Added note in PWM driver and example about PM0

cpu/cc2538/dev/pwm.h

@@ -43,9 +43,13 @@
  * easily changed (see PWM_FREQ_MIN and PWM_FREQ_MAX values).  The reason it is
  * fixed to these frequencies is to have a consistent duty cycle
  * implementation.
+ *
  * Depending on the specific needs these limits can be changed to meet a given
  * duty cycle and lower frequencies by using the prescaler (GPTIMER_TnPR).
  *
+ * The PWM timer is stopped when dropping below PM0, alternatively you can set
+ * LPM_CONF_MAX_PM to zero, or call lpm_max_pm(0)
+ *
  * @{
  *
  * \file
@@ -174,4 +178,4 @@ int8_t pwm_toggle_direction(uint8_t timer, uint8_t ab);
 /**
  * @}
  * @}
- */
+ */

examples/zolertia/zoul/test-pwm.c

@@ -35,6 +35,11 @@
  * \defgroup remote-test-pwm Test the CC2538 PWM driver
  *
  * Demonstrates the use of the CC2538 PWM driver for the Zolertia's Zoul boards
+ * The PWM timer is stopped when dropping below PM0, alternatively you can set
+ * LPM_CONF_MAX_PM to zero, or call lpm_max_pm(0).  In this example is not 
+ * needed as we disable RDC in the Makefile, and the CC2538 never drops below
+ * PM0
+ *
  * @{
  *
  * \file