1 files changed, 63 insertions, 0 deletions
diff --git a/host/docs/x400_gpio_api.dox b/host/docs/x400_gpio_api.dox
index 80c59cae7..592bff4ba 100644
--- a/host/docs/x400_gpio_api.dox
+++ b/host/docs/x400_gpio_api.dox
@@ -130,5 +130,68 @@ Valid values can be enumerated with the
 uhd::features::gpio_power_iface::supported_voltages() call, and are "1V8",
 "2V5", and "3V3".
 
+\section x4x0_spi_iface The x4x0 SPI Mode
+
+The GPIO ports of the x4x0 can be used with the Serial Peripheral Interface (SPI) to control 
+external components. To use SPI mode, set the pins you need on the desired GPIO port to be 
+controlled by the SPI engine and configure the data direction.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
+auto usrp = uhd::usrp::multi_usrp::make(args);
+auto& spi_getter_iface =
+        usrp->get_radio_control().get_feature<uhd::features::spi_getter_iface>();
+usrp->set_gpio_src("GPIO0",
+    {"DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI",
+        "DB0_SPI"});
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The example shows the usage of GPIO port 0 (GPIO0) for SPI and needs to be run for 
+GPIO1 again to use that port with SPI, too.
+
+\subsection x4x0_spi_cfg Configuration of SPI lines
+
+The x4x0 SPI mode supports up to 4 slaves. All of these slaves may have a different SPI pin 
+configuration. The pins available for the usage with SPI are listed in \ref x4x0gpio_fpanel_pins.
+For GPIO0 the available pins are enumerated from 0 through 11, for GPIO1 the available pins are 
+from 12 through 23.
+The vector of slave configurations is passed to the spi_iface_getter to get the reference:
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
+    uhd::features::spi_slave_config_t slave_cfg;
+    slave_cfg.slave_clk  = 0;
+    slave_cfg.slave_miso = 1;
+    slave_cfg.slave_mosi = 2;
+    slave_cfg.slave_ss   = 3;
+    std::vector<uhd::features::spi_slave_config_t> slave_cfgs;
+    slave_cfgs.push_back(slave_cfg);
+    auto spi_ref = spi_getter_iface.get_spi_ref(slave_cfgs);
+
+    // Set data direction register (set all to outgoing except for MISO)
+    usrp->set_gpio_attr("GPIOA", "DDR", 0xD, 0xF);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+\subsection x4x0_spi_r_w Write and read on SPI
+
+With the SPI reference read and write operations can be performed. For doing this, 
+some characteristics of the SPI need to be configured:
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~{.cpp}
+   uhd::spi_config_t config;
+   config.divider = 4;
+   config.miso_edge = config.EDGE_RISE;
+   ...
+   spi_ref->write_spi(0, config, 0xFEFE, 32);
+   uint32_t read_data = spi_ref->read_spi(0, config, 0xFEFE, 32);
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The SPI clock \f$SCLK\f$ is derived from the Radio clock and the SPI clock divider as follows: 
+
+\f[SCLK = \frac{Radio\_Clk}{SPI\_CLK\_DIV + 1}\f]
+
 */
 // vim:ft=doxygen: