Add signal source mode to hackrf_transfer

Now hackrf_transfer can act as an analog signal source to generate a
pure SINE wave at the given frequency.

host/hackrf-tools/src/hackrf_transfer.c

@@ -171,7 +171,9 @@ t_wav_file_hdr wave_file_hdr =
 typedef enum {
 	TRANSCEIVER_MODE_OFF = 0,
 	TRANSCEIVER_MODE_RX = 1,
-	TRANSCEIVER_MODE_TX = 2
+	TRANSCEIVER_MODE_TX = 2,
+	TRANSCEIVER_MODE_SS = 3
+
 } transceiver_mode_t;
 static transceiver_mode_t transceiver_mode = TRANSCEIVER_MODE_RX;
 
@@ -297,6 +299,9 @@ volatile bool do_exit = false;
 FILE* fd = NULL;
 volatile uint32_t byte_count = 0;
 
+bool signalsource = false;
+uint32_t power_level = 1;
+
 bool receive = false;
 bool receive_wav = false;
 
@@ -367,6 +372,7 @@ int rx_callback(hackrf_transfer* transfer) {
 
 int tx_callback(hackrf_transfer* transfer) {
 	size_t bytes_to_read;
+	int i;
 
 	if( fd != NULL )
 	{
@@ -390,8 +396,27 @@ int tx_callback(hackrf_transfer* transfer) {
 		} else {
 			return 0;
 		}
-	} else {
+	} else if (transceiver_mode == TRANSCEIVER_MODE_SS) {
+		/* Transmit SINE signal with specific powerlevel */
+		ssize_t bytes_read;
+		byte_count += transfer->valid_length;
+		bytes_to_read = transfer->valid_length;
+		if (limit_num_samples) {
+			if (bytes_to_read >= bytes_to_xfer) {
+				bytes_to_read = bytes_to_xfer;
+			}
+			bytes_to_xfer -= bytes_to_read;
+		}
+
+		for(i = 0;i<bytes_to_read;i++)
+			transfer->buffer[i] = power_level;
+
+		if (limit_num_samples && (bytes_to_xfer == 0)) {
 			return -1;
+		} else {
+			return 0;
+		}
+
 	}
 }
 
@@ -419,6 +444,7 @@ static void usage() {
 	printf("\t[-s sample_rate_hz] # Sample rate in Hz (8/10/12.5/16/20MHz, default %sMHz).\n",
 		u64toa((DEFAULT_SAMPLE_RATE_HZ/FREQ_ONE_MHZ),&ascii_u64_data1));
 	printf("\t[-n num_samples] # Number of samples to transfer (default is unlimited).\n");
+	printf("\t[-c power_level] # Signal source mode, power level, 0-255, dc value to DAC (default is 1).\n");
 	printf("\t[-b baseband_filter_bw_hz] # Set baseband filter bandwidth in MHz.\n\tPossible values: 1.75/2.5/3.5/5/5.5/6/7/8/9/10/12/14/15/20/24/28MHz, default < sample_rate_hz.\n" );
 }
 
@@ -460,7 +486,7 @@ int main(int argc, char** argv) {
 	float time_diff;
 	unsigned int lna_gain=8, vga_gain=20, txvga_gain=0;
   
-	while( (opt = getopt(argc, argv, "wr:t:f:i:o:m:a:p:s:n:b:l:g:x:")) != EOF )
+	while( (opt = getopt(argc, argv, "wr:t:f:i:o:m:a:p:s:n:b:l:g:x:z:")) != EOF )
 	{
 		result = HACKRF_SUCCESS;
 		switch( opt ) 
@@ -537,6 +563,12 @@ int main(int argc, char** argv) {
 			result = parse_u32(optarg, &baseband_filter_bw_hz);
 			break;
 
+		case 'c':
+			transmit = true;
+			signalsource = true;
+			result = parse_u32(optarg, &power_level);
+			break;
+
 		default:
 			printf("unknown argument '-%c %s'\n", opt, optarg);
 			usage();
@@ -707,6 +739,15 @@ int main(int argc, char** argv) {
 		transceiver_mode = TRANSCEIVER_MODE_TX;
 	}
 
+	if (signalsource) {
+		transceiver_mode = TRANSCEIVER_MODE_SS;
+		if (power_level >255) {
+			printf("argument error: power level shall be in between 0 and 255.\n");
+			usage();
+			return EXIT_FAILURE;
+		}
+	}
+
 	if( receive_wav )
 	{
 		time (&rawtime);
@@ -719,11 +760,14 @@ int main(int argc, char** argv) {
 		printf("Receive wav file: %s\n", path);
 	}	
 
+	// In signal source mode, the PATH argument is neglected.
+	if (transceiver_mode != TRANSCEIVER_MODE_SS) {
 		if( path == NULL ) {
 			printf("specify a path to a file to transmit/receive\n");
 			usage();
 			return EXIT_FAILURE;
 		}
+	}
 
 	result = hackrf_init();
 	if( result != HACKRF_SUCCESS ) {
@@ -739,6 +783,7 @@ int main(int argc, char** argv) {
 		return EXIT_FAILURE;
 	}
 	
+	if (transceiver_mode != TRANSCEIVER_MODE_SS) {
 		if( transceiver_mode == TRANSCEIVER_MODE_RX )
 		{
 			fd = fopen(path, "wb");
@@ -757,6 +802,7 @@ int main(int argc, char** argv) {
 			usage();
 			return EXIT_FAILURE;
 		}
+	}
 
 	/* Write Wav header */
 	if( receive_wav )