Add software PWM support and test app

utils/spwm.c

@@ -0,0 +1,48 @@
+/** @file   spwm.c
+    @author M. P. Hayes, UCECE
+    @date   25 Sep 2013
+    @brief  These routines are for software pulse width modulation.  They are
+    useful for modulating the luiminance of an LED or beeping a piezo
+    tweeter. 
+*/
+#include "system.h"
+#include "spwm.h"
+
+
+/** Set the period.  This must be greater than the duty.  */
+void
+spwm_period_set (spwm_t *spwm, spwm_period_t period)
+{
+    spwm->period = period;
+}
+
+
+/** Set the duty.  This must be less than the period.  */
+void
+spwm_duty_set (spwm_t *spwm, spwm_period_t duty)
+{
+    spwm->duty = duty;
+}
+
+
+/** Return whether spwm output should be high or low.
+    For example, to control the flashing of a LED use:
+
+    led_set (led, spwm_update (spwm));  */
+bool
+spwm_update (spwm_t *spwm)
+{
+    spwm->count++;
+    if (spwm->count >= spwm->period)
+        spwm->count = 0;
+
+    return spwm->count < spwm->duty;
+}
+
+
+/* Reset spwm.  */
+void
+spwm_reset (spwm_t *spwm)
+{
+    spwm->count = 0;
+}

utils/spwm.h

@@ -0,0 +1,45 @@
+/** @file   spwm.h
+    @author M. P. Hayes, UCECE
+    @date   25 Sep 2013
+    @brief  These routines are for software pulse width modulation.  They are
+    useful for modulating the luiminance of an LED or beeping a piezo
+    tweeter. 
+*/
+#ifndef SPWM_H
+#define SPWM_H
+
+#include "system.h"
+
+
+typedef uint8_t spwm_period_t;
+
+
+/* This structure is defined here so the compiler can allocate enough
+   memory for it.  However, its fields should be treated as
+   private.  */
+typedef struct spwm_struct
+{
+    spwm_period_t period;
+    spwm_period_t duty;
+    spwm_period_t count;
+} spwm_t;
+
+
+/** Set the period.  This must be greater than the duty.  */
+void spwm_period_set (spwm_t *spwm, spwm_period_t period);
+
+
+/** Set the duty.  This must be less than the period.  */
+void spwm_duty_set (spwm_t *spwm, spwm_period_t duty);
+
+
+/** Return whether spwm output should be high or low.
+    For example, to control the flashing of a LED use:
+
+    led_set (led, spwm_update (spwm));  */
+bool spwm_update (spwm_t *spwm);
+
+
+/* Reset spwm.  */
+void spwm_reset (spwm_t *spwm);
+#endif