E220-400M22S (LLCC68) RXEN TXEN

[thanosbi]

E220-400M22S has RXEN TXEN these pins. Can someone explain what is their role?
Ebyte info is very little, says for RXEN: "RF switch receive control pin, connect external MCU IO, high level effective"
and "RF switch emission control pin, connected to external MCU IO or dio2, high level effective" for the TXEN.

[HeadBoffin]

Some similar discussion here: #1265

Found with a search of just RXEN with many other hits, so there may be some other info to be looked at.

[S5NC]

Explanation highly adapted from from https://github.com/meshtastic/firmware/blob/master/variants/EBYTE_ESP32-S3/variant.h (I wrote the text about the options in that file)

The module connects the exposed TXEN and RXEN connections to an RF switch which connects the antenna to the transmission output of the SX1262 (the LoRa IC in the module) or the receiving input.

Some modules also have Low Noise Amplifiers (LNA) and Power Amplifiers (PA) but this module doesn't have this.

In your case, the antenna can only be connected to the transmission output of the SX1262 or its receiving input. Maybe the RF switch disconnects the antenna completely when RXEN and TXEN are the same, but I don't know this as I don't have the datasheet for the RF switch used.

The SX126x's function "DIO2 as RF Switch" sets DIO2 to high when it is about to transmit. You can configure the delay between setting DIO2 high and starting the transmission.
If you don't use this function, DIO2 can be used as an interrupt pin (in addition to DIO1), however one interrupt pin is generally enough.

Your module's datasheet says when transmitting, have TXEN HIGH and RXEN LOW. When receiving, have TXEN LOW and RXEN HIGH.

You can do this multiple ways:

1. (Commonly used, used to have more manual control over the switching process or to maintain access to DIO2 to use as an additional interrupt pin) Connect TXEN to an MCU pin. Connect RXEN to an MCU pin. Let RadioLib control TXEN and RXEN manually.
2. (Commonly used to save 2 MCU pins when DIO2 is not needed as an interrupt) Connect DIO2 to TXEN. Invert DIO2 using hardware and connect that inverted DIO2 signal to RXEN. Use the function of the SX1262 called DIO2 as RF Switch which you can access via RadioLib.
3. (Commonly used, used to save one MCU pin when DIO2 is not needed as an interrupt) Connect DIO2 to TXEN. Use DIO2 as RF Switch which you can access via RadioLib. Connect RXEN to an MCU pin. Let RadioLib control RXEN manually.
4. (Crazy but you could do it if you really wanted to...) Connect TXEN to an MCU pin. Let RadioLib control TXEN manually. Invert DIO2 using hardware and connect that inverted DIO2 signal to RXEN. Use DIO2 as RF Switch which you can access via RadioLib.

Use option 1 if you can use 2 MCU pins for RF switching.
Use option 2 if you can use 1 MCU pin for RF switching and don't need DIO2 as an interrupt.
Use option 3 if you can use no MCU pins for RF switching and can add a hardware logical inversion and don't need DIO2 as an interrupt.
Use option 4 if you can use 1 MCU pin for RF switching and can add a hardware logical inversion and don't need DIO2 as an interrupt. You should never have to use this option, because you could always use option 2 instead and avoid adding extra hardware.

For your module you don't need this, but for information: If you ever need TXEN and RXEN to be both off (or both on) always use option 1. For example, for modules with PAs and LNAs you might want to be able to do this (when you are neither TXing or RXing to save power if the PA is enabled by TXEN and the LNA is enabled by TXEN).

Personally for your module I would use option 1 for the most flexibility, option 2 if I am slightly short on MCU pins, and option 3 if I am even more short on MCU pins are can add a hardware inverter, or if I am designing the PCB and want to be elegant or be sure I avoid undefined behaviour cases with an RF switch controlled by TXEN and RXEN

[thanosbi]

I think you are absolutely right! Already tried "option 1" seems to work ok (almost for a week now) need to check it at long distance.
The on board RFswitch(?) is marked 404 2220 in sot23-6 package searched but cant find anything. Generally these modules have weird behavior. Their chips supposed to be llcc68 but they are reporting as sx1261 https://github.com/jgromes/RadioLib/issues/1329