| Posted: 05:36am 02 Dec 2025 |
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Hello All,
I would like to use the Modbus CRC in a Micromite MK2. I am aware of the MMBasic versions on The Fruit Of The Shed (I put them there) but I would like to use a CFunction for extra speed. I also Know that it is available in the PicoMite but I want to use the Micomite.
Claude AI has written the C code for me but I don't have the software or the knowledge to compile the code.
If there is someone out there who could do that for I would be very grateful.
The code:
/*
* Modbus CRC-16 CFunction for Micromite Mk2 MMBasic
*
* Compile this with MPLAB X and XC32 compiler using the settings:
* - Position independent code (-mno-abicalls -fno-pic)
* - Optimization level -O1 or -O2
* - Target: PIC32MX470F512H (for Micromite Mk2)
*
* After compiling, extract the hex codes and insert into MMBasic as:
*
* CFunction modbus_crc(STRING) INTEGER
* [hex codes here]
* End CFunction
*
* Usage in MMBasic:
* data$ = CHR$(&h01) + CHR$(&h03) + CHR$(&h00) + CHR$(&h00) + CHR$(&h00) + CHR$(&h0A)
* crc% = modbus_crc(data$)
* PRINT "CRC: "; HEX$(crc%, 4)
*/
#include "CFunction.h"
/*
* Calculate Modbus CRC-16
* Input: Pointer to MMBasic string (first byte = length, followed by data)
* Returns: 16-bit CRC value as long long int
*/
long long int modbus_crc(unsigned char *str) {
unsigned char len;
unsigned char *data;
unsigned short crc;
int i, j;
// Get string length (first byte)
len = *str;
// Point to actual data (skip length byte)
data = str + 1;
// Initialize CRC to 0xFFFF
crc = 0xFFFF;
// Process each byte
for (i = 0; i < len; i++) {
crc = crc ^ data[i];
// Process each bit
for (j = 0; j < 8; j++) {
if (crc & 0x0001) {
crc = (crc >> 1) ^ 0xA001;
} else {
crc = crc >> 1;
}
}
}
// Return as long long int (MMBasic integer type)
return (long long int)crc;
}
/*
* Example usage in MMBasic:
*
* ' Define the CFunction (after compiling and extracting hex codes)
* CFunction modbus_crc(STRING) INTEGER
* 00000000
* [your hex codes here]
* End CFunction
*
* ' Use it in your program
* data$ = CHR$(&h01) + CHR$(&h03) + CHR$(&h00) + CHR$(&h00) + CHR$(&h00) + CHR$(&h0A)
* crc% = modbus_crc(data$)
*
* PRINT "CRC value: "; HEX$(crc%, 4)
*
* ' Append CRC to message (Modbus uses little-endian: low byte first)
* message$ = data$ + CHR$(crc% AND &hFF) + CHR$((crc% >> 8) AND &hFF)
*
* ' Send the complete message with CRC
* PRINT "Complete message with CRC: ";
* FOR i = 1 TO LEN(message$)
* PRINT HEX$(ASC(MID$(message$, i, 1)), 2); " ";
* NEXT i
* PRINT
*/
If someone can do it for me I will ask Geoff to include it in the documentation and/or put it on The Fruit Of The Shed.
Thank you in anticipation.
Bill
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