Fixup CRC16 to detect missing sequences of zeros

SpectrumScan.py

@@ -50,6 +50,7 @@ def parse_line(line):
 
 POLY = 0x1021
 def crc16(s, c):
+    c = c ^ 0xffff
     for ch in s:
         c = c ^ (ord(ch) << 8)
         for i in range(8):
@@ -58,7 +59,7 @@ def crc16(s, c):
             else:
                 c = (c << 1) & 0xffff
 
-    return c
+    return c ^ 0xffff
 
 def main():
     parser = argparse.ArgumentParser(formatter_class=RawTextHelpFormatter, description='''\

lib/comms/comms.cpp

@@ -123,6 +123,7 @@ String _wrap_str(String);
 #define POLY 0x1021
 uint16_t crc16(String v, uint16_t c)
 {
+    c ^= 0xffff;
     for (int i = 0; i < v.length(); i++)
     {
         uint16_t ch = v.charAt(i);
@@ -140,7 +141,7 @@ uint16_t crc16(String v, uint16_t c)
         }
     }
 
-    return c;
+    return c ^ 0xffff;
 }
 
 void ReadlineComms::_onReceive()

spectrum_scan.c

@@ -13,6 +13,7 @@ typedef struct {
 
 #define POLY 0x1021
 uint16_t crc16(char *p, char *end, uint16_t c) {
+    c ^= 0xffff;
     if (end == NULL) {
         end = strchr(p, 0);
     }
@@ -29,7 +30,7 @@ uint16_t crc16(char *p, char *end, uint16_t c) {
         }
     }
 
-    return c;
+    return c ^ 0xffff;
 }
 
 #define BUFSIZE 102400