B-WL5M-SUBG1 - RF Switch

[fabltd]

Hi All

I have the B-WL5M-SUBG1 board this uses the STM32WL5MOC mcu.

The datasheet for the B-WL5M-SUBG1 board says that the RF switch uses PINS:

• SW_RF
• PC_5
• PC_3

In the datasheet for the STM32WL5MOC I cannot locate SW_RF it does not exsit. 

Also in the datasheet for the B-WL5M-SUBG1 Table 18 I/O assignment there is no reference to SW_RF. I do see SW_CTL?

PC_5 and PC_3 are not in the table. I assume these are interal. 

Help...

[STTwo-32 · Accepted solution]

Hello @fabltd 

The SW_RF is the pin SW_CTL. (Just compare the description of both pins).

The PC5 and PC3 are IOs inside the STM32WL5MOCH module and dedicated to the RF front-end switch control. 

More details available on the schemtic pack here.

Best Regards.

STTwo-32 

[fabltd]

Thanks

Can you please point to where this is referenced in the datasheet?

Does SW_CTL have a pin name in example code I have seen this referenced as PC2 ?

[Tesla DeLorean]

https://www.st.com/resource/en/schematic_pack/mb1779-high-band-b02-schematic.pdf#page=5

I'd assume it connects directly to the RF Baseband, and indicates if the High or Low Power output is selected. So look at registers in the radio to control, not the STM32 MCU GPIO pins.

[fabltd]

Thanks for the schematic. Thats been driving me crazy.

Not sure why its missing from the docs. 

[STTwo-32]

Hello @fabltd 

Everything is mentionned on the schematic pack that i've attached to may answer and @Tesla DeLorean mentionned also.

For the schematic, you can always find it on the CAD RESSOURCES tab for all ST evaluation boards on the website.

Best Regards.

STTwo-32 

[Erlkoenig]

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@Tesla DeLorean wrote:

I'd assume it connects directly to the RF Baseband, and indicates if the High or Low Power output is selected.

---

I don't think this is the case, unfortunately. The STM32CubeWL HAL driver's source code reveals the assignment of the internal pins:

SW_CTRL = PC4 = RADIO_SWITCH_CTRL1
PC5 = RADIO_SWITCH_CTRL2
PC3 = RADIO_SWITCH_CTRL3

So this means we have to set PC4 manually/via the HAL driver in order to indicate RX/LP/HP mode, exactly as mentioned in the B-WL5M-SUBG1 schematic: 0 = HP transmit, 1 = LP transmit or receive

I think we even have to do this even if no external DC switches are used, and we do not dynamically switch PA outputs, in order to control the internal RF switch correctly (which is implemented very similarly to the NUCLEO-WL55JC board, and also has SW_CTRL as an input - see UM3127 page 17).

This also means we can't use PC4/SW_CTRL as a GPIO even when no external DC switches are used, and PC3/PC5 aren't available at all. This also means we have no direct external indication that the SubGHz radio is enabled at all (to control an antenna tuning switch or similar), in contrast to using the MCU directly without the module, where we can use the FE_CTRLx or perhaps PB0=TCXO power signals (as on the NUCLEO-WL55JC) for such a purpose.

I also think that PC3 is used to power the internal RF switch, similarly to FE_CTRL3 on the NUCLEO-WL55JC. This means setting PC3=0 will make sure the switch doesn't consume any power when SubGHz radio isn't used. However note that the truth table for FE_CTRL1-3 and PC3-5 differs.