diff --git a/sys/fmt/fmt.c b/sys/fmt/fmt.c
index 11eddf3cfa..78adb8afe6 100644
--- a/sys/fmt/fmt.c
+++ b/sys/fmt/fmt.c
@@ -470,14 +470,13 @@ uint32_t scn_u32_dec(const char *str, size_t n)
     uint32_t res = 0;
 
     while (n--) {
-        char c = *str++;
-        if (!fmt_is_digit(c)) {
+        unsigned c = *str++;
+        unsigned d = c - (unsigned)'0';
+        if ( !( '0'<= c && c <= '9') ) {
             break;
         }
-        else {
-            res *= 10;
-            res += (c - '0');
-        }
+        res *= 10U;
+        res += d;
     }
     return res;
 }
@@ -487,21 +486,23 @@ uint32_t scn_u32_hex(const char *str, size_t n)
     uint32_t res = 0;
 
     while (n--) {
-        char c = *str++;
-        if (!fmt_is_digit(c)) {
-            if (fmt_is_upper(c)) {
-                c = _to_lower(c);
-            }
-            if (c == '\0' || c > 'f') {
-                break;
-            }
-            res <<= 4;
-            res |= c - 'a' + 0xa;
+        unsigned c = *str++;
+        unsigned d;
+        if (('0'<= c) && (c <= '9')){
+            d = c - (unsigned)'0';
+        }
+        else if (('A' <= c) && (c <= 'F')) {
+            d = c - (unsigned)'A' + 0xaU;
+        }
+        else if (('a' <= c) && (c <= 'f')) {
+            d = c - (unsigned)'a' + 0xaU;
         }
         else {
-            res <<= 4;
-            res |= c - '0';
+            break;
         }
+        res <<= 4U;
+        res |= d;
+
     }
     return res;
 }