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

[Susheel Nuguru]

It looks like SKYWorks have other pins CSD/CPS/CHL other than CTX and CRX that needs to be controlled for proper functionality. We provide FEM support for skyworks through simple_gpio. We control crx and ctx pins automatically.Other pins whatever they are, are to be controlled by the user application, as gpios.

[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. 

[PabloL]

Hello Susheel,

It would be a great help for developers.
Unfortunately the documentation is a bit confusing.
I'm using a Fanstel EV-BT40X board (nRF52840 + SKY66112).

I was able to make the example ble_app_uart_c work successfully with the Softdevice S140, using the control codes from Fanstel (https://www.fanstel.com/s/BT840X_SKY66112-Codes.zip), but not for Zephyr with NCS 2.5.0.

// BT840X SKY66112 FEM ble_app_uart_c example
// main.c
...
#include "pa_lna.h"

#define APP_PA_LAN

#ifdef APP_PA_LAN
#define APP_PA_PIN      17      // CTX pin
#define APP_LNA_PIN     19      // CRX pin
#define APP_CHL_PIN		8       // CHL pin (0: TX low pwer, 1: TX high power)
#define APP_CPS_PIN		6       // CPS pin (0: LNA/PA On, 1: Bypass)
#endif
...

int main(void)
{
...
#ifdef APP_PA_LAN
	nrf_gpio_cfg_output(APP_CPS_PIN);
	nrf_gpio_cfg_output(APP_CHL_PIN);
	nrf_gpio_pin_clear(APP_CPS_PIN);    // LNA/PA On
	nrf_gpio_pin_set(APP_CHL_PIN);      // TX High Power
	pa_lna_init(APP_PA_PIN,APP_LNA_PIN);    
#endif
...
}

Please let us know any news.

Thank you.

Pablo

[Susheel Nuguru]

Pablo,  I was assuming that you were using NCS as well. Is this for the nRF5SDK based solution? If it is, then I think the support of FEM in our softdevice was very limited. We know that nRF5 based SDK solution might not be working with Skyworks FEM as we only supported generic gpio based solution in that.

Regarding the NCS support, Currently released support for (simple GPIO) Skyworks FEM does not allow run-time control. Decision to use PA or bypass is taken at compile-time.