diff --git a/conf/autopilot/subsystems/fixedwing/autopilot.makefile b/conf/autopilot/subsystems/fixedwing/autopilot.makefile
index b3a2ae5f62..3a398196d5 100644
--- a/conf/autopilot/subsystems/fixedwing/autopilot.makefile
+++ b/conf/autopilot/subsystems/fixedwing/autopilot.makefile
@@ -70,7 +70,7 @@ endif
 ifndef PERIODIC_FREQUENCY
 PERIODIC_FREQUENCY = 60
 endif
-$(TARGET).CFLAGS += -DPERIODIC_TASK_PERIOD='CPU_TICKS_OF_SEC((1./$(PERIODIC_FREQUENCY).))' -DPERIODIC_FREQUENCY=$(PERIODIC_FREQUENCY)
+$(TARGET).CFLAGS += -DPERIODIC_FREQUENCY=$(PERIODIC_FREQUENCY)
 $(TARGET).srcs += mcu_periph/sys_time.c $(SRC_ARCH)/mcu_periph/sys_time_arch.c
 
 #
diff --git a/sw/airborne/firmwares/fixedwing/main_ap.c b/sw/airborne/firmwares/fixedwing/main_ap.c
index 2a613b27da..7decc8025d 100644
--- a/sw/airborne/firmwares/fixedwing/main_ap.c
+++ b/sw/airborne/firmwares/fixedwing/main_ap.c
@@ -528,7 +528,7 @@ void periodic_task_ap( void ) {
 void init_ap( void ) {
 #ifndef SINGLE_MCU /** init done in main_fbw in single MCU */
   mcu_init();
-  sys_time_register_timer(SYS_TIME_TIMER_S(1./PERIODIC_FREQUENCY), NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
 #endif /* SINGLE_MCU */
 
   /************* Sensors initialization ***************/
diff --git a/sw/airborne/firmwares/fixedwing/main_fbw.c b/sw/airborne/firmwares/fixedwing/main_fbw.c
index 914bb01951..23555723af 100644
--- a/sw/airborne/firmwares/fixedwing/main_fbw.c
+++ b/sw/airborne/firmwares/fixedwing/main_fbw.c
@@ -61,7 +61,7 @@ void init_fbw( void ) {
 
   mcu_init();
 
-  sys_time_register_timer(SYS_TIME_TIMER_S(1./PERIODIC_FREQUENCY), NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
 
   electrical_init();
 
diff --git a/sw/airborne/firmwares/motor_bench/main_motor_bench.c b/sw/airborne/firmwares/motor_bench/main_motor_bench.c
index 44e4999e35..3b3afe9f38 100644
--- a/sw/airborne/firmwares/motor_bench/main_motor_bench.c
+++ b/sw/airborne/firmwares/motor_bench/main_motor_bench.c
@@ -41,7 +41,7 @@ int main( void ) {
 static inline void main_init( void ) {
 
   mcu_init();
-  sys_time_register_timer(PERIODIC_TASK_PERIOD, NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
   mb_tacho_init();
 
 #if defined USE_TWI_CONTROLLER
diff --git a/sw/airborne/firmwares/motor_bench/main_turntable.c b/sw/airborne/firmwares/motor_bench/main_turntable.c
index 0b5ad71a2d..576a9583f1 100644
--- a/sw/airborne/firmwares/motor_bench/main_turntable.c
+++ b/sw/airborne/firmwares/motor_bench/main_turntable.c
@@ -35,7 +35,7 @@ int main( void ) {
 
 static inline void main_init( void ) {
   mcu_init();
-  sys_time_register_timer(PERIODIC_TASK_PERIOD, NULL);
+  sys_time_register_timer(1./PERIODIC_FREQUENCY, NULL);
   main_init_tacho();
   mcu_int_enable();
 }
diff --git a/sw/airborne/firmwares/rotorcraft/main.c b/sw/airborne/firmwares/rotorcraft/main.c
index 3893773dd3..26f0336067 100644
--- a/sw/airborne/firmwares/rotorcraft/main.c
+++ b/sw/airborne/firmwares/rotorcraft/main.c
@@ -87,7 +87,7 @@ STATIC_INLINE void main_init( void ) {
 
   mcu_init();
 
-  sys_time_register_timer(SYS_TIME_TIMER_S(1./PERIODIC_FREQUENCY), NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
 
   electrical_init();
 
diff --git a/sw/airborne/firmwares/tutorial/main_demo2.c b/sw/airborne/firmwares/tutorial/main_demo2.c
index e69b857d7d..23950fb208 100644
--- a/sw/airborne/firmwares/tutorial/main_demo2.c
+++ b/sw/airborne/firmwares/tutorial/main_demo2.c
@@ -18,7 +18,7 @@ int main( void ) {
 
 static inline void main_init( void ) {
   mcu_init();
-  sys_time_register_timer(PERIODIC_TASK_PERIOD, NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
 }
 
 static inline void main_periodic_task( void ) {
diff --git a/sw/airborne/firmwares/tutorial/main_demo3.c b/sw/airborne/firmwares/tutorial/main_demo3.c
index a2eb80c7f1..47527879d3 100644
--- a/sw/airborne/firmwares/tutorial/main_demo3.c
+++ b/sw/airborne/firmwares/tutorial/main_demo3.c
@@ -20,7 +20,7 @@ int main( void ) {
 
 static inline void main_init( void ) {
   mcu_init();
-  sys_time_register_timer(PERIODIC_TASK_PERIOD, NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
   uart0_init_tx();
   mcu_int_enable();
 }
diff --git a/sw/airborne/firmwares/tutorial/main_demo4.c b/sw/airborne/firmwares/tutorial/main_demo4.c
index 5194eef92a..b0a91d0d8d 100644
--- a/sw/airborne/firmwares/tutorial/main_demo4.c
+++ b/sw/airborne/firmwares/tutorial/main_demo4.c
@@ -21,7 +21,7 @@ int main( void ) {
 
 static inline void main_init( void ) {
   mcu_init();
-  sys_time_register_timer(PERIODIC_TASK_PERIOD, NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
   uart0_init_tx();
   mcu_int_enable();
 }
diff --git a/sw/airborne/firmwares/tutorial/main_demo5.c b/sw/airborne/firmwares/tutorial/main_demo5.c
index eab0c4c31a..fe85cd36e0 100644
--- a/sw/airborne/firmwares/tutorial/main_demo5.c
+++ b/sw/airborne/firmwares/tutorial/main_demo5.c
@@ -25,7 +25,7 @@ int main( void ) {
 
 static inline void main_init( void ) {
   mcu_init();
-  sys_time_register_timer(PERIODIC_TASK_PERIOD, NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
   uart0_init_tx();
   mcu_int_enable();
 }
diff --git a/sw/airborne/firmwares/tutorial/main_demo6.c b/sw/airborne/firmwares/tutorial/main_demo6.c
index 8f950e8c7f..7703f13f1c 100644
--- a/sw/airborne/firmwares/tutorial/main_demo6.c
+++ b/sw/airborne/firmwares/tutorial/main_demo6.c
@@ -22,7 +22,7 @@ int main( void ) {
 
 static inline void main_init( void ) {
   mcu_init();
-  sys_time_register_timer(PERIODIC_TASK_PERIOD, NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
   usb_serial_init();
   mcu_int_enable();
 }
diff --git a/sw/airborne/lisa/test_uart_lisam.c b/sw/airborne/lisa/test_uart_lisam.c
index 664f3efe83..5ef4c0dad9 100644
--- a/sw/airborne/lisa/test_uart_lisam.c
+++ b/sw/airborne/lisa/test_uart_lisam.c
@@ -31,6 +31,7 @@
 #include "mcu.h"
 #include "mcu_periph/uart.h"
 #include "mcu_periph/sys_time.h"
+#include "led.h"
 
 static inline void main_init( void );
 static inline void main_periodic( void );
@@ -40,15 +41,16 @@ int main(void) {
   main_init();
 
   while (1) {
-    if (sys_time_periodic())
+    if (sys_time_check_and_ack_timer(0))
       main_periodic();
   }
+
   return 0;
 }
 
 static inline void main_init( void ) {
   mcu_init();
-  sys_time_init();
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
 }
 
 static inline void main_periodic( void ) {
diff --git a/sw/airborne/lisa/tunnel_hw.c b/sw/airborne/lisa/tunnel_hw.c
index 0cd55fd838..6f9df43608 100644
--- a/sw/airborne/lisa/tunnel_hw.c
+++ b/sw/airborne/lisa/tunnel_hw.c
@@ -53,7 +53,7 @@ void Delay(__IO uint32_t nCount) {
 int main(void) {
 
   mcu_init();
-  sys_time_register_timer(SYS_TIME_TIMER_S(1./512.), NULL);
+  sys_time_register_timer((1./PERIODIC_FREQUENCY), NULL);
 
   /* init RCC */
   RCC_APB2PeriphClockCmd(A_PERIPH , ENABLE);
@@ -83,7 +83,8 @@ int main(void) {
     if (sys_time_check_and_ack_timer(0))
       main_periodic();
     main_event();
- };
+  }
+
   return 0;
 }
 
diff --git a/sw/airborne/mcu_periph/sys_time.c b/sw/airborne/mcu_periph/sys_time.c
index 9e5b4e49fa..944c4b2784 100644
--- a/sw/airborne/mcu_periph/sys_time.c
+++ b/sw/airborne/mcu_periph/sys_time.c
@@ -32,15 +32,15 @@
 
 struct sys_time sys_time;
 
-uint8_t sys_time_register_timer(uint32_t duration, sys_time_cb cb) {
+uint8_t sys_time_register_timer(float duration, sys_time_cb cb) {
 
   uint32_t start_time = sys_time.nb_tick;
   for (int i = 0; i< SYS_TIME_NB_TIMER; i++) {
     if (!sys_time.timer[i].in_use) {
       sys_time.timer[i].cb         = cb;
       sys_time.timer[i].elapsed    = FALSE;
-      sys_time.timer[i].end_time   = start_time + duration;
-      sys_time.timer[i].duration   = duration;
+      sys_time.timer[i].end_time   = start_time + SYS_TIME_TICKS_OF_SEC(duration);
+      sys_time.timer[i].duration   = SYS_TIME_TICKS_OF_SEC(duration);
       sys_time.timer[i].in_use     = TRUE;
       return i;
     }
@@ -59,10 +59,10 @@ void sys_time_cancel_timer(uint8_t id) {
 }
 
 // FIXME: race condition ??
-void sys_time_update_timer(uint8_t id, uint32_t duration) {
+void sys_time_update_timer(uint8_t id, float duration) {
   mcu_int_disable();
-  sys_time.timer[id].end_time -= (sys_time.timer[id].duration - duration);
-  sys_time.timer[id].duration = duration;
+  sys_time.timer[id].end_time -= (sys_time.timer[id].duration - SYS_TIME_TICKS_OF_SEC(duration));
+  sys_time.timer[id].duration = SYS_TIME_TICKS_OF_SEC(duration);
   mcu_int_enable();
 }
 
diff --git a/sw/airborne/mcu_periph/sys_time.h b/sw/airborne/mcu_periph/sys_time.h
index 3d0865b914..2c0993175d 100644
--- a/sw/airborne/mcu_periph/sys_time.h
+++ b/sw/airborne/mcu_periph/sys_time.h
@@ -64,9 +64,26 @@ extern struct sys_time sys_time;
 #define cpu_time_ticks sys_time.nb_sec_rem
 
 extern void sys_time_init(void);
-extern uint8_t sys_time_register_timer(uint32_t duration, sys_time_cb cb);
+
+/**
+ * Register a new system timer.
+ * @param duration Duration in seconds until the timer elapses.
+ * @param cb Callback function that is called from the ISR when timer elapses, or NULL
+ */
+extern uint8_t sys_time_register_timer(float duration, sys_time_cb cb);
+
+/**
+ * Cancel a system timer by id.
+ * @param id Timer id.
+ */
 extern void    sys_time_cancel_timer(uint8_t id);
-extern void    sys_time_update_timer(uint8_t id, uint32_t duration);
+
+/**
+ * Update the duration until a timer elapses.
+ * @param id Timer id
+ * @param duration Duration in seconds until the timer elapses.
+ */
+extern void    sys_time_update_timer(uint8_t id, float duration);
 
 static inline bool_t sys_time_check_and_ack_timer( uint8_t id ) {
   if (sys_time.timer[id].elapsed) {
@@ -94,8 +111,6 @@ static inline bool_t sys_time_check_and_ack_timer( uint8_t id ) {
 #endif
 #define SYS_TIME_RESOLUTION_CPU_TICKS CPU_TICKS_OF_SEC(SYS_TIME_RESOLUTION)
 
-#define SYS_TIME_TIMER_S(_s) ((_s)/SYS_TIME_RESOLUTION)
-
 #define SYS_TIME_TICKS_OF_SEC(s) (uint32_t)((s) / SYS_TIME_RESOLUTION + 0.5)
 #define SYS_TIME_TICKS_OF_USEC(us) SYS_TIME_TICKS_OF_SEC((us) * 1e-6)
 #define SYS_TIME_TICKS_OF_NSEC(ns) SYS_TIME_TICKS_OF_SEC((ns) * 1e-9)
diff --git a/sw/airborne/test/mcu_periph/test_sys_time.c b/sw/airborne/test/mcu_periph/test_sys_time.c
index ec1b48d8bc..95147a13cc 100644
--- a/sw/airborne/test/mcu_periph/test_sys_time.c
+++ b/sw/airborne/test/mcu_periph/test_sys_time.c
@@ -11,9 +11,9 @@ static inline void main_event( void );
 int main(void) {
 
   mcu_init();
-  unsigned int tmr_02 = sys_time_register_timer(SYS_TIME_TIMER_S(0.2), NULL);
-  unsigned int tmr_03 = sys_time_register_timer(SYS_TIME_TIMER_S(0.3), NULL);
-  sys_time_register_timer(SYS_TIME_TIMER_S(0.5), main_periodic_05);
+  unsigned int tmr_02 = sys_time_register_timer(0.2, NULL);
+  unsigned int tmr_03 = sys_time_register_timer(0.3, NULL);
+  sys_time_register_timer(0.5, main_periodic_05);
 
   while(1) {
     if (sys_time_check_and_ack_timer(tmr_02))