diff --git a/firmware/common/hackrf_core.c b/firmware/common/hackrf_core.c
index 1c05e221..d1e4d71d 100644
--- a/firmware/common/hackrf_core.c
+++ b/firmware/common/hackrf_core.c
@@ -498,8 +498,6 @@ void cpu_clock_init(void)
 
 	si5351c_set_clock_source(&clock_gen, PLL_SOURCE_XTAL);
 	// soft reset
-	// uint8_t resetdata[] = { 177, 0xac };
-	// si5351c_write(&clock_gen, resetdata, sizeof(resetdata));
 	si5351c_reset_pll(&clock_gen);
 	si5351c_enable_clock_outputs(&clock_gen);
 
diff --git a/firmware/common/si5351c.c b/firmware/common/si5351c.c
index 3bdefcda..a217d6d5 100644
--- a/firmware/common/si5351c.c
+++ b/firmware/common/si5351c.c
@@ -197,12 +197,27 @@ void si5351c_configure_clock_control(si5351c_driver_t* const drv, const enum pll
 	si5351c_write(drv, data, sizeof(data));
 }
 
+#define SI5351C_CLK_ENABLE(x)	(0<<x)
+#define SI5351C_CLK_DISABLE(x)	(1<<x)
+#define SI5351C_REG_OUTPUT_EN	(3)
+#define SI5351C_REG_CLK3_CTRL	(19)
+
 void si5351c_enable_clock_outputs(si5351c_driver_t* const drv)
 {
 	/* Enable CLK outputs 0, 1, 2, 4, 5 only. */
 	/* 7: Clock to CPU is deactivated as it is not used and creates noise */
 	/* 3: External clock output is deactivated by default */
-	uint8_t data[] = { 3, ~((1 << 0) | (1 << 1) | (1 << 2) | (1 << 4) | (1 << 5))};
+	// uint8_t data[] = { 3, ~((1 << 0) | (1 << 1) | (1 << 2) | (1 << 4) | (1 << 5))};
+	uint8_t data[] = { SI5351C_REG_OUTPUT_EN,
+		SI5351C_CLK_ENABLE(0) |
+		SI5351C_CLK_ENABLE(1) |
+		SI5351C_CLK_ENABLE(2) |
+		SI5351C_CLK_DISABLE(3) |
+		SI5351C_CLK_ENABLE(4) |
+		SI5351C_CLK_ENABLE(5) |
+		SI5351C_CLK_DISABLE(6) |
+		SI5351C_CLK_DISABLE(7)
+	};
 	si5351c_write(drv, data, sizeof(data));
 }
 
@@ -244,3 +259,43 @@ void si5351c_activate_best_clock_source(si5351c_driver_t* const drv)
 		}
 	}
 }
+
+void si5351c_clkout_enable(si5351c_driver_t* const drv, uint8_t enable)
+{
+	/* Set optput in output enable register */
+	uint8_t output_enable = si5351c_read_single(drv, 3);
+	output_enable = output_enable & !SI5351C_CLK_DISABLE(3);
+	if(enable)
+		output_enable = output_enable | SI5351C_CLK_ENABLE(3);
+	else
+		output_enable = output_enable | SI5351C_CLK_DISABLE(3);
+	uint8_t oe_data[] = {SI5351C_REG_OUTPUT_EN, output_enable};
+	si5351c_write(drv, oe_data, 2);
+
+	/* Configure clock to 10MHz (TODO customisable?) */
+	si5351c_configure_multisynth(drv, 3, 80*128-512, 0, 1, 0);
+
+	/* Set power up/doen in CLK3 control register*/
+	uint8_t pll;
+	#ifdef RAD1O
+		/* PLLA on XTAL */
+		pll = SI5351C_CLK_PLL_SRC_A;
+	#endif
+	
+	#if (defined JAWBREAKER || defined HACKRF_ONE)
+		if (active_clock_source == PLL_SOURCE_CLKIN) {
+			/* PLLB on CLKIN */
+			pll = SI5351C_CLK_PLL_SRC_B;
+		} else {
+			/* PLLA on XTAL */
+			pll = SI5351C_CLK_PLL_SRC_A;
+		}
+	#endif
+	uint8_t clk3_ctrl;
+	if(enable)
+		clk3_ctrl = SI5351C_CLK_INT_MODE | SI5351C_CLK_PLL_SRC(pll) | SI5351C_CLK_SRC(SI5351C_CLK_SRC_MULTISYNTH_SELF) | SI5351C_CLK_IDRV(SI5351C_CLK_IDRV_8MA);
+	else
+		clk3_ctrl = SI5351C_CLK_POWERDOWN | SI5351C_CLK_INT_MODE;
+	uint8_t clk3_data[] = {SI5351C_REG_CLK3_CTRL, clk3_ctrl};
+	si5351c_write(drv, clk3_data, 2);
+}
diff --git a/firmware/common/si5351c.h b/firmware/common/si5351c.h
index d10a0a93..533c5830 100644
--- a/firmware/common/si5351c.h
+++ b/firmware/common/si5351c.h
@@ -89,6 +89,7 @@ void si5351c_activate_best_clock_source(si5351c_driver_t* const drv);
 void si5351c_write_single(si5351c_driver_t* const drv, uint8_t reg, uint8_t val);
 uint8_t si5351c_read_single(si5351c_driver_t* const drv, uint8_t reg);
 void si5351c_write(si5351c_driver_t* const drv, const uint8_t* const data, const size_t data_count);
+void si5351c_clkout_enable(si5351c_driver_t* const drv, uint8_t enable);
 
 #ifdef __cplusplus
 }
diff --git a/firmware/hackrf_usb/hackrf_usb.c b/firmware/hackrf_usb/hackrf_usb.c
index 5d7467e4..d99b3529 100644
--- a/firmware/hackrf_usb/hackrf_usb.c
+++ b/firmware/hackrf_usb/hackrf_usb.c
@@ -89,7 +89,8 @@ static const usb_request_handler_fn vendor_request_handler[] = {
 	usb_vendor_request_operacake_set_ports,
 	usb_vendor_request_set_hw_sync_mode,
 	usb_vendor_request_reset,
-	usb_vendor_request_operacake_set_ranges
+	usb_vendor_request_operacake_set_ranges,
+	usb_vendor_request_set_clkout_enable
 };
 
 static const uint32_t vendor_request_handler_count =
diff --git a/firmware/hackrf_usb/usb_api_register.c b/firmware/hackrf_usb/usb_api_register.c
index 0fd8bfc2..eaeadece 100644
--- a/firmware/hackrf_usb/usb_api_register.c
+++ b/firmware/hackrf_usb/usb_api_register.c
@@ -149,3 +149,14 @@ usb_request_status_t usb_vendor_request_read_rffc5071(
 	}
 }
 #endif
+
+usb_request_status_t usb_vendor_request_set_clkout_enable(
+	usb_endpoint_t* const endpoint,
+	const usb_transfer_stage_t stage
+) {
+	if (stage == USB_TRANSFER_STAGE_SETUP) {
+		si5351c_clkout_enable(&clock_gen, endpoint->setup.value);
+		usb_transfer_schedule_ack(endpoint->in);
+	}
+	return USB_REQUEST_STATUS_OK;
+}
diff --git a/firmware/hackrf_usb/usb_api_register.h b/firmware/hackrf_usb/usb_api_register.h
index 06eb8df7..9fef6eed 100644
--- a/firmware/hackrf_usb/usb_api_register.h
+++ b/firmware/hackrf_usb/usb_api_register.h
@@ -50,5 +50,9 @@ usb_request_status_t usb_vendor_request_read_rffc5071(
 	usb_endpoint_t* const endpoint,
 	const usb_transfer_stage_t stage
 );
+usb_request_status_t usb_vendor_request_set_clkout_enable(
+	usb_endpoint_t* const endpoint,
+	const usb_transfer_stage_t stage
+);
 
 #endif /* end of include guard: __USB_API_REGISTER_H__ */
diff --git a/host/CMakeLists.txt b/host/CMakeLists.txt
index 34136f09..03e31ee7 100644
--- a/host/CMakeLists.txt
+++ b/host/CMakeLists.txt
@@ -3,6 +3,8 @@
 cmake_minimum_required(VERSION 2.8)
 project (HackRF)
 
+set(CMAKE_C_FLAGS "$ENV{CFLAGS}" CACHE STRING "C Flags")
+
 add_subdirectory(libhackrf)
 add_subdirectory(hackrf-tools)
 
diff --git a/host/hackrf-tools/src/CMakeLists.txt b/host/hackrf-tools/src/CMakeLists.txt
index ea396455..40f11dc0 100644
--- a/host/hackrf-tools/src/CMakeLists.txt
+++ b/host/hackrf-tools/src/CMakeLists.txt
@@ -31,6 +31,7 @@ SET(TOOLS
 	hackrf_cpldjtag
 	hackrf_info
 	hackrf_debug
+	hackrf_clock
 	hackrf_sweep
 	hackrf_operacake
 )
diff --git a/host/hackrf-tools/src/hackrf_clock.c b/host/hackrf-tools/src/hackrf_clock.c
new file mode 100644
index 00000000..bd3344dd
--- /dev/null
+++ b/host/hackrf-tools/src/hackrf_clock.c
@@ -0,0 +1,337 @@
+/*
+ * Copyright 2017 Dominic Spill <dominicgs@gmail.com>
+ *
+ * This file is part of HackRF.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2, or (at your option)
+ * any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include <hackrf.h>
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <getopt.h>
+
+#ifndef bool
+typedef int bool;
+#define true 1
+#define false 0
+#endif
+
+#define CLOCK_UNDEFINED 0xFF
+#define REGISTER_INVALID 32767
+
+int parse_int(char* s, uint16_t* const value) {
+	uint_fast8_t base = 10;
+	char* s_end;
+	long long_value;
+
+	if( strlen(s) > 2 ) {
+		if( s[0] == '0' ) {
+			if( (s[1] == 'x') || (s[1] == 'X') ) {
+				base = 16;
+				s += 2;
+			} else if( (s[1] == 'b') || (s[1] == 'B') ) {
+				base = 2;
+				s += 2;
+			}
+		}
+	}
+
+	s_end = s;
+	long_value = strtol(s, &s_end, base);
+	if( (s != s_end) && (*s_end == 0) ) {
+		*value = (uint16_t)long_value;
+		return HACKRF_SUCCESS;
+	} else {
+		return HACKRF_ERROR_INVALID_PARAM;
+	}
+}
+
+int si5351c_read_register(hackrf_device* device, const uint16_t register_number) {
+	uint16_t register_value;
+	int result = hackrf_si5351c_read(device, register_number, &register_value);
+	
+	if( result == HACKRF_SUCCESS ) {
+		printf("[%3d] -> 0x%02x\n", register_number, register_value);
+	} else {
+		printf("hackrf_si5351c_read() failed: %s (%d)\n", hackrf_error_name(result), result);
+	}
+	
+	return result;
+}
+
+int si5351c_write_register(
+	hackrf_device* device,
+	const uint16_t register_number,
+	const uint16_t register_value
+) {
+	int result = HACKRF_SUCCESS;
+	result = hackrf_si5351c_write(device, register_number, register_value);
+	
+	if( result == HACKRF_SUCCESS ) {
+		printf("0x%2x -> [%3d]\n", register_value, register_number);
+	} else {
+		printf("hackrf_max2837_write() failed: %s (%d)\n", hackrf_error_name(result), result);
+	}
+	
+	return result;
+}
+
+#define SI5351C_CLK_POWERDOWN (1<<7)
+#define SI5351C_CLK_INT_MODE  (1<<6)
+#define SI5351C_CLK_PLL_SRC   (1<<5)
+#define SI5351C_CLK_INV       (1<<4)
+#define SI5351C_CLK_SRC_XTAL            0
+#define SI5351C_CLK_SRC_CLKIN           1
+#define SI5351C_CLK_SRC_MULTISYNTH_0_4  2
+#define SI5351C_CLK_SRC_MULTISYNTH_SELF 3
+
+void print_clk_control(uint8_t clk_ctrl) {
+	uint8_t clk_src, clk_pwr;
+	printf("\tclock control = ");
+	if(clk_ctrl & SI5351C_CLK_POWERDOWN)
+		printf("Down, ");
+	else
+		printf("Up, ");
+	if(clk_ctrl & SI5351C_CLK_INT_MODE)
+		printf("Int Mode, ");
+	else
+		printf("Frac Mode, ");
+	if(clk_ctrl & SI5351C_CLK_PLL_SRC)
+		printf("PLL src B, ");
+	else
+		printf("PLL src A, ");
+	if(clk_ctrl & SI5351C_CLK_INV)
+		printf("Inverted, ");
+	clk_src = (clk_ctrl >> 2) & 0x3;
+	switch (clk_src) {
+		case 0:
+			printf("XTAL, ");
+			break;
+		case 1:
+			printf("CLKIN, ");
+			break;
+		case 2:
+			printf("MULTISYNTH 0 4, ");
+			break;
+		case 3:
+			printf("MULTISYNTH SELF, ");
+			break;
+	}
+	clk_pwr = clk_ctrl & 0x3;
+	switch (clk_pwr) {
+		case 0:
+			printf("2 mA\n");
+			break;
+		case 1:
+			printf("4 mA\n");
+			break;
+		case 2:
+			printf("6 mA\n");
+			break;
+		case 3:
+			printf("8 mA\n");
+			break;
+	}
+}
+
+int si5351c_read_multisynth_config(hackrf_device* device, const uint_fast8_t ms_number) {
+	uint_fast8_t i, reg_base, reg_number;
+	uint16_t parameters[8], clk_control;
+	uint32_t p1,p2,p3,r_div;
+	uint_fast8_t div_lut[] = {1,2,4,8,16,32,64,128};
+	int result;
+
+	printf("MS%d:\n", ms_number);
+	result = hackrf_si5351c_read(device, 16+ms_number, &clk_control);
+	if( result != HACKRF_SUCCESS ) {
+		return result;
+	}
+	print_clk_control(clk_control);
+	if(ms_number <6){
+		reg_base = 42 + (ms_number * 8);
+		for(i=0; i<8; i++) {
+			reg_number = reg_base + i;
+			result = hackrf_si5351c_read(device, reg_number, &parameters[i]);
+			if( result != HACKRF_SUCCESS ) {
+				return result;
+			}
+		}
+
+		p1 = ((parameters[2] & 0x03) << 16)
+			| (parameters[3] << 8)
+			| parameters[4];
+		p2 = ((parameters[5] & 0x0F) << 16)
+			| (parameters[6] << 8)
+			| parameters[7];
+		p3 = ((parameters[5] & 0xF0) << 12)
+			| (parameters[0] << 8)
+			|  parameters[1];
+		r_div = (parameters[2] >> 4) & 0x7;
+
+		printf("\tp1 = %u\n", p1);
+		printf("\tp2 = %u\n", p2);
+		printf("\tp3 = %u\n", p3);
+		if(p3)
+			printf("\tOutput (800Mhz PLL): %#.10f Mhz\n", ((double)800 / (double)(((double)p1*p3 + p2 + 512*p3)/(double)(128*p3))) / div_lut[r_div] );
+	} else {
+		// MS6 and 7 are integer only
+		unsigned int parms;
+		reg_base = 90;
+
+		for(i=0; i<3; i++) {
+			uint_fast8_t reg_number = reg_base + i;
+			int result = hackrf_si5351c_read(device, reg_number, &parameters[i]);
+			if( result != HACKRF_SUCCESS ) {
+				return result;
+			}
+		}
+		r_div = (ms_number == 6) ? parameters[2] & 0x7 : (parameters[2] & 0x70) >> 4 ;
+		parms = (ms_number == 6) ? parameters[0] : parameters[1];
+		printf("\tp1_int = %u\n", parms);
+		if(parms)
+			printf("\tOutput (800Mhz PLL): %#.10f Mhz\n", (800.0f / parms) / div_lut[r_div] );
+	}
+	printf("\toutput divider = %u\n", div_lut[r_div]);
+	return HACKRF_SUCCESS;
+}
+
+int si5351c_read_configuration(hackrf_device* device) {
+	uint_fast8_t ms_number;
+	int result;
+			
+	for(ms_number=0; ms_number<8; ms_number++) {
+		result = si5351c_read_multisynth_config(device, ms_number);
+		if( result != HACKRF_SUCCESS ) {
+			return result;
+		}
+	}
+	return HACKRF_SUCCESS;
+}
+
+static void usage() {
+	printf("hackrf_clock - HackRF clock configuration utility\n");
+	printf("Usage:\n");
+	printf("\t-h, --help: this help\n");
+	printf("\t-r, --read <clock_num>: read settings for clock_num\n");
+	printf("\t-a, --all: read settings for all clocks\n");
+	printf("\t-o, --clkout <clkout_enable>: enable/disable CLKOUT\n");
+	printf("\t-d, --device <serial_number>: Serial number of desired HackRF.\n");
+	printf("\nExamples:\n");
+	printf("\thackrf_clock -r 3 : prints settings for CLKOUT\n");
+}
+
+static struct option long_options[] = {
+	{ "help", no_argument, 0, 'h' },
+	{ "read", required_argument, 0, 'r' },
+	{ "all", no_argument, 0, 'a' },
+	{ "clkout", required_argument, 0, 'o' },
+	{ "device", required_argument, 0, 'd' },
+	{ 0, 0, 0, 0 },
+};
+
+int main(int argc, char** argv) {
+	hackrf_device* device = NULL;
+	int opt, option_index = 0;
+	bool read = false;
+	uint16_t clock = CLOCK_UNDEFINED;
+	bool clkout = false;
+	uint16_t clkout_enable;
+	const char* serial_number = NULL;
+
+	int result = hackrf_init();
+	if(result) {
+		printf("hackrf_init() failed: %s (%d)\n", hackrf_error_name(result), result);
+		return EXIT_FAILURE;
+	}
+
+	while( (opt = getopt_long(argc, argv, "r:ao:d:h?", long_options, &option_index)) != EOF ) {
+		switch( opt ) {
+		case 'r':
+			read = true;
+			result = parse_int(optarg, &clock);
+			break;
+
+		case 'a':
+			read = true;
+			break;
+
+		case 'o':
+			clkout = true;
+			result = parse_int(optarg, &clkout_enable);
+			break;
+
+		case 'd':
+			serial_number = optarg;
+			break;
+
+		case 'h':
+		case '?':
+			usage();
+			return EXIT_SUCCESS;
+		default:
+			fprintf(stderr, "unknown argument '-%c %s'\n", opt, optarg);
+			usage();
+			return EXIT_FAILURE;
+		}
+
+		if(result != HACKRF_SUCCESS) {
+			printf("argument error: %s (%d)\n", hackrf_error_name(result), result);
+			usage();
+			return EXIT_FAILURE;
+		}
+	}
+
+	if(!clkout && !read) {
+		fprintf(stderr, "Either read or enable CLKOUT option must be specified.\n");
+		usage();
+		return EXIT_FAILURE;
+	}
+
+	result = hackrf_open_by_serial(serial_number, &device);
+	if(result) {
+		printf("hackrf_open() failed: %s (%d)\n", hackrf_error_name(result), result);
+		return EXIT_FAILURE;
+	}
+
+	if(clkout) {
+		result = hackrf_set_clkout_enable(device, clkout_enable);
+		if(result) {
+			printf("hackrf_set_clkout_enable() failed: %s (%d)\n", hackrf_error_name(result), result);
+			return EXIT_FAILURE;
+		}
+	}
+
+	if(read) {
+		if(clock == CLOCK_UNDEFINED)
+			si5351c_read_configuration(device);
+		else {
+			printf("%d\n", clock);
+			si5351c_read_multisynth_config(device, clock);
+		}
+	}
+
+	result = hackrf_close(device);
+	if(result) {
+		printf("hackrf_close() failed: %s (%d)\n", hackrf_error_name(result), result);
+		return EXIT_FAILURE;
+	}
+
+	hackrf_exit();
+    return EXIT_SUCCESS;
+}
diff --git a/host/libhackrf/src/hackrf.c b/host/libhackrf/src/hackrf.c
index 4fb28406..e0a9f527 100644
--- a/host/libhackrf/src/hackrf.c
+++ b/host/libhackrf/src/hackrf.c
@@ -79,6 +79,7 @@ typedef enum {
 	HACKRF_VENDOR_REQUEST_SET_HW_SYNC_MODE = 29,
 	HACKRF_VENDOR_REQUEST_RESET = 30,
 	HACKRF_VENDOR_REQUEST_OPERACAKE_SET_RANGES = 31,
+	HACKRF_VENDOR_REQUEST_CLKOUT_ENABLE = 32,
 } hackrf_vendor_request;
 
 #define USB_CONFIG_STANDARD 0x1
@@ -1991,6 +1992,31 @@ int ADDCALL hackrf_set_operacake_ranges(hackrf_device* device, uint8_t* ranges,
 		return HACKRF_SUCCESS;
 	}
 }
+
+int ADDCALL hackrf_set_clkout_enable(hackrf_device* device, const uint8_t value)
+{
+	USB_API_REQUIRED(device, 0x0103)
+	int result;
+	result = libusb_control_transfer(
+		device->usb_device,
+		LIBUSB_ENDPOINT_OUT | LIBUSB_REQUEST_TYPE_VENDOR | LIBUSB_RECIPIENT_DEVICE,
+		HACKRF_VENDOR_REQUEST_CLKOUT_ENABLE,
+		value,
+		0,
+		NULL,
+		0,
+		0
+	);
+
+	if (result != 0)
+	{
+		last_libusb_error = result;
+		return HACKRF_ERROR_LIBUSB;
+	} else {
+		return HACKRF_SUCCESS;
+	}
+}
+
 #ifdef __cplusplus
 } // __cplusplus defined.
 #endif
diff --git a/host/libhackrf/src/hackrf.h b/host/libhackrf/src/hackrf.h
index c2b28206..2e335c01 100644
--- a/host/libhackrf/src/hackrf.h
+++ b/host/libhackrf/src/hackrf.h
@@ -246,6 +246,8 @@ extern ADDAPI int ADDCALL hackrf_set_operacake_ranges(hackrf_device* device,
                                                       uint8_t* ranges,
                                                       uint8_t num_ranges);
 
+extern ADDAPI int ADDCALL hackrf_set_clkout_enable(hackrf_device* device, const uint8_t value);
+
 #ifdef __cplusplus
 } // __cplusplus defined.
 #endif