Add pwm module.

src/include/pwm.h

@@ -0,0 +1,5 @@
+#define OMEGA 314.1592654
+
+void pwm_init(void);
+void pwm_worker(void);
+void test_pwm(void);

src/pwm.c

@@ -0,0 +1,45 @@
+#include <stdio.h>
+#include <stdint.h>
+#include <math.h>
+
+#include "pwm.h"
+#include "timer.h"
+
+int interrupt_received = 0;
+uint32_t interrupt_counter = 0;
+
+void pwm_init(void)
+{
+	timer_subToT1Overflow(test_pwm);
+}
+
+void pwm_worker()
+{
+	if (!interrupt_received)
+		return;
+
+	double amp = 0.0;
+
+	/* t_n is the time of nth pulse */
+	/* factor 0.00005 because of 1/20 kHz */
+	double t_n = interrupt_counter * 0.00005;
+
+	amp = sin(2*M_PI*50*t_n);
+