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Forum Index : Microcontroller and PC projects : Checksum and CRC
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lew247 Guru  Joined: 23/12/2015 Location: United KingdomPosts: 1702 |
Sorry for posting so much but I need your help again, and I am very grateful for the help I have received so far. HOW do I create a Checksum figure? THEN how do I do a CRC to check if that Checksum is correct? An example of how I want to use it - A micromite in the Fridge just sending the temperature once every 15 minutes A micromite in 2 seperate Freezers sending the temperature once every 15 minutes This is how I envisage the Fridge Micromite program OPTION BREAK 0 OPTION AUTORUN ON WATCHDOG 86400 'Reset the Micromite once a day OPEN "COM1:9600" AS #1 'OPEN port PRINT #1 "FRIDGE: " TEMPR(pin)"C" checksum 'Send data via HC-12 IF #1="1" then 'Check for "1" to prove the data was recieved correctly ELSE PRINT #1 "Fridge" checksum 'Resend the data and check it was recieved correctly ENDIF CLOSE #1 CPU SLEEP 900 *** The same program would be on the Freezer Micromites but obviously they would send Freezer 1 or Freezer 2 instead of Fridge*** On the Micromite that monitors the temperatures, it will be doing other things but when it gets an interupt from COM2 it will then process the data coming from the Fridge/freezers and display the temperature This is how I see this section of program IF #2= "Fridge" and CRC Is correct then 'Data coming from Fridge Sensor **NO idea how to check Checksum is correct** PRINT #2 "1" 'COM2 saying the data is correct ELSE PRINT #2 "2" 'CRC is incorrect Please resend IF #2= "FREEZER 1" and CRC Is correct then 'Data coming from Freezer Sensor PRINT #2 "1" 'COM2 saying the data is correct ELSE PRINT #2 "2" 'CRC is incorrect Please resend IF #2= "FREEZER 2" and CRC Is correct then 'Data coming from Freezer Sensor PRINT #2 "1" 'COM2 saying the data is correct ELSE PRINT #2 "2" 'CRC is incorrect Please resend ENDIF PRINT the information from #2 'NOT SURE HOW TO DO THIS I am totally blank on how to create and check the checksum on both micromites Can anyone help with correcting what I have above and show me how to create and check a checksum? It's not in the manual I've read both the basic and Micromite+ manuals |
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| paceman Guru Joined: 07/10/2011 Location: AustraliaPosts: 1329 |
A quick Google of "checksum" throws this up as the second hit. Greg |
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panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1114 |
Lew, CRC (Cyclic Redundancy Check) and checksum are basically the same. Generally, they are the sum of the bytes in the message - try Googleing CRC for a more technical description - in order to determine if there has been an error. Reading the HC-12 doco by Robert Rozee from the Backshed (search the Backshed for HC-12) it seems that the HC-12 does not generate or check CRC by default. So, if you want to have some form of checksum, you would need to write your own CRC generator and then checker. In order to know what is data and what is CRC in the data stream, you would need a set length for data (ie. a set number of bytes ) so you know when the CRC is. Regards, Doug. Edit: Do a search in the Backshed - there are several code examples of CRC checking. ... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it! |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6266 |
Lew, To expand on the code in another post, DO ' sending attaches the checksum to the string INPUT txt$ sum = chksum(txt$) txt$ = txt$+CHR$(sum) PRINT "Sending: ";sum,txt$ 'receiving tests the checksum. A zero indicates that all is well if chksum(txt$) = 0 then txt$ = left$(txt$,len(txt$)-1) else txt$= "error!!!" endif print "Received: ";txt$ LOOP FUNCTION chksum(report$) LOCAL n, ch$ chksum=0 FOR n = 1 TO LEN(report$) ch$=MID$(report$,n,1) chksum=chksum XOR ASC(ch$) NEXT n chksum = chksum MOD 256 'print chksum END FUNCTION Anything you are going to send, add the checksum character to the end. Anything you are receiving, check the checksum of the 'string including the checksum'. If it is zero, the string is intact and simply remove the last character (the checksum) What you do with a bad checksum is up to you. If it's not critical, print/log the error and carry on. Otherwise, send a reply requesting a resend. One alternative is to always send and acknowledgement and the sender resends if it doesn't receive the ACK There are lots of methods and it depends on your overall design. For basic environment logging, I would stick with the log the error and carry on approach. The checksum method I have shown you finds a single error. It is possible that two errors in the one string will give the same checksum. There are other more complex checksum/CRC methods that find more errors and even correct them but the overheads are higher. Jim VK7JH MMedit |
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| lew247 Guru Joined: 23/12/2015 Location: United KingdomPosts: 1702 |
Thanks Jim |
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| Greg Fordyce Senior Member  Joined: 16/09/2011 Location: United KingdomPosts: 153 |
Here's a good app note from Maxim explaining CRC checking Maxim app note 3749 |
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