diff --git a/src/platforms/stm32/serialno.c b/src/platforms/stm32/serialno.c
index a8b2898b..a5c03efa 100644
--- a/src/platforms/stm32/serialno.c
+++ b/src/platforms/stm32/serialno.c
@@ -23,44 +23,42 @@
 char *serial_no_read(char *s)
 {
 #if DFU_SERIAL_LENGTH == 9
-	int i;
-	volatile uint32_t *unique_id_p = (volatile uint32_t *)DESIG_UNIQUE_ID_BASE;
-	uint32_t unique_id = *unique_id_p +
+	const volatile uint32_t *const unique_id_p = (volatile uint32_t *)DESIG_UNIQUE_ID_BASE;
+	const uint32_t unique_id = *unique_id_p +
 			*(unique_id_p + 1) +
 			*(unique_id_p + 2);
 	/* Fetch serial number from chip's unique ID */
-	for(i = 0; i < 8; i++) {
-		s[7-i] = ((unique_id >> (4*i)) & 0xF) + '0';
+	for(size_t i = 0; i < 8U; ++i) {
+		s[7U - i] = ((unique_id >> (i * 4U)) & 0x0FU) + '0';
+		/* If the character is something above 9, then add the offset to make it ASCII A-F */
+		if (s[i] > '9')
+			s[i] += 16; /* 'A' - '9' = 17, less 1 gives 16. */
 	}
-	for(i = 0; i < 8; i++)
-		if(s[i] > '9')
-			s[i] += 'A' - '9' - 1;
 #elif DFU_SERIAL_LENGTH == 13
 	/* Use the same serial number as the ST DFU Bootloader.*/
-	uint16_t *uid = (uint16_t *)DESIG_UNIQUE_ID_BASE;
+	const uint16_t *const uid = (uint16_t *)DESIG_UNIQUE_ID_BASE;
 # if defined(STM32F4) || defined(STM32F7)
 	int offset = 3;
 # elif defined(STM32L0) ||  defined(STM32F0) || defined(STM32F3)
 	int offset = 5;
 # endif
-	sprintf(s, "%04X%04X%04X",
-            uid[1] + uid[5], uid[0] + uid[4], uid[offset]);
+	sprintf(s, "%04X%04X%04X", uid[1] + uid[5], uid[0] + uid[4], uid[offset]);
 #elif DFU_SERIAL_LENGTH == 25
-	int i;
 	uint32_t unique_id[3];
-	memcpy(unique_id, (uint32_t *)DESIG_UNIQUE_ID_BASE, 12);
-	for(i = 0; i < 8; i++) {
-		s[ 7-i] = ((unique_id[0] >> (4*i)) & 0xF) + '0';
-		s[15-i] = ((unique_id[1] >> (4*i)) & 0xF) + '0';
-		s[23-i] = ((unique_id[2] >> (4*i)) & 0xF) + '0';
+	memcpy(unique_id, (char *)DESIG_UNIQUE_ID_BASE, 12);
+	for(size_t i = 0; i < 8U; ++i) {
+		s[ 7U - i] = ((unique_id[0] >> (i * 4)) & 0x0F) + '0';
+		s[15U - i] = ((unique_id[1] >> (i * 4)) & 0x0F) + '0';
+		s[23U - i] = ((unique_id[2] >> (i * 4)) & 0x0F) + '0';
+	}
+	for (size_t i = 0; i < 24U; ++i) {
+		/* If the character is something above 9, then add the offset to make it ASCII A-F */
+		if (s[i] > '9')
+			s[i] += 16; /* 'A' - '9' = 17, less 1 gives 16. */
 	}
-	for (i = 0; i < 24; i++)
-		if(s[i] > '9')
-			s[i] += 'A' - '9' - 1;
 #else
 # WARNING "Unhandled DFU_SERIAL_LENGTH"
 #endif
-	s[DFU_SERIAL_LENGTH - 1] = 0;
+	s[DFU_SERIAL_LENGTH - 1] = '\0';
 	return s;
 }
-