diff --git a/firmware/common/hackrf_core.c b/firmware/common/hackrf_core.c
index 3de6dc15..3ff0318a 100644
--- a/firmware/common/hackrf_core.c
+++ b/firmware/common/hackrf_core.c
@@ -136,9 +136,6 @@ bool sample_rate_frac_set(uint32_t rate_num, uint32_t rate_denom)
 	/* MS0/CLK2 is the source for SGPIO (CODEC_X2_CLK) */
 	si5351c_configure_multisynth(2, 0, 0, 0, 0);//p1 doesn't matter
 
-	/* MS0/CLK3 is the source for the external clock output. */
-	//si5351c_configure_multisynth(3, p1, 0, 1, 0); // no clk out
-
 	return true;
 }
 
@@ -248,9 +245,6 @@ bool sample_rate_set(const uint32_t sample_rate_hz) {
 	/* MS0/CLK2 is the source for SGPIO (CODEC_X2_CLK) */
 	si5351c_configure_multisynth(2, p1, 0, 1, 0);//p1 doesn't matter
 
-	/* MS0/CLK3 is the source for the external clock output. */
-	//si5351c_configure_multisynth(3, p1, 0, 1, 0); // no clk out
-
 	return true;
 #endif
 }
@@ -274,8 +268,7 @@ void cpu_clock_init(void)
 	si5351c_power_down_all_clocks();
 	si5351c_set_crystal_configuration();
 	si5351c_enable_xo_and_ms_fanout();
-	si5351c_configure_pll_sources_for_xtal();
-	si5351c_configure_pll1_multisynth();
+	si5351c_set_clock_source(PLL_SOURCE_XTAL);
 
 #ifdef JELLYBEAN
 	/*
@@ -313,6 +306,9 @@ void cpu_clock_init(void)
 	 *   CLK7 -> LPC4330 (but LPC4330 starts up on its own crystal)
 	 */
 
+	/* MS3/CLK3 is the source for the external clock output. */
+	si5351c_configure_multisynth(3, 80*128-512, 0, 1, 0); /* 800/80 = 10MHz */
+
 	/* MS4/CLK4 is the source for the RFFC5071 mixer. */
 	si5351c_configure_multisynth(4, 16*128-512, 0, 1, 0); /* 800/16 = 50MHz */
  
diff --git a/firmware/common/si5351c.c b/firmware/common/si5351c.c
index c976cf0e..dd96605c 100644
--- a/firmware/common/si5351c.c
+++ b/firmware/common/si5351c.c
@@ -23,6 +23,8 @@
 #include "si5351c.h"
 #include <libopencm3/lpc43xx/i2c.h>
 
+enum pll_sources active_clock_source;
+
 /* FIXME return i2c0 status from each function */
 
 /* write to single register */
@@ -117,7 +119,7 @@ void si5351c_set_crystal_configuration()
  */
 void si5351c_enable_xo_and_ms_fanout()
 {
-	uint8_t data[] = { 187, 0x50 };
+	uint8_t data[] = { 187, 0xD0 };
 	si5351c_write(data, sizeof(data));
 }
 
@@ -127,18 +129,29 @@ void si5351c_enable_xo_and_ms_fanout()
  * PLLB_SRC=0 (XTAL input)
  * PLLA_SRC=0 (XTAL input)
  */
-void si5351c_configure_pll_sources_for_xtal()
+void si5351c_configure_pll_sources(const enum pll_sources source)
 {
 	uint8_t data[] = { 15, 0x00 };
+
+	if (source == PLL_SOURCE_CLKIN) {
+		data[1] = 0x0C;
+	}
+
 	si5351c_write(data, sizeof(data));
 }
 
 /* MultiSynth NA (PLL1) */
-void si5351c_configure_pll1_multisynth()
+void si5351c_configure_pll1_multisynth(const enum pll_sources source)
 {
-	//init plla and pllb to (0x0e00+512)/128*25mhz xtal = 800mhz -> int mode
- 	uint8_t data[] = { 26, 0x00, 0x01, 0x00, 0x0E, 0x00, 0x00, 0x00, 0x00 };
- 	si5351c_write(data, sizeof(data));
+	//init plla to (0x0e00+512)/128*25mhz xtal = 800mhz -> int mode
+	uint8_t data[] = { 26, 0x00, 0x01, 0x00, 0x0E, 0x00, 0x00, 0x00, 0x00 };
+
+	if (source == PLL_SOURCE_CLKIN) {
+		/* 10 MHz input on CLKIN instead of 25 MHz XTAL */
+		data[4] = 0x26;
+	}
+
+	si5351c_write(data, sizeof(data));
 	//~ data[0] =34;// pllb
 	//~ si5351c_write(data, sizeof(data));
 }
@@ -233,7 +246,7 @@ void si5351c_configure_clock_control()
 	,SI5351C_CLK_FRAC_MODE | SI5351C_CLK_PLL_SRC(SI5351C_CLK_PLL_SRC_A) | SI5351C_CLK_SRC(SI5351C_CLK_SRC_MULTISYNTH_SELF) | SI5351C_CLK_IDRV(SI5351C_CLK_IDRV_2MA)
 	,SI5351C_CLK_INT_MODE | SI5351C_CLK_PLL_SRC(SI5351C_CLK_PLL_SRC_A) | SI5351C_CLK_SRC(SI5351C_CLK_SRC_MULTISYNTH_0_4) | SI5351C_CLK_IDRV(SI5351C_CLK_IDRV_2MA)
 	,SI5351C_CLK_INT_MODE | SI5351C_CLK_PLL_SRC(SI5351C_CLK_PLL_SRC_A) | SI5351C_CLK_SRC(SI5351C_CLK_SRC_MULTISYNTH_0_4) | SI5351C_CLK_IDRV(SI5351C_CLK_IDRV_2MA)
-	,SI5351C_CLK_POWERDOWN /*not connected, clock out*/
+	,SI5351C_CLK_INT_MODE | SI5351C_CLK_PLL_SRC(SI5351C_CLK_PLL_SRC_A) | SI5351C_CLK_SRC(SI5351C_CLK_SRC_MULTISYNTH_SELF) | SI5351C_CLK_IDRV(SI5351C_CLK_IDRV_8MA)
 	,SI5351C_CLK_INT_MODE | SI5351C_CLK_PLL_SRC(SI5351C_CLK_PLL_SRC_A) | SI5351C_CLK_SRC(SI5351C_CLK_SRC_MULTISYNTH_SELF) | SI5351C_CLK_IDRV(SI5351C_CLK_IDRV_6MA)
 	,SI5351C_CLK_INT_MODE | SI5351C_CLK_PLL_SRC(SI5351C_CLK_PLL_SRC_A) | SI5351C_CLK_SRC(SI5351C_CLK_SRC_MULTISYNTH_SELF) | SI5351C_CLK_IDRV(SI5351C_CLK_IDRV_4MA)
 	,SI5351C_CLK_POWERDOWN | SI5351C_CLK_INT_MODE /*not connected, but: plla int mode*/
@@ -243,10 +256,10 @@ void si5351c_configure_clock_control()
 }
 #endif
 
-/* Enable CLK outputs 0, 1, 2, 4, 5, 7 only. */
+/* Enable CLK outputs 0, 1, 2, 3, 4, 5, 7 only. */
  void si5351c_enable_clock_outputs()
  {
-	uint8_t data[] = { 3, 0x48 };
+	uint8_t data[] = { 3, 0x40 };
  	si5351c_write(data, sizeof(data));
  }
 
@@ -267,3 +280,28 @@ void si5351c_configure_clock_control()
   }
 
  }
+
+void si5351c_set_clock_source(const enum pll_sources source)
+{
+	si5351c_configure_pll_sources(source);
+	si5351c_configure_pll1_multisynth(source);
+
+	active_clock_source = source;
+}
+
+void si5351c_activate_best_clock_source(void)
+{
+	uint8_t device_status = si5351c_read_single(0);
+
+	if (device_status & SI5351C_LOS) {
+		/* CLKIN not detected */
+		if (active_clock_source == PLL_SOURCE_CLKIN) {
+			si5351c_set_clock_source(PLL_SOURCE_XTAL);
+		}
+	} else {
+		/* CLKIN detected */
+		if (active_clock_source == PLL_SOURCE_XTAL) {
+			si5351c_set_clock_source(PLL_SOURCE_CLKIN);
+		}
+	}
+}
diff --git a/firmware/common/si5351c.h b/firmware/common/si5351c.h
index 92946c70..562c37b7 100644
--- a/firmware/common/si5351c.h
+++ b/firmware/common/si5351c.h
@@ -55,13 +55,20 @@ extern "C"
 #define SI5351C_CLK_IDRV_6MA 2
 #define SI5351C_CLK_IDRV_8MA 3
 
+#define SI5351C_LOS (1<<4)
+
+enum pll_sources {
+	PLL_SOURCE_XTAL = 0,
+	PLL_SOURCE_CLKIN = 1,
+};
+
 void si5351c_disable_all_outputs();
 void si5351c_disable_oeb_pin_control();
 void si5351c_power_down_all_clocks();
 void si5351c_set_crystal_configuration();
 void si5351c_enable_xo_and_ms_fanout();
-void si5351c_configure_pll_sources_for_xtal();
-void si5351c_configure_pll1_multisynth();
+void si5351c_configure_pll_sources(const enum pll_sources source);
+void si5351c_configure_pll1_multisynth(const enum pll_sources source);
 void si5351c_configure_multisynth(const uint_fast8_t ms_number,
     	const uint32_t p1, const uint32_t p2, const uint32_t p3,
     	const uint_fast8_t r_div);
@@ -72,6 +79,8 @@ void si5351c_set_int_mode(const uint_fast8_t ms_number, const uint_fast8_t on);
 void si5351c_write_single(uint8_t reg, uint8_t val);
 uint8_t si5351c_read_single(uint8_t reg);
 void si5351c_write(uint8_t* const data, const uint_fast8_t data_count);
+void si5351c_set_clock_source(const enum pll_sources source);
+void si5351c_activate_best_clock_source(void);
 
 #ifdef __cplusplus
 }
diff --git a/firmware/hackrf_usb/hackrf_usb.c b/firmware/hackrf_usb/hackrf_usb.c
index d2605f1a..3daf0b6c 100644
--- a/firmware/hackrf_usb/hackrf_usb.c
+++ b/firmware/hackrf_usb/hackrf_usb.c
@@ -43,6 +43,7 @@
 #include "rf_path.h"
 #include "sgpio_isr.h"
 #include "usb_bulk_buffer.h"
+#include "si5351c.h"
  
 static volatile transceiver_mode_t _transceiver_mode = TRANSCEIVER_MODE_OFF;
 
@@ -210,12 +211,18 @@ int main(void) {
 
 	rf_path_init();
 
+	uint16_t periodic_event_counter = 0;
+
 	unsigned int phase = 0;
 	while(true) {
 		// Check whether we need to initiate a CPLD update
 		if (start_cpld_update)
 			cpld_update();
 
+		if (++periodic_event_counter == 0) {
+			si5351c_activate_best_clock_source();
+		}
+
 		// Set up IN transfer of buffer 0.
 		if ( usb_bulk_buffer_offset >= 16384
 		     && phase == 1