diff --git a/sonoff/support.ino b/sonoff/support.ino
index f060a551b..388ebacfc 100644
--- a/sonoff/support.ino
+++ b/sonoff/support.ino
@@ -123,6 +123,89 @@ size_t strcspn(const char *str1, const char *str2)
   }
   return ret;
 }
+
+/*
+ * Convert a string to an unsigned long long integer.
+ *
+ * Assumes that the upper and lower case
+ * alphabets and digits are each contiguous.
+ * https://opensource.apple.com/source/Libc/Libc-583/stdlib/FreeBSD/strtoull.c
+ */
+
+#ifndef __LONG_LONG_MAX__
+#define __LONG_LONG_MAX__ 9223372036854775807LL
+#endif
+#undef ULLONG_MAX
+#define ULLONG_MAX (__LONG_LONG_MAX__ * 2ULL + 1)
+
+unsigned long long strtoull(const char *__restrict nptr, char **__restrict endptr, int base)
+{
+	const char *s;
+	unsigned long long acc;
+	char c;
+	unsigned long long cutoff;
+	int neg, any, cutlim;
+
+	/*
+	 * See strtoq for comments as to the logic used.
+	 */
+	s = nptr;
+	do {
+		c = *s++;
+	} while (isspace((unsigned char)c));
+	if (c == '-') {
+		neg = 1;
+		c = *s++;
+	} else {
+		neg = 0;
+		if (c == '+')
+			c = *s++;
+	}
+	if ((base == 0 || base == 16) &&
+	    c == '0' && (*s == 'x' || *s == 'X')) {
+		c = s[1];
+		s += 2;
+		base = 16;
+	}
+	if (base == 0)
+		base = c == '0' ? 8 : 10;
+	acc = any = 0;
+	if (base < 2 || base > 36)
+		goto noconv;
+
+	cutoff = ULLONG_MAX / base;
+	cutlim = ULLONG_MAX % base;
+	for ( ; ; c = *s++) {
+		if (c >= '0' && c <= '9')
+			c -= '0';
+		else if (c >= 'A' && c <= 'Z')
+			c -= 'A' - 10;
+		else if (c >= 'a' && c <= 'z')
+			c -= 'a' - 10;
+		else
+			break;
+		if (c >= base)
+			break;
+		if (any < 0 || acc > cutoff || (acc == cutoff && c > cutlim))
+			any = -1;
+		else {
+			any = 1;
+			acc *= base;
+			acc += c;
+		}
+	}
+	if (any < 0) {
+		acc = ULLONG_MAX;
+	} else if (!any) {
+noconv:
+    uint8_t dummy = 0;
+	} else if (neg)
+		acc = -acc;
+	if (endptr != nullptr)
+		*endptr = (char *)(any ? s - 1 : nptr);
+	return (acc);
+}
+
 #endif  // ARDUINO_ESP8266_RELEASE_2_3_0
 
 // Get span until single character in string