nRF21540 implementation questions

[Angelo]

In the nRF21540 datasheet I see various extra communication pins but we are tight on pins and space I'm trying to determine the minimum number of connections required to make the FEM work.  For example if I connect the TX_EN/RX_EN pins, do I still need to use SPI?  Do I need to use the UICR mode?  Also, I'm not clear how the TX and RX paths are enabled on the BLE chip side.  Is that done programmatically by our application firmware, or is it handled somewhere at a lower level in the BLE stack?  I.e. how do we link the TX/RX enable pins to BLE communications?

I was also trying to find a datasheet or schematic for the nRF21540 DK board but wasn't able. Is there one somewhere?

Thanks

[Andreas]

Hi,

 

You do not need to use SPI, but you will then not be able to tune the TX gain, but if you do not want/need to do this you can use the TXEN/RXEN pins.

 

You would need your stack to control the pins as you say to have it toggle between TX and RX. The NCS BLE controller must be configured to do this. Some pins can also be tied high or low if a static state is wanted, saving pins. This is also the case for TXEN or RXEN if you want the FEM to only stay in TX or RX for your application.

 

Best regards,

Andreas

[Angelo]

Thanks Andreas.  If I'm only using one antenna, can I just leave ANT_SEL pulled low, and ANT2 unterminated?

[Andreas]

Hi,

 

Yes that is fine.

 

Best regards,

Andreas

[Angelo]

I noticed that some pins on the nRF52840 are recommended for SPI, but for optimal layout I would like to use different digital pins (particularly P0.06, P0.08, P1.08, P1.09).  Are there any concerns doing this?

[Andreas]

Hi,

 

The 'recommendation' is only if the pin is configured as high-drive and there is high-frequency toggling during radio operation, on some pins this may affect some radio performance parametres like e.g. RX sensitivity. Apart from this we recommend using the pins that are optimal for your layout, and when using the TXEN/RXEN control this will not be a concern, so using the pins you mention is perfectly fine. 

 

Best regards,

Andreas