Max output power in BLE direct test mode

[Rantanplan]

Hello,

we are using the direct test mode example of NCS with an nRF5340.

Now we got a question from the testlab, about the transmit power parameter.

The setting in the GUI is in dBm and goes up to 8 dBm, but the nRF5340 has 3 dBm max. TX power.

Now, just to cross-check, my question is what is the correct setting to achieve the max. TX power? Everything larger or equal 3 dBm or something else?

Many thanks for your answers in advance.

Kind regards,

Christian

[JONATHAN LL]

Hi, 

Max TX power is +3dBm so set the UI to +3 dBm, adjusting the slider higher should still leave the TX power at largest possible value, but I have not tested this so can not confirm. 

What you can do is read the TXPOWER register to see what value it has. 

Regards,
Jonathan

[Rantanplan]

Hi,

sorry for the stupid question, but how to access the TXPOWER and VREQCTRL correctly from the application or network core of the nRF5340?

Many thanks and kind regards,

Christian

[Rantanplan]

Hi,
I can confirm that higher TXPOWER values than +3 dBm in the GUI are simply ignored (the old setting persists).

To get the highest output power with the nRF5340, + 3dBm must be selected, which results in:

TXPOWER = 0x0 and

VREQCTRL = 0x1.

Just to explain:
VREQCTRL = 0x1 enables +3 dBm output

TXPOWER is typically interpreted as negative value.

So, e.g. -2 dBm can be achieved with

VREQCTRL = 0x1 (3 dBm)

and

TXPOWER = 0xFF (-1)

Kind regards,

Christian

 PS: Unfortunately, the ignoring of unsupported high power values is not reflected in the GUI. The GUI shows a bar with 8 dBm for the nRF5340 if the slider is set to 8 dBm. A error message would be highly appreciated here.

[JONATHAN LL]

You can use the following nrfutil command:


nrfutil device x-read --address <address> --direct --core Network

Base address network RADIO
0x41008000
TXPOWER
0x50C

-----------------------

Base address network VREQCTRL
0x41004000
VREGRADIO.VREQH
0x500
VREGRADIO.VREQHREADY
0x508


nrfutil device x-read --address <address> --direct --core Network

So example: 

C:\>nrfutil device x-read --address 0x4100850C --direct --core Network
1050053641
0x4100850C: 00000000


So did test and if you set the TX power to more then 3 dBm in the DTM app then it actually sets the power to -8dBm. So dont do that. 

Regards,
Jonathan