public CRC_Low, CRC_High As Byte

Public Sub CRC_16(ByVal Data As String, Length As Integer)
CRC_Low = &HFF
CRC_High = &HFF

For i = 1 To Length
Index = CRC_High Xor Asc(Mid(Data, i, 1))
CRC_High = CRC_Low Xor CRCTable(Index)
CRC_Low = CRCTable(Index + 256)
Next i

End Sub

LOOKUP TABLE
CRCTable(0) = &H0
CRCTable(1) = &HC1
CRCTable(2) = &H81
CRCTable(3) = &H40
CRCTable(4) = &H1
CRCTable(5) = &HC0
CRCTable(6) = &H80
CRCTable(7) = &H41
CRCTable(8) = &H1
CRCTable(9) = &HC0
CRCTable(10) = &H80
CRCTable(11) = &H41
CRCTable(12) = &H0
CRCTable(13) = &HC1
CRCTable(14) = &H81
CRCTable(15) = &H40
CRCTable(16) = &H1
CRCTable(17) = &HC0
CRCTable(18) = &H80
CRCTable(19) = &H41
CRCTable(20) = &H0
CRCTable(21) = &HC1
CRCTable(22) = &H81
CRCTable(23) = &H40
CRCTable(24) = &H0
CRCTable(25) = &HC1
CRCTable(26) = &H81
CRCTable(27) = &H40
CRCTable(28) = &H1
CRCTable(29) = &HC0
CRCTable(30) = &H80
CRCTable(31) = &H41
CRCTable(32) = &H1
CRCTable(33) = &HC0
CRCTable(34) = &H80
CRCTable(35) = &H41
CRCTable(36) = &H0
CRCTable(37) = &HC1
CRCTable(38) = &H81
CRCTable(39) = &H40
CRCTable(40) = &H0
CRCTable(41) = &HC1
CRCTable(42) = &H81
CRCTable(43) = &H40
CRCTable(44) = &H1
CRCTable(45) = &HC0
CRCTable(46) = &H80
CRCTable(47) = &H41
CRCTable(48) = &H0
CRCTable(49) = &HC1
CRCTable(50) = &H81
CRCTable(51) = &H40
CRCTable(52) = &H1
CRCTable(53) = &HC0
CRCTable(54) = &H80
CRCTable(55) = &H41
CRCTable(56) = &H1
CRCTable(57) = &HC0
CRCTable(58) = &H80
CRCTable(59) = &H41
CRCTable(60) = &H0
CRCTable(61) = &HC1
CRCTable(62) = &H81
CRCTable(63) = &H40
CRCTable(64) = &H1
CRCTable(65) = &HC0
CRCTable(66) = &H80
CRCTable(67) = &H41
CRCTable(68) = &H0
CRCTable(69) = &HC1
CRCTable(70) = &H81
CRCTable(71) = &H40
CRCTable(72) = &H0
CRCTable(73) = &HC1
CRCTable(74) = &H81
CRCTable(75) = &H40
CRCTable(76) = &H1
CRCTable(77) = &HC0
CRCTable(78) = &H80
CRCTable(79) = &H41
CRCTable(80) = &H0
CRCTable(81) = &HC1
CRCTable(82) = &H81
CRCTable(83) = &H40
CRCTable(84) = &H1
CRCTable(85) = &HC0
CRCTable(86) = &H80
CRCTable(87) = &H41
CRCTable(88) = &H1
CRCTable(89) = &HC0
CRCTable(90) = &H80
CRCTable(91) = &H41
CRCTable(92) = &H0
CRCTable(93) = &HC1
CRCTable(94) = &H81
CRCTable(95) = &H40
CRCTable(96) = &H0
CRCTable(97) = &HC1
CRCTable(98) = &H81
CRCTable(99) = &H40
CRCTable(100) = &H1
CRCTable(101) = &HC0
CRCTable(102) = &H80
CRCTable(103) = &H41
CRCTable(104) = &H1
CRCTable(105) = &HC0
CRCTable(106) = &H80
CRCTable(107) = &H41
CRCTable(108) = &H0
CRCTable(109) = &HC1
CRCTable(110) = &H81
CRCTable(111) = &H40
CRCTable(112) = &H1
CRCTable(113) = &HC0
CRCTable(114) = &H80
CRCTable(115) = &H41
CRCTable(116) = &H0
CRCTable(117) = &HC1
CRCTable(118) = &H81
CRCTable(119) = &H40
CRCTable(120) = &H0
CRCTable(121) = &HC1
CRCTable(122) = &H81
CRCTable(123) = &H40
CRCTable(124) = &H1
CRCTable(125) = &HC0
CRCTable(126) = &H80
CRCTable(127) = &H41
CRCTable(128) = &H1
CRCTable(129) = &HC0
CRCTable(130) = &H80
CRCTable(131) = &H41
CRCTable(132) = &H0
CRCTable(133) = &HC1
CRCTable(134) = &H81
CRCTable(135) = &H40
CRCTable(136) = &H0
CRCTable(137) = &HC1
CRCTable(138) = &H81
CRCTable(139) = &H40
CRCTable(140) = &H1
CRCTable(141) = &HC0
CRCTable(142) = &H80
CRCTable(143) = &H41
CRCTable(144) = &H0
CRCTable(145) = &HC1
CRCTable(146) = &H81
CRCTable(147) = &H40
CRCTable(148) = &H1
CRCTable(149) = &HC0
CRCTable(150) = &H80
CRCTable(151) = &H41
CRCTable(152) = &H1
CRCTable(153) = &HC0
CRCTable(154) = &H80
CRCTable(155) = &H41
CRCTable(156) = &H0
CRCTable(157) = &HC1
CRCTable(158) = &H81
CRCTable(159) = &H40
CRCTable(160) = &H0
CRCTable(161) = &HC1
CRCTable(162) = &H81
CRCTable(163) = &H40
CRCTable(164) = &H1
CRCTable(165) = &HC0
CRCTable(166) = &H80
CRCTable(167) = &H41
CRCTable(168) = &H1
CRCTable(169) = &HC0
CRCTable(170) = &H80
CRCTable(171) = &H41
CRCTable(172) = &H0
CRCTable(173) = &HC1
CRCTable(174) = &H81
CRCTable(175) = &H40
CRCTable(176) = &H1
CRCTable(177) = &HC0
CRCTable(178) = &H80
CRCTable(179) = &H41
CRCTable(180) = &H0
CRCTable(181) = &HC1
CRCTable(182) = &H81
CRCTable(183) = &H40
CRCTable(184) = &H0
CRCTable(185) = &HC1
CRCTable(186) = &H81
CRCTable(187) = &H40
CRCTable(188) = &H1
CRCTable(189) = &HC0
CRCTable(190) = &H80
CRCTable(191) = &H41
CRCTable(192) = &H0
CRCTable(193) = &HC1
CRCTable(194) = &H81
CRCTable(195) = &H40
CRCTable(196) = &H1
CRCTable(197) = &HC0
CRCTable(198) = &H80
CRCTable(199) = &H41
CRCTable(200) = &H1
CRCTable(201) = &HC0
CRCTable(202) = &H80
CRCTable(203) = &H41
CRCTable(204) = &H0
CRCTable(205) = &HC1
CRCTable(206) = &H81
CRCTable(207) = &H40
CRCTable(208) = &H1
CRCTable(209) = &HC0
CRCTable(210) = &H80
CRCTable(211) = &H41
CRCTable(212) = &H0
CRCTable(213) = &HC1
CRCTable(214) = &H81
CRCTable(215) = &H40
CRCTable(216) = &H0
CRCTable(217) = &HC1
CRCTable(218) = &H81
CRCTable(219) = &H40
CRCTable(220) = &H1
CRCTable(221) = &HC0
CRCTable(222) = &H80
CRCTable(223) = &H41
CRCTable(224) = &H1
CRCTable(225) = &HC0
CRCTable(226) = &H80
CRCTable(227) = &H41
CRCTable(228) = &H0
CRCTable(229) = &HC1
CRCTable(230) = &H81
CRCTable(231) = &H40
CRCTable(232) = &H0
CRCTable(233) = &HC1
CRCTable(234) = &H81
CRCTable(235) = &H40
CRCTable(236) = &H1
CRCTable(237) = &HC0
CRCTable(238) = &H80
CRCTable(239) = &H41
CRCTable(240) = &H0
CRCTable(241) = &HC1
CRCTable(242) = &H81
CRCTable(243) = &H40
CRCTable(244) = &H1
CRCTable(245) = &HC0
CRCTable(246) = &H80
CRCTable(247) = &H41
CRCTable(248) = &H1
CRCTable(249) = &HC0
CRCTable(250) = &H80
CRCTable(251) = &H41
CRCTable(252) = &H0
CRCTable(253) = &HC1
CRCTable(254) = &H81
CRCTable(255) = &H40
CRCTable(256) = &H0
CRCTable(257) = &HC0
CRCTable(258) = &HC1
CRCTable(259) = &H1
CRCTable(260) = &HC3
CRCTable(261) = &H3
CRCTable(262) = &H2
CRCTable(263) = &HC2
CRCTable(264) = &HC6
CRCTable(265) = &H6
CRCTable(266) = &H7
CRCTable(267) = &HC7
CRCTable(268) = &H5
CRCTable(269) = &HC5
CRCTable(270) = &HC4
CRCTable(271) = &H4
CRCTable(272) = &HCC
CRCTable(273) = &HC
CRCTable(274) = &HD
CRCTable(275) = &HCD
CRCTable(276) = &HF
CRCTable(277) = &HCF
CRCTable(278) = &HCE
CRCTable(279) = &HE
CRCTable(280) = &HA
CRCTable(281) = &HCA
CRCTable(282) = &HCB
CRCTable(283) = &HB
CRCTable(284) = &HC9
CRCTable(285) = &H9
CRCTable(286) = &H8
CRCTable(287) = &HC8
CRCTable(288) = &HD8
CRCTable(289) = &H18
CRCTable(290) = &H19
CRCTable(291) = &HD9
CRCTable(292) = &H1B
CRCTable(293) = &HDB
CRCTable(294) = &HDA
CRCTable(295) = &H1A
CRCTable(296) = &H1E
CRCTable(297) = &HDE
CRCTable(298) = &HDF
CRCTable(299) = &H1F
CRCTable(300) = &HDD
CRCTable(301) = &H1D
CRCTable(302) = &H1C
CRCTable(303) = &HDC
CRCTable(304) = &H14
CRCTable(305) = &HD4
CRCTable(306) = &HD5
CRCTable(307) = &H15
CRCTable(308) = &HD7
CRCTable(309) = &H17
CRCTable(310) = &H16
CRCTable(311) = &HD6
CRCTable(312) = &HD2
CRCTable(313) = &H12
CRCTable(314) = &H13
CRCTable(315) = &HD3
CRCTable(316) = &H11
CRCTable(317) = &HD1
CRCTable(318) = &HD0
CRCTable(319) = &H10
CRCTable(320) = &HF0
CRCTable(321) = &H30
CRCTable(322) = &H31
CRCTable(323) = &HF1
CRCTable(324) = &H33
CRCTable(325) = &HF3
CRCTable(326) = &HF2
CRCTable(327) = &H32
CRCTable(328) = &H36
CRCTable(329) = &HF6
CRCTable(330) = &HF7
CRCTable(331) = &H37
CRCTable(332) = &HF5
CRCTable(333) = &H35
CRCTable(334) = &H34
CRCTable(335) = &HF4
CRCTable(336) = &H3C
CRCTable(337) = &HFC
CRCTable(338) = &HFD
CRCTable(339) = &H3D
CRCTable(340) = &HFF
CRCTable(341) = &H3F
CRCTable(342) = &H3E
CRCTable(343) = &HFE
CRCTable(344) = &HFA
CRCTable(345) = &H3A
CRCTable(346) = &H3B
CRCTable(347) = &HFB
CRCTable(348) = &H39
CRCTable(349) = &HF9
CRCTable(350) = &HF8
CRCTable(351) = &H38
CRCTable(352) = &H28
CRCTable(353) = &HE8
CRCTable(354) = &HE9
CRCTable(355) = &H29
CRCTable(356) = &HEB
CRCTable(357) = &H2B
CRCTable(358) = &H2A
CRCTable(359) = &HEA
CRCTable(360) = &HEE
CRCTable(361) = &H2E
CRCTable(362) = &H2F
CRCTable(363) = &HEF
CRCTable(364) = &H2D
CRCTable(365) = &HED
CRCTable(366) = &HEC
CRCTable(367) = &H2C
CRCTable(368) = &HE4
CRCTable(369) = &H24
CRCTable(370) = &H25
CRCTable(371) = &HE5
CRCTable(372) = &H27
CRCTable(373) = &HE7
CRCTable(374) = &HE6
CRCTable(375) = &H26
CRCTable(376) = &H22
CRCTable(377) = &HE2
CRCTable(378) = &HE3
CRCTable(379) = &H23
CRCTable(380) = &HE1
CRCTable(381) = &H21
CRCTable(382) = &H20
CRCTable(383) = &HE0
CRCTable(384) = &HA0
CRCTable(385) = &H60
CRCTable(386) = &H61
CRCTable(387) = &HA1
CRCTable(388) = &H63
CRCTable(389) = &HA3
CRCTable(390) = &HA2
CRCTable(391) = &H62
CRCTable(392) = &H66
CRCTable(393) = &HA6
CRCTable(394) = &HA7
CRCTable(395) = &H67
CRCTable(396) = &HA5
CRCTable(397) = &H65
CRCTable(398) = &H64
CRCTable(399) = &HA4
CRCTable(400) = &H6C
CRCTable(401) = &HAC
CRCTable(402) = &HAD
CRCTable(403) = &H6D
CRCTable(404) = &HAF
CRCTable(405) = &H6F
CRCTable(406) = &H6E
CRCTable(407) = &HAE
CRCTable(408) = &HAA
CRCTable(409) = &H6A
CRCTable(410) = &H6B
CRCTable(411) = &HAB
CRCTable(412) = &H69
CRCTable(413) = &HA9
CRCTable(414) = &HA8
CRCTable(415) = &H68
CRCTable(416) = &H78
CRCTable(417) = &HB8
CRCTable(418) = &HB9
CRCTable(419) = &H79
CRCTable(420) = &HBB
CRCTable(421) = &H7B
CRCTable(422) = &H7A
CRCTable(423) = &HBA
CRCTable(424) = &HBE
CRCTable(425) = &H7E
CRCTable(426) = &H7F
CRCTable(427) = &HBF
CRCTable(428) = &H7D
CRCTable(429) = &HBD
CRCTable(430) = &HBC
CRCTable(431) = &H7C
CRCTable(432) = &HB4
CRCTable(433) = &H74
CRCTable(434) = &H75
CRCTable(435) = &HB5
CRCTable(436) = &H77
CRCTable(437) = &HB7
CRCTable(438) = &HB6
CRCTable(439) = &H76
CRCTable(440) = &H72
CRCTable(441) = &HB2
CRCTable(442) = &HB3
CRCTable(443) = &H73
CRCTable(444) = &HB1
CRCTable(445) = &H71
CRCTable(446) = &H70
CRCTable(447) = &HB0
CRCTable(448) = &H50
CRCTable(449) = &H90
CRCTable(450) = &H91
CRCTable(451) = &H51
CRCTable(452) = &H93
CRCTable(453) = &H53
CRCTable(454) = &H52
CRCTable(455) = &H92
CRCTable(456) = &H96
CRCTable(457) = &H56
CRCTable(458) = &H57
CRCTable(459) = &H97
CRCTable(460) = &H55
CRCTable(461) = &H95
CRCTable(462) = &H94
CRCTable(463) = &H54
CRCTable(464) = &H9C
CRCTable(465) = &H5C
CRCTable(466) = &H5D
CRCTable(467) = &H9D
CRCTable(468) = &H5F
CRCTable(469) = &H9F
CRCTable(470) = &H9E
CRCTable(471) = &H5E
CRCTable(472) = &H5A
CRCTable(473) = &H9A
CRCTable(474) = &H9B
CRCTable(475) = &H5B
CRCTable(476) = &H99
CRCTable(477) = &H59
CRCTable(478) = &H58
CRCTable(479) = &H98
CRCTable(480) = &H88
CRCTable(481) = &H48
CRCTable(482) = &H49
CRCTable(483) = &H89
CRCTable(484) = &H4B
CRCTable(485) = &H8B
CRCTable(486) = &H8A
CRCTable(487) = &H4A
CRCTable(488) = &H4E
CRCTable(489) = &H8E
CRCTable(490) = &H8F
CRCTable(491) = &H4F
CRCTable(492) = &H8D
CRCTable(493) = &H4D
CRCTable(494) = &H4C
CRCTable(495) = &H8C
CRCTable(496) = &H44
CRCTable(497) = &H84
CRCTable(498) = &H85
CRCTable(499) = &H45
CRCTable(500) = &H87
CRCTable(501) = &H47
CRCTable(502) = &H46
CRCTable(503) = &H86
CRCTable(504) = &H82
CRCTable(505) = &H42
CRCTable(506) = &H43
CRCTable(507) = &H83
CRCTable(508) = &H41
CRCTable(509) = &H81
CRCTable(510) = &H80
CRCTable(511) = &H40

*********************************************
espero te sirva

[Charlie]

Interesting methof of doing CRC, but completely unrelated to the original question.

[falingtrea]

Take the ASCII value, subtract 0x30, if results is >9 then subtract 0x07. Value should then be in hex. If you want to bounds check then test result to make sure it is >= 0 and < 0x10. Then take the first packet hex value and leftshift by 4 and then add to hex value of second packet. Bitwise AND your calculated checksum with 0xFF and compare with the transmitted checksum.

[scrwld]

hi wasilus, tanks for you answer, these are examples used to calculate CRC16.

try this:

1.- $50 + $31 + $32 + $33 + $34 = $11A

2.- $11A / $100 = $1

3.- the carry = A

4.- Your checksum = 1A