diff --git a/src/stm32f103/backup.c b/src/stm32f103/backup.c
index 18a4c20..1137568 100644
--- a/src/stm32f103/backup.c
+++ b/src/stm32f103/backup.c
@@ -24,18 +24,15 @@
 
 #define RTC_BKP_DR(reg)  MMIO16(BACKUP_REGS_BASE + 4 + (4 * (reg)))
 
-void backup_write(enum BackupRegister reg, uint32_t value) {
+void backup_write(enum BackupRegister reg, uint16_t value) {
     rcc_periph_clock_enable(RCC_PWR);
     rcc_periph_clock_enable(RCC_BKP);
 
     pwr_disable_backup_domain_write_protect();
-    RTC_BKP_DR((int)reg*2) = value & 0xFFFFUL;
-    RTC_BKP_DR((int)reg*2+1) = (value & 0xFFFF0000UL) >> 16;
+    RTC_BKP_DR((int)reg) = value;
     pwr_enable_backup_domain_write_protect();
 }
 
-uint32_t backup_read(enum BackupRegister reg) {
-    uint32_t value = ((uint32_t)RTC_BKP_DR((int)reg*2+1) << 16)
-                   | ((uint32_t)RTC_BKP_DR((int)reg*2) << 0);
-    return value;
+uint16_t backup_read(enum BackupRegister reg) {
+    return RTC_BKP_DR((int)reg);
 }
diff --git a/src/stm32f103/backup.h b/src/stm32f103/backup.h
index c2fe2b0..5e265a9 100644
--- a/src/stm32f103/backup.h
+++ b/src/stm32f103/backup.h
@@ -20,14 +20,19 @@
 #define BACKUP_H_INCLUDED
 
 enum BackupRegister {
-    BKP0 = 0,
     BKP1,
     BKP2,
     BKP3,
     BKP4,
+    BKP5,
+    BKP6,
+    BKP7,
+    BKP8,
+    BKP9,
+    BKP10,
 };
 
-extern void backup_write(enum BackupRegister reg, uint32_t value);
-extern uint32_t backup_read(enum BackupRegister reg);
+extern void backup_write(enum BackupRegister reg, uint16_t value);
+extern uint16_t backup_read(enum BackupRegister reg);
 
 #endif
diff --git a/src/stm32f103/target_stm32f103.c b/src/stm32f103/target_stm32f103.c
index 6948482..557f630 100644
--- a/src/stm32f103/target_stm32f103.c
+++ b/src/stm32f103/target_stm32f103.c
@@ -55,7 +55,13 @@ _Static_assert((FLASH_BASE + FLASH_SIZE_OVERRIDE >= APP_BASE_ADDRESS),
                "Incompatible flash size");
 #endif
 
-static const uint32_t CMD_BOOT = 0x544F4F42UL;
+#ifndef REG_BOOT
+#define REG_BOOT BKP1
+#endif
+
+#ifndef CMD_BOOT
+#define CMD_BOOT 0x4F42UL
+#endif
 
 void target_clock_setup(void) {
 #ifdef USE_HSI
@@ -160,13 +166,13 @@ const usbd_driver* target_usb_init(void) {
 bool target_get_force_bootloader(void) {
     bool force = false;
     /* Check the RTC backup register */
-    uint32_t cmd = backup_read(BKP0);
+    uint16_t cmd = backup_read(REG_BOOT);
     if (cmd == CMD_BOOT) {
         force = true;
     }
 
     /* Clear the RTC backup register */
-    backup_write(BKP0, 0);
+    backup_write(REG_BOOT, 0);
 
 #if HAVE_BUTTON
     /* Wait some time in case the button has some debounce capacitor */
diff --git a/src/stm32l1/target_stm32l1.c b/src/stm32l1/target_stm32l1.c
index 57b4825..f94bd90 100644
--- a/src/stm32l1/target_stm32l1.c
+++ b/src/stm32l1/target_stm32l1.c
@@ -27,8 +27,15 @@
 #include "config.h"
 #include "backup.h"
 
+#ifndef REG_BOOT
+#define REG_BOOT BKP1
+#endif
+
+#ifndef CMD_BOOT
+#define CMD_BOOT 0x544F4F42UL
+#endif
+
 //#define CMD_FAST_BOOT 0xfa57b007
-static const uint32_t CMD_BOOT = 0x544F4F42UL;
 
 void target_clock_setup(void) {
 
@@ -82,11 +89,11 @@ const usbd_driver* target_usb_init(void)
 bool target_get_force_bootloader(void)
 {
 	bool enter_bl = false;
-	uint32_t cmd = backup_read(BKP0);
+	uint32_t cmd = backup_read(REG_BOOT);
 	if (cmd == CMD_BOOT) {
 		enter_bl = true;
 	}
-	backup_write(BKP0, 0);
+	backup_write(REG_BOOT, 0);
 
 #if HAVE_BUTTON
 #warning HAVE_BUTTON not implemented for L1