BT840X Low Transmit Power with Skyworks FEM

[edmcnicholas]

Hello,

I have read through many threads and am trying to understand how the Skyworks fem module works as there is not a good example.  I will start by saying that I am seeing more power with the FEM but it is now what was expected. This is all being used for BLE.

Currently I have 2 boards and several different advertising sets and one has the module with the skyworks FEM.  When I set both to 8db using set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_ADV, 2, 8); I am not seeing the same rssi.  The unit with the FEM is about 6-10 DB low so I am going to say something is being left on the table for link budget.  I would expect to see the same RSSI when using the same antenna and not moving it.  I can see the the spike on the power profiler is about the same as well which is why it is a bit confusing.

Device Tree:

    nrf_radio_fem: skyFem {
        compatible = "skyworks,sky66112-11", "generic-fem-two-ctrl-pins";
        ctx-gpios = <&gpio0 17 GPIO_ACTIVE_HIGH>;
        crx-gpios = <&gpio0 19 GPIO_ACTIVE_HIGH>;
        cps-gpios = <&gpio0 6 GPIO_ACTIVE_HIGH>;
        chl-gpios = <&gpio0 8 GPIO_ACTIVE_HIGH>;
        };

&radio {
    fem = <&nrf_radio_fem>;
    };

Init code

gpio_pin_configure_dt(&CHL, GPIO_OUTPUT | GPIO_OUTPUT_INIT_HIGH);
		gpio_pin_configure_dt(&CPS, GPIO_OUTPUT | GPIO_OUTPUT_INIT_LOW);

		// mpsl_fem_gain_t fem = {.gain_db=22};
		// int ret = mpsl_fem_pa_gain_set(&fem);
		// if(ret != 0)
		// {
		// 	LOG_ERR("FEM");
		// }

		tx_power = 22;
		set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_ADV, 0, tx_power);
		set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_ADV, 1, tx_power);
		set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_ADV, 2, 8);
		set_tx_power(BT_HCI_VS_LL_HANDLE_TYPE_ADV, 3, 8);

Config

CONFIG_MPSL=y
CONFIG_MPSL_FEM=y
CONFIG_MPSL_FEM_SIMPLE_GPIO=y
CONFIG_BT_CTLR_TX_PWR_ANTENNA=22

[edmcnicholas]

Yes that is what I explained to the other engineer but i think the gain setup is incorrect by default

[Susheel Nuguru]

If the other pins are not controlled by the gpio and are set to default float then the behavior of the default value SKYWorks FEM is unpredictable. 

[edmcnicholas]

If you review the post you will see that they are set in software.  What I am stating is that your gain settings for the driver are incorrect.  I expect a setting of 8db to have the same rssi with clear line of sight with or without a FEM.

[PabloL]

Hi, 

Same for me. I could not get it to work.

 If you see the nrf52840dk_nrf52840.dts, in the included file nrf52840dk_nrf52840-pinctrl.dtsi

uart0_default: uart0_default {
    group1 {
            psels = <NRF_PSEL(UART_TX, 0, 6)>,
            <NRF_PSEL(UART_RTS, 0, 5)>;
        };
    group2 {
            psels = <NRF_PSEL(UART_RX, 0, 8)>,
            <NRF_PSEL(UART_CTS, 0, 7)>;
            bias-pull-up;
        };
};

uart0_sleep: uart0_sleep {
    group1 {
            psels = <NRF_PSEL(UART_TX, 0, 6)>,
                    <NRF_PSEL(UART_RX, 0, 8)>,
                    <NRF_PSEL(UART_RTS, 0, 5)>,
                    <NRF_PSEL(UART_CTS, 0, 7)>;
            low-power-enable;
    };
};

The uart0 uses pins P0.6 and P0.8 as TX and RX.
Does the new definitions in the device tree override the pin functions?

[Susheel Nuguru]

OK, it looks like something is clearly wrong then if it is more than one who is seeing the same thing. I will try to find Skyworkorks FEM Module or order it to test this myself.