lib: Use GPIO P0 to toggle an (optional) bias-t

At least one distributor of rtl-sdr dongles (rtl-sdr.com) added
a bias-t to their dongles which could be toggled via GPIO P0 of the
RTL2832U chip.

source: http://www.rtl-sdr.com/rtl-sdr-blog-v-3-dongles-user-guide/

Signed-off-by: Steve Markgraf <steve@steve-m.de>

include/rtl-sdr.h

@@ -380,6 +380,15 @@ RTLSDR_API int rtlsdr_read_async(rtlsdr_dev_t *dev,
  */
 RTLSDR_API int rtlsdr_cancel_async(rtlsdr_dev_t *dev);
 
+/*!
+ * Enable or disable the bias tee on GPIO PIN 0.
+ *
+ * \param dev the device handle given by rtlsdr_open()
+ * \param on  1 for Bias T on. 0 for Bias T off.
+ * \return -1 if device is not initialized. 0 otherwise.
+ */
+RTLSDR_API int rtlsdr_set_bias_tee(rtlsdr_dev_t *dev, int on);
+
 #ifdef __cplusplus
 }
 #endif

src/librtlsdr.c

@@ -1937,3 +1937,14 @@ int rtlsdr_i2c_read_fn(void *dev, uint8_t addr, uint8_t *buf, int len)
 
 	return -1;
 }
+
+int rtlsdr_set_bias_tee(rtlsdr_dev_t *dev, int on)
+{
+	if (!dev)
+		return -1;
+
+	rtlsdr_set_gpio_output(dev, 0);
+	rtlsdr_set_gpio_bit(dev, 0, on);
+
+	return 0;
+}