[New Feature] Use an interrupt that isn't part of the M0+ core for the RTTY interrupt. This allows sleep mode

firmware/inc/telemetry.h

@@ -26,5 +26,6 @@
 #define TELEMETRY_H
 
 uint16_t crc_checksum(char *string);
+void timer0_tick_init(uint32_t frequency);
 
 #endif /* TELEMETRY_H */

firmware/src/main.c

@@ -326,15 +326,14 @@ int main(void)
   SystemCoreClock = system_cpu_clock_get_hz();
 
   /* Configure Sleep Mode */
-  system_set_sleepmode(SYSTEM_SLEEPMODE_STANDBY);
-  system_set_sleepmode(SYSTEM_SLEEPMODE_IDLE_0); /* Disable CPU, AHB. APB still runs */
+  //system_set_sleepmode(SYSTEM_SLEEPMODE_STANDBY);
+  system_set_sleepmode(SYSTEM_SLEEPMODE_IDLE_2); /* Disable CPU, AHB and APB */
 
   /* Configure the Power Manager */
   powermananger_init();
 
-  /* Configure the SysTick for 50Hz triggering */
-  SysTick_Config(SystemCoreClock / 50);
-
+  /* Timer 0 for 50Hz triggering */
+  timer0_tick_init(50);
 
   /**
    * System initialisation
@@ -365,11 +364,13 @@ int main(void)
   si4060_gpio_init();
   si4060_start_tx(0);
 
+  led_on();
+
   while (1) {
     /* Watchdog */
     wdt_reset_count();
 
-    /* Set the next packet */
+    /* Send the next packet */
     output_telemetry_string();
   }
 }
@@ -377,8 +378,11 @@ int main(void)
 /**
  * Called at 50Hz
  */
-void SysTick_Handler(void)
+void TC0_Handler(void)
 {
-  /* Output RTTY */
-  rtty_tick();
+  if (tc_get_status(TC0) & TC_STATUS_CHANNEL_0_MATCH) {
+    tc_clear_status(TC0, TC_STATUS_CHANNEL_0_MATCH);
+
+    rtty_tick();
+  }
 }

firmware/src/telemetry.c

@@ -26,6 +26,10 @@
 #include <string.h>
 
 #include "samd20.h"
+#include "system/gclk.h"
+#include "system/interrupt.h"
+#include "tc/tc_driver.h"
+
 
 /**
  * CRC Function for the XMODEM protocol.
@@ -67,3 +71,46 @@ uint16_t crc_checksum(char *string)
 
   return crc;
 }
+
+/**
+ * Initialises a timer interupt at the given frequency
+ */
+void timer0_tick_init(uint32_t frequency)
+{
+  /* Calculate the wrap value for the given frequency */
+  uint32_t gclk0_frequency = system_gclk_chan_get_hz(0);
+  uint32_t count = gclk0_frequency / frequency;
+
+  /* Configure Timer 0 */
+  bool t0_capture_channel_enables[]    = {false, false};
+  uint32_t t0_compare_channel_values[] = {count, 0x0000};
+  tc_init(TC0,
+	  GCLK_GENERATOR_0,
+	  TC_COUNTER_SIZE_32BIT,
+	  TC_CLOCK_PRESCALER_DIV1,
+	  TC_WAVE_GENERATION_MATCH_FREQ,
+	  TC_RELOAD_ACTION_GCLK,
+	  TC_COUNT_DIRECTION_UP,
+	  TC_WAVEFORM_INVERT_OUTPUT_NONE,
+	  false,			/* Oneshot  */
+	  true,				/* Run in standby */
+	  0x0000,			/* Initial value */
+	  count,			/* Top value */
+	  t0_capture_channel_enables,	/* Capture Channel Enables */
+	  t0_compare_channel_values);	/* Compare Channels Values */
+
+  /* Enable Events */
+  struct tc_events event;
+  memset(&event, 0, sizeof(struct tc_events));
+  event.generate_event_on_compare_channel[0] = true;
+  event.event_action = TC_EVENT_ACTION_RETRIGGER;
+  tc_enable_events(TC0, &event);
+
+  /* Enable Interrupt */
+  TC0->COUNT32.INTENSET.reg = (1 << 4);
+  irq_register_handler(TC0_IRQn, 0); /* Highest Priority */
+
+  /* Enable Timer */
+  tc_enable(TC0);
+  tc_start_counter(TC0);
+}