Setting TX power with BT840XE

RE: Setting TX power with BT840XE

Hung Bui — Tue, 20 Jan 2026 09:09:17 GMT

Hi Gus,
I don't think we have sample code for the specific module. But from what you showed it should work. Have you tried to measure the output power ?
Please try for example CONFIG_BT_CTLR_TX_PWR_ANTENNA=20 and measure the output power.

RE: Setting TX power with BT840XE

GusRS — Mon, 19 Jan 2026 16:07:46 GMT

Is there some kind of sample code for specifically using fem and bt840xe for testing?

RE: Setting TX power with BT840XE

Hung Bui — Mon, 19 Jan 2026 09:55:50 GMT

Hi Gus,

I will double check internally, but as far as I know CONFIG_BT_CTLR_TX_PWR_PLUS_3 will override CONFIG_BT_CTLR_TX_PWR_ANTENNA when you don't use FEM.
You can take a look at \nrf\subsys\bluetooth\controller\radio_nrf5_txp.h file, quoted here:

#if defined(CONFIG_BT_CTLR_TX_PWR_0) && CONFIG_BT_CTLR_TX_PWR_ANTENNA == 0
#define RADIO_TXP_DEFAULT  0
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_10)
#define RADIO_TXP_DEFAULT  10
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_8)
#define RADIO_TXP_DEFAULT  8
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_7)
#define RADIO_TXP_DEFAULT  7
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_6)
#define RADIO_TXP_DEFAULT  6
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_5)
#define RADIO_TXP_DEFAULT  5
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_4)
#define RADIO_TXP_DEFAULT  4
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_3)
#define RADIO_TXP_DEFAULT  3
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_2)
#define RADIO_TXP_DEFAULT  2
#elif defined(CONFIG_BT_CTLR_TX_PWR_PLUS_1)
#define RADIO_TXP_DEFAULT  1
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_1)
#define RADIO_TXP_DEFAULT -1
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_2)
#define RADIO_TXP_DEFAULT -2
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_3)
#define RADIO_TXP_DEFAULT -3
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_4)
#define RADIO_TXP_DEFAULT -4
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_5)
#define RADIO_TXP_DEFAULT -5
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_6)
#define RADIO_TXP_DEFAULT -6
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_7)
#define RADIO_TXP_DEFAULT -7
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_8)
#define RADIO_TXP_DEFAULT -8
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_9)
#define RADIO_TXP_DEFAULT -9
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_10)
#define RADIO_TXP_DEFAULT -10
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_12)
#define RADIO_TXP_DEFAULT -12
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_14)
#define RADIO_TXP_DEFAULT -14
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_16)
#define RADIO_TXP_DEFAULT -16
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_20)
#define RADIO_TXP_DEFAULT -20
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_26)
#define RADIO_TXP_DEFAULT -26
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_30)
#define RADIO_TXP_DEFAULT -30
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_40)
#define RADIO_TXP_DEFAULT -40
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_46)
#define RADIO_TXP_DEFAULT -46
#elif defined(CONFIG_BT_CTLR_TX_PWR_MINUS_70)
#define RADIO_TXP_DEFAULT -70
#else
#define RADIO_TXP_DEFAULT CONFIG_BT_CTLR_TX_PWR_ANTENNA
#endif

RE: Setting TX power with BT840XE

GusRS — Fri, 16 Jan 2026 13:11:52 GMT

Hello, thank you for your answer.

Forgetting about the fem for a second, let's assume it's a normal nrf52840.So,if I don't configure CONFIG_BT_CTLR_TX_PWR_ANTENNA, it gets 0 as default value. So, even if I use CONFIG_BT_CTLR_TX_PWR_PLUS_3 or another, it will overwrite it, and become dbm 0?

RE: Setting TX power with BT840XE

Hung Bui — Fri, 16 Jan 2026 11:42:24 GMT

Hi Gus,

[quote user=""]and somewhere in my runtime, I need to use the following lines to enable the pins for transmit high power, since the driver only automatically enables the rx and tx

const struct device* gpio0_dev = device_get_binding("gpio@50000000"); // gpio 0

gpio_pin_configure(gpio0_dev, 6, GPIO_OUTPUT); // gpio 0.06 - CPS

gpio_pin_set(gpio0_dev, 6, 0);

gpio_pin_configure(gpio0_dev, 8, GPIO_OUTPUT); // gpio 0.08 - CHL

gpio_pin_set(gpio0_dev, 8, 1);

[/quote]

Yes correct.

[quote user=""]CONFIG_BT_CTLR_TX_PWR_* that define which kind of dbm you want to use, positive,negative, and how much

CONFIG_BT_CTLR_TX_PWR_ANTENNA that defines how much power you want at the antenna and lets the software set the values for the nrf and fem based on that

CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL that allows dynamic control of the tx power.

I want to use the +21bdm the bt840xe offers. I set tx-gain to 18, rx-gain to 11, following the specification, set the high tx mode using CPS and CHL, set

CONFIG_BT_CTLR_TX_PWR_PLUS_3 and did not set CONFIG_BT_CTLR_TX_PWR_ANTENNA or CONFIG_BT_CTLR_TX_PWR_DYNAMIC_CONTROL.

Since tx-gain and rx-gain don't actually control the gain,instead are used to inform the software regarding the gains from fem so it can adjust itself,and I want to divide between the nrf52840 and fem, since I read this is the way to do it to avoid overheating of the nrf52840,I did this way.

I wanted to confirm if what I did is correct

[/quote]

My suggestion is only use CONFIG_BT_CTLR_TX_PWR_ANTENNA, by using that it will always try to use the lowest chip's TXPower combine with the TXgain of the FEM to achieve the desired CONFIG_BT_CTLR_TX_PWR_ANTENNA. So if you want +3 dBm on the chip you set tx-gain-db = CONFIG_BT_CTLR_TX_PWR_ANTENNA - 3

Even if you don't configure CONFIG_BT_CTLR_TX_PWR_ANTENNA, it will get the value of 0 as the default.
See this ticket: nRF21540-DK how to get good results?