diff --git a/cpu/cc2538/dev/gpio.c b/cpu/cc2538/dev/gpio.c
index 111946001..2961a2eb0 100644
--- a/cpu/cc2538/dev/gpio.c
+++ b/cpu/cc2538/dev/gpio.c
@@ -83,10 +83,10 @@ notify(uint8_t mask, uint8_t port)
 void
 gpio_port_a_isr()
 {
-  ENERGEST_ON(ENERGEST_TYPE_IRQ);
-
   lpm_exit();
 
+  ENERGEST_ON(ENERGEST_TYPE_IRQ);
+
   notify(REG(GPIO_A_BASE | GPIO_MIS), GPIO_A_NUM);
 
   GPIO_CLEAR_INTERRUPT(GPIO_A_BASE, 0xFF);
@@ -99,10 +99,10 @@ gpio_port_a_isr()
 void
 gpio_port_b_isr()
 {
-  ENERGEST_ON(ENERGEST_TYPE_IRQ);
-
   lpm_exit();
 
+  ENERGEST_ON(ENERGEST_TYPE_IRQ);
+
   notify(REG(GPIO_B_BASE | GPIO_MIS), GPIO_B_NUM);
 
   GPIO_CLEAR_INTERRUPT(GPIO_B_BASE, 0xFF);
@@ -115,10 +115,10 @@ gpio_port_b_isr()
 void
 gpio_port_c_isr()
 {
-  ENERGEST_ON(ENERGEST_TYPE_IRQ);
-
   lpm_exit();
 
+  ENERGEST_ON(ENERGEST_TYPE_IRQ);
+
   notify(REG(GPIO_C_BASE | GPIO_MIS), GPIO_C_NUM);
 
   GPIO_CLEAR_INTERRUPT(GPIO_C_BASE, 0xFF);
@@ -131,10 +131,10 @@ gpio_port_c_isr()
 void
 gpio_port_d_isr()
 {
-  ENERGEST_ON(ENERGEST_TYPE_IRQ);
-
   lpm_exit();
 
+  ENERGEST_ON(ENERGEST_TYPE_IRQ);
+
   notify(REG(GPIO_D_BASE | GPIO_MIS), GPIO_D_NUM);
 
   GPIO_CLEAR_INTERRUPT(GPIO_D_BASE, 0xFF);
diff --git a/cpu/cc2538/lpm.c b/cpu/cc2538/lpm.c
index bbc035219..80b536998 100644
--- a/cpu/cc2538/lpm.c
+++ b/cpu/cc2538/lpm.c
@@ -158,16 +158,28 @@ enter_pm0(void)
 static void
 select_32_mhz_xosc(void)
 {
+  /*First, make sure there is no ongoing clock source change */
+  while((REG(SYS_CTRL_CLOCK_STA) & SYS_CTRL_CLOCK_STA_SOURCE_CHANGE) != 0);
+
   /* Turn on the 32 MHz XOSC and source the system clock on it. */
   REG(SYS_CTRL_CLOCK_CTRL) &= ~SYS_CTRL_CLOCK_CTRL_OSC;
 
   /* Wait for the switch to take place */
   while((REG(SYS_CTRL_CLOCK_STA) & SYS_CTRL_CLOCK_STA_OSC) != 0);
+
+  /* Power down the unused oscillator. */
+  REG(SYS_CTRL_CLOCK_CTRL) |= SYS_CTRL_CLOCK_CTRL_OSC_PD;
 }
 /*---------------------------------------------------------------------------*/
 static void
 select_16_mhz_rcosc(void)
 {
+  /*
+   * Power up both oscillators in order to speed up the transition to the 32-MHz
+   * XOSC after wake up.
+   */
+  REG(SYS_CTRL_CLOCK_CTRL) &= ~SYS_CTRL_CLOCK_CTRL_OSC_PD;
+
   /*First, make sure there is no ongoing clock source change */
   while((REG(SYS_CTRL_CLOCK_STA) & SYS_CTRL_CLOCK_STA_SOURCE_CHANGE) != 0);
 
@@ -187,6 +199,16 @@ lpm_exit()
     return;
   }
 
+  /*
+   * When returning from PM1/2, the sleep timer value (used by RTIMER_NOW()) is
+   * not up-to-date until a positive edge on the 32-kHz clock has been detected
+   * after the system clock restarted. To ensure an updated value is read, wait
+   * for a positive transition on the 32-kHz clock by polling the
+   * SYS_CTRL_CLOCK_STA.SYNC_32K bit, before reading the sleep timer value.
+   */
+  while(REG(SYS_CTRL_CLOCK_STA) & SYS_CTRL_CLOCK_STA_SYNC_32K);
+  while(!(REG(SYS_CTRL_CLOCK_STA) & SYS_CTRL_CLOCK_STA_SYNC_32K));
+
   LPM_STATS_ADD(REG(SYS_CTRL_PMCTL) & SYS_CTRL_PMCTL_PM3,
                 RTIMER_NOW() - sleep_enter_time);
 
@@ -301,6 +323,11 @@ lpm_enter()
     select_32_mhz_xosc();
 
     REG(SYS_CTRL_PMCTL) = SYS_CTRL_PMCTL_PM0;
+
+    /* Remember IRQ energest for next pass */
+    ENERGEST_IRQ_SAVE(irq_energest);
+    ENERGEST_ON(ENERGEST_TYPE_CPU);
+    ENERGEST_OFF(ENERGEST_TYPE_LPM);
   } else {
     /* All clear. Assert WFI and drop to PM1/2. This is now un-interruptible */
     assert_wfi();
diff --git a/cpu/cc2538/lpm.h b/cpu/cc2538/lpm.h
index f4ca3366f..0c497b5be 100644
--- a/cpu/cc2538/lpm.h
+++ b/cpu/cc2538/lpm.h
@@ -150,6 +150,12 @@ void lpm_enter(void);
  * interrupt. This may lead to other parts of the code trying to use the RF,
  * so we need to switch the clock source \e before said code gets executed.
  *
+ * This function also makes sure that the sleep timer value is up-to-date
+ * following wake-up from PM1/2 so that RTIMER_NOW() works.
+ *
+ * \note This function should be called at the very beginning of ISRs waking up
+ * the SoC in order to restore all clocks and timers.
+ *
  * \sa lpm_enter(), rtimer_isr()
  */
 void lpm_exit(void);
diff --git a/cpu/cc2538/rtimer-arch.c b/cpu/cc2538/rtimer-arch.c
index 2407b64b6..ffb221024 100644
--- a/cpu/cc2538/rtimer-arch.c
+++ b/cpu/cc2538/rtimer-arch.c
@@ -136,13 +136,6 @@ rtimer_arch_now()
 void
 rtimer_isr()
 {
-  ENERGEST_ON(ENERGEST_TYPE_IRQ);
-
-  next_trigger = 0;
-
-  nvic_interrupt_unpend(NVIC_INT_SM_TIMER);
-  nvic_interrupt_disable(NVIC_INT_SM_TIMER);
-
   /*
    * If we were in PM1+, call the wake-up sequence first. This will make sure
    * that the 32MHz OSC is selected as the clock source. We need to do this
@@ -150,6 +143,13 @@ rtimer_isr()
    */
   lpm_exit();
 
+  ENERGEST_ON(ENERGEST_TYPE_IRQ);
+
+  next_trigger = 0;
+
+  nvic_interrupt_unpend(NVIC_INT_SM_TIMER);
+  nvic_interrupt_disable(NVIC_INT_SM_TIMER);
+
   rtimer_run_next();
 
   ENERGEST_OFF(ENERGEST_TYPE_IRQ);