imet54: CRC

demod/mod/imet54mod.c

@@ -224,6 +224,61 @@ static ui8_t hamming(int opt_ecc, ui8_t *cwb, ui8_t *sym) {
     return ret; // { 0, 1, 0xF0 }
 }
 
+static int crc32ok(ui8_t *bytes, int len) {
+    ui32_t poly0 = 0x0EDB;
+    ui32_t poly1 = 0x8260;
+    //[105 , 7, 0x8EDB, 0x8260],
+    //[104 , 0, 0x48EB, 0x1ACA]
+    int n = 104;
+    int b = 0;
+    ui32_t c0 = 0x48EB;
+    ui32_t c1 = 0x1ACA;
+    ui32_t nx_c0 = c0;
+    ui32_t nx_c1 = c1;
+
+    ui32_t data_c0 = (bytes[100]<<8) | bytes[101];
+    ui32_t data_c1 = (bytes[106]<<8) | bytes[107];
+
+    ui32_t crc0 = 0;
+    ui32_t crc1 = 0;
+
+    if (len < 108) return 0;
+
+    while (n >= 0) {
+
+        if (n < 100 || (n > 101 && n < 106)) {
+            if ((bytes[n]>>b) & 1) {
+                crc0 ^= c0;
+                crc1 ^= c1;
+            }
+        }
+
+        if (c1 & 0x8000) {
+            nx_c0 ^= poly0;
+            nx_c1 ^= poly1;
+        }
+        nx_c0 <<= 1;
+        nx_c1 <<= 1;
+        if ( c1     & 0x8000) nx_c0 |= 1;
+        if ((c1^c0) & 0x8000) nx_c1 |= 1;
+        nx_c0 &= 0xFFFF;
+        c0 = nx_c0;
+        c1 = nx_c1;
+
+        if (b < 7) b += 1;
+        else {
+            b = 0;
+            if (n % 4 == 3) n -= 7;
+            else            n += 1;
+        }
+    }
+
+    crc0 ^= data_c0^0x5000;
+    crc1 ^= data_c1^0x1DAD;
+
+    if (crc1 == 0 && (crc0 & 0xF000) == 0) return 1;
+    return 0;
+}
 
 /* ------------------------------------------------------------------------------------ */
 
@@ -406,9 +461,11 @@ static int print_position(gpx_t *gpx, int len, int ecc_frm, int ecc_gps) {
     int ptu1e9 = 0;
     int tp_err = 0;
     int pos_ok = 0,
-        frm_ok = 0;
+        frm_ok = 0,
+        crc_ok = 0;
     int rs_type = 54;
 
+    crc_ok = crc32ok(gpx->frame, len);
     pos_ok = (ecc_frm >= 0  &&  len > pos_STATUS+2);
     frm_ok = (ecc_frm >= 0  &&  len > pos_F8);
 
@@ -458,6 +515,10 @@ static int print_position(gpx_t *gpx, int len, int ecc_frm, int ecc_gps) {
             if (gpx->RH > -0.5f)   fprintf(stdout, " RH=%.0f%% ", gpx->RH);
         }
 
+        if (gpx->option.vbs) {
+            if ( crc_ok ) fprintf(stdout, " [OK]"); else fprintf(stdout, " [NO]");
+        }
+
         // (imet54:GPS+PTU) status: 003E , (imet50:GPS); 0030
         if (gpx->option.vbs && prnSTS) {
             fprintf(stdout, "  [%04X] ", gpx->status);
@@ -473,7 +534,7 @@ static int print_position(gpx_t *gpx, int len, int ecc_frm, int ecc_gps) {
     }
 
     // prnGPS,prnTPU
-    if (gpx->option.jsn && frm_ok && (gpx->status&0x30)==0x30) {
+    if (gpx->option.jsn && frm_ok && crc_ok && (gpx->status&0x30)==0x30) {
         char *ver_jsn = NULL;
         char *subtype = (rs_type == 54) ? "IMET54" : "IMET50";
         unsigned long count_day = (unsigned long)(gpx->std*3600 + gpx->min*60 + gpx->sek+0.5);  // (gpx->timems/1e3+0.5) has gaps
@@ -556,6 +617,7 @@ static void print_frame(gpx_t *gpx, int len, int b2B) {
                 if (gpx->option.raw == 4 && i % 4 == 3) fprintf(stdout, " ");
             }
         }
+        if ( crc32ok(gpx->frame, len) ) fprintf(stdout, " [OK]"); else fprintf(stdout, " [NO]");
         if (gpx->option.ecc && ecc_frm != 0) {
             fprintf(stdout, " # (%d)", ecc_frm);
             fprintf(stdout, " [%d]", ecc_gps);

imet/imet54.txt

@@ -12,7 +12,7 @@ data: 27x64 bits, i.e. 27 raw blocks
 block:
 8 Hamming[8,4] codewords, interleaved 8x8 bits;
 8 codewords -> 8 msg-nibbles, i.e. 32 bits (4 bytes)
-
+crc
 
 
 SN         | GPS_time   | GPS_lat    | GPS_lon    | GPS_alt    |                         | f32 T      | f32 _RH    | f32 Trh    |      | status?