lora: Note known issue with STM32WL5 HP antenna.

For unknown reason, power output in this configuration is lower than it
should be (including when compared to the STM32Cube C libraries running on
the same board.

Suspect either the Nucleo board antenna switch or the power amplifier
registers are being set wrong, but the actual root cause remains elusive...

Signed-off-by: Angus Gratton <angus@redyak.com.au>

micropython/lora/README.md

@@ -444,9 +444,11 @@ datasheet Table 32 "Frequency Bands" for more details.
 | `"PA_BOOST"`    | RFO_HP pin (high power) |
 | Any other value | RFO_LP pin (low power)  |
 
+**NOTE**: Currently the `PA_BOOST` HP antenna output is lower than it should be
+on this board, due to an unknown driver bug.
 
-**Important**: If setting `tx_ant` value, also set `output_power` at the same
-time or again before transmitting.
+If setting `tx_ant` value, also set `output_power` at the same time or again
+before transmitting.
 
 #### `output_power` - Transmit output power level
 Type: `int`

micropython/lora/lora-stm32wl5/lora/stm32wl5.py

@@ -101,6 +101,8 @@ class _WL55SubGhzModem(sx126x._SX126x):
         # power).
 
         if tx_ant is not None:
+            # Note: currently HP antenna power output is less than it should be,
+            # due to some (unknown) bug.
             self._hp = tx_ant == "PA_BOOST"
 
         # Update the OCP register to match the maximum power level