added pwm code

cubesatsim/cubesatsim.ino

@@ -28,6 +28,11 @@
 #include <MPU6050_tockn.h>
 #include <EEPROM.h>
 #include <Arduino-APRS-Library.h>
+#include <stdio.h>
+#include "pico/stdlib.h"   // stdlib 
+#include "hardware/irq.h"  // interrupts
+#include "hardware/pwm.h"  // pwm 
+#include "hardware/sync.h" // wait for interrupt 
 
 Adafruit_INA219 ina219_1_0x40;
 Adafruit_INA219 ina219_1_0x41(0x41);
@@ -1972,3 +1977,54 @@ void start_ina219() {
   ina219_2_0x44.setCalibration_16V_400mA(); 
   ina219_2_0x45.setCalibration_16V_400mA(); 	
 }
+
+void start_pwm() {
+// based on code https://github.com/rgrosset/pico-pwm-audio
+//
+  set_sys_clock_khz(125000, true); 
+  gpio_set_function(AUDIO_OUT_PIN, GPIO_FUNC_PWM);
+
+  int audio_pin_slice = pwm_gpio_to_slice_num(AUDIO_PIN);
+
+  // Setup PWM interrupt to fire when PWM cycle is complete
+  pwm_clear_irq(audio_pin_slice);
+  pwm_set_irq_enabled(audio_pin_slice, true);
+  // set the handle function above
+  irq_set_exclusive_handler(PWM_IRQ_WRAP, pwm_interrupt_handler); 
+  irq_set_enabled(PWM_IRQ_WRAP, true);	
+	
+  // Setup PWM for audio output
+    pwm_config config = pwm_get_default_config();
+    /* Base clock 176,000,000 Hz divide by wrap 250 then the clock divider further divides
+     * to set the interrupt rate. 
+     * 
+     * 11 KHz is fine for speech. Phone lines generally sample at 8 KHz
+     * 
+     * 
+     * So clkdiv should be as follows for given sample rate
+     *  8.0f for 11 KHz
+     *  4.0f for 22 KHz
+     *  2.0f for 44 KHz etc
+     */
+    pwm_config_set_clkdiv(&config, 16.0);   // 8.0f); 
+    pwm_config_set_wrap(&config, 250); 
+    pwm_init(audio_pin_slice, &config, true);
+
+    pwm_set_gpio_level(AUDIO_OUT_PIN, 0);
+	
+}
+
+void pwm_interrupt_handler() {
+// based on code https://github.com/rgrosset/pico-pwm-audio
+//	
+    pwm_clear_irq(pwm_gpio_to_slice_num(AUDIO_OUT_PIN));    
+    if (wav_position < (WAV_DATA_LENGTH<<3) - 1) { 
+        // set pwm level 
+        // allow the pwm value to repeat for 8 cycles this is >>3 
+        pwm_set_gpio_level(AUDIO_OUT_PIN, WAV_DATA[wav_position>>3]);  
+        wav_position++;
+    } else {
+        // reset to start
+        wav_position = 0;
+    }
+}