Enabling GPIO+SPI mode support for nRF21540

The nRF21540 device is a range extender that you can use with nRF52 and nRF53 Series devices. The nRF21540 features an SPI interface. You can use it to fully control your front-end module or you can use a combination of SPI and GPIO interface. The SPI interface enables you, for example, to set the output power of the nRF21540.

The nRF Connect SDK provides code that configures FEM based on devicetree (DTS) and Kconfig information using the Multiprotocol Service Layer (MPSL) library. The FEM hardware description in the application’s devicetree file is an essential part of the configuration. To enable FEM support, an nrf_radio_fem node must be present in the application’s devicetree file. The node can be provided by the devicetree file of the target board, by an overlay file, or through the SHIELD CMake variable (see Providing CMake options). See Devicetree Guide for more information about the DTS data structure, and Devicetree versus Kconfig for information about differences between DTS and Kconfig.

Note

See also Optional FEM properties for nRF21540 GPIO and GPIO+SPI when enabling support for nRF21540.

To use nRF21540 in GPIO+SPI mode, complete the following steps:

1. Add the following node in the devicetree file:

   / {
         nrf_radio_fem: name_of_fem_node {
            compatible  = "nordic,nrf21540-fem";
            tx-en-gpios = <&gpio0 13 GPIO_ACTIVE_HIGH>;
            rx-en-gpios = <&gpio0 14 GPIO_ACTIVE_HIGH>;
            pdn-gpios   = <&gpio0 15 GPIO_ACTIVE_HIGH>;
            spi-if = <&nrf_radio_fem_spi>
      };
   };
   

2. Optionally replace the device name name_of_fem_node.

3. Replace the pin numbers provided for each of the required properties:

   • tx-en-gpios - GPIO characteristic of the device that controls the TX_EN signal of nRF21540.

   • rx-en-gpios - GPIO characteristic of the device that controls the RX_EN signal of nRF21540.

   • pdn-gpios - GPIO characteristic of the device that controls the PDN signal of nRF21540.

   These properties correspond to TX_EN, RX_EN, and PDN pins of nRF21540 that are supported by software FEM.

   The``phandle-array`` type is commonly used for describing GPIO signals in Zephyr’s devicetree. The first element &gpio0 refers to the GPIO port (“port 0” has been selected in the example shown). The second element is the pin number on that port. The last element must be GPIO_ACTIVE_HIGH for nRF21540, but for a different FEM module you can use GPIO_ACTIVE_LOW.

   Set the state of the remaining control pins according to the nRF21540 Product Specification.

4. Add a following SPI bus device node on the devicetree file:

   &pinctrl {
      spi3_default_alt: spi3_default_alt {
         group1 {
            psels = <NRF_PSEL(SPI_SCK, 1, 15)>,
                    <NRF_PSEL(SPI_MISO, 1, 14)>,
                    <NRF_PSEL(SPI_MOSI, 1, 13)>;
         };
      };
   
      spi3_sleep_alt: spi3_sleep_alt {
         group1 {
            psels = <NRF_PSEL(SPI_SCK, 1, 15)>,
                    <NRF_PSEL(SPI_MISO, 1, 14)>,
                    <NRF_PSEL(SPI_MOSI, 1, 13)>;
            low-power-enable;
         };
      };
   };
   
   fem_spi: &spi3 {
      status = "okay";
      pinctrl-0 = <&spi3_default_alt>;
      pinctrl-1 = <&spi3_sleep_alt>;
      pinctrl-names = "default", "sleep";
      cs-gpios = <&gpio0 21 GPIO_ACTIVE_LOW>;
   
      nrf_radio_fem_spi: nrf21540_fem_spi@0 {
         compatible = "nordic,nrf21540-fem-spi";
         status = "okay";
         reg = <0>;
         spi-max-frequency = <8000000>;
      };
   };
   

   In this example, the nRF21540 is controlled by the spi3 bus. Replace the SPI bus according to your hardware design, and create alternative entries for SPI3 with different pinctrl-N and pinctrl-names properties.

5. On nRF53 devices, the devicetree nodes described above must be added to the network core. For the application core, you must also add a GPIO forwarder node to its devicetree file:

   &gpio_fwd {
      nrf21540-gpio-if {
         gpios = <&gpio0 11 0>,   /* tx-en-gpios */
                 <&gpio0 12 0>,   /* rx-en-gpios */
                 <&gpio0 13 0>,   /* pdn-gpios */
                 <&gpio0 14 0>,   /* ant-sel-gpios */
                 <&gpio0 15 0>;   /* mode-gpios */
      };
      nrf21540-spi-if {
         gpios = <&gpio0 21 0>,   /* cs-gpios */
                 <&gpio1 15 0>,   /* SPIM_SCK */
                 <&gpio1 14 0>,   /* SPIM_MISO */
                 <&gpio1 13 0>;   /* SPIM_MOSI */
      };
   };
   

   The pins defined in the GPIO forwarder node in the application core’s devicetree file must match the pins defined in the FEM nodes in the network core’s devicetree file.

6. On nRF53 devices, SPIM0 and UARTE0 are mutually exclusive AHB bus masters on the network core as described in the Product Specification, Section 6.4.3.1, Table 22. As a result, they cannot be used simultaneously. For the SPI part of the nRF21540 interface to be functional, you must disable the UARTE0 node in the network core’s devicetree file.

   &uart0 {
      status = "disabled";
   };
   

Note

The nRF21540 GPIO-only mode of operation is selected by default in Kconfig when an nRF21540 node is provided in devicetree, as mentioned in the Enabling FEM support section. To enable the GPIO+SPI mode of operation, you must explicitly set the CONFIG_MPSL_FEM_NRF21540_GPIO_SPI Kconfig option to y.