diff --git a/examples/stm32/f4/stm32f4-discovery/timer/timer.c b/examples/stm32/f4/stm32f4-discovery/timer/timer.c
index 927228d..b9cf04c 100644
--- a/examples/stm32/f4/stm32f4-discovery/timer/timer.c
+++ b/examples/stm32/f4/stm32f4-discovery/timer/timer.c
@@ -18,14 +18,18 @@
  * along with this library.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <libopencm3/cm3/nvic.h>
 #include <libopencm3/stm32/rcc.h>
 #include <libopencm3/stm32/gpio.h>
 #include <libopencm3/stm32/timer.h>
-#include <libopencm3/cm3/nvic.h>
 #include <libopencm3/stm32/exti.h>
 
 #include <libopencmsis/core_cm3.h>
 
+#ifndef ARRAY_LEN
+#define ARRAY_LEN(array) (sizeof((array))/sizeof((array)[0]))
+#endif
+
 uint16_t frequency_sequence[18] = {
 	1000,
 	500,
@@ -80,75 +84,41 @@ static void tim_setup(void)
 	/* Enable TIM2 interrupt. */
 	nvic_enable_irq(NVIC_TIM2_IRQ);
 
-	/* Reset TIM2 peripheral. */
+	/* Reset TIM2 peripheral to defaults. */
 	rcc_periph_reset_pulse(RST_TIM2);
 
 	/* Timer global mode:
 	 * - No divider
 	 * - Alignment edge
 	 * - Direction up
+	 * (These are actually default values after reset above, so this call
+	 * is strictly unnecessary, but demos the api for alternative settings)
 	 */
 	timer_set_mode(TIM2, TIM_CR1_CKD_CK_INT,
 		       TIM_CR1_CMS_EDGE, TIM_CR1_DIR_UP);
-
-	/* Reset prescaler value.
-	 * Running the clock at 10kHz.
-	 */
-	/*
-	 * On STM32F4 the timers are not running directly from pure APB1 or
-	 * APB2 clock busses.  The APB1 and APB2 clocks used for timers might
-	 * be the double of the APB1 and APB2 clocks.  This depends on the
-	 * setting in DCKCFGR register. By default the behaviour is the
-	 * following: If the Prescaler APBx is greater than 1 the derived timer
-	 * APBx clocks will be double of the original APBx frequencies. Only if
-	 * the APBx prescaler is set to 1 the derived timer APBx will equal the
-	 * original APBx frequencies.
-	 *
-	 * In our case here the APB1 is system frequency divided by 4 and APB2
-	 * is system frequency divided by 2. This means APB1 timer will be 2 x
-	 * APB1 and APB2 will be 2 x APB2. So when we try to calculate the
-	 * prescaler value we have to use rcc_apb1_freqency * 2!!!
-	 *
-	 * For additional information see reference manual for the stm32f4
-	 * familiy of chips. Page 204 and 213
-	 */
+        /*
+         * Please take note that the clock source for STM32F4 timers
+         * might not be the raw APB1/APB2 clocks.  In various conditions they
+         * are doubled.  See the Reference Manual for full details!
+         * In our case, TIM2 on APB1 is running at double frequency, so this
+         * sets the prescaler to have the timer run at 10kHz
+         */
 	timer_set_prescaler(TIM2, ((rcc_apb1_frequency * 2) / 10000));
 
 	/* Disable preload. */
 	timer_disable_preload(TIM2);
-
-	/* Continous mode. */
 	timer_continuous_mode(TIM2);
 
-	/* Period (36kHz). */
+	/* count full range, as we'll update compare value continuously */
 	timer_set_period(TIM2, 65535);
 
-	/* Disable outputs. */
-	timer_disable_oc_output(TIM2, TIM_OC1);
-	timer_disable_oc_output(TIM2, TIM_OC2);
-	timer_disable_oc_output(TIM2, TIM_OC3);
-	timer_disable_oc_output(TIM2, TIM_OC4);
-
-	/* -- OC1 configuration -- */
-
-	/* Configure global mode of line 1. */
-	timer_disable_oc_clear(TIM2, TIM_OC1);
-	timer_disable_oc_preload(TIM2, TIM_OC1);
-	timer_set_oc_slow_mode(TIM2, TIM_OC1);
-	timer_set_oc_mode(TIM2, TIM_OC1, TIM_OCM_FROZEN);
-
-	/* Set the capture compare value for OC1. */
+	/* Set the initual output compare value for OC1. */
 	timer_set_oc_value(TIM2, TIM_OC1, 1000);
 
-	/* ---- */
-
-	/* ARR reload enable. */
-	timer_disable_preload(TIM2);
-
 	/* Counter enable. */
 	timer_enable_counter(TIM2);
 
-	/* Enable commutation interrupt. */
+	/* Enable Channel 1 compare interrupt to recalculate compare values */
 	timer_enable_irq(TIM2, TIM_DIER_CC1IE);
 }
 
@@ -170,8 +140,9 @@ void tim2_isr(void)
 		new_time = compare_time + frequency;
 
 		timer_set_oc_value(TIM2, TIM_OC1, new_time);
-		if (frequency_sel == 18)
+		if (frequency_sel == ARRAY_LEN(frequency_sequence)) {
 			frequency_sel = 0;
+		}
 
 		/* Toggle LED to indicate compare event. */
 		gpio_toggle(GPIOD, GPIO12);