diff --git a/cpu/nrf5x_common/include/periph_cpu_common.h b/cpu/nrf5x_common/include/periph_cpu_common.h
index 7e43a7dd09..c6fa1cfc4e 100644
--- a/cpu/nrf5x_common/include/periph_cpu_common.h
+++ b/cpu/nrf5x_common/include/periph_cpu_common.h
@@ -187,7 +187,14 @@ typedef enum {
  */
 typedef struct {
     NRF_TIMER_Type *dev;    /**< timer device */
-    uint8_t channels;       /**< number of channels available */
+    /**
+     * @brief   number of hardware channels ***minus one***
+     *
+     * The last hardware channels is implicitly used by timer_read() and not
+     * available to the user. This value, hence, is the number of channels
+     * available to the user.
+     */
+    uint8_t channels;
     uint8_t bitmode;        /**< counter width */
     uint8_t irqn;           /**< IRQ number of the timer device */
 } timer_conf_t;