Hardware Serial only in RX (or TX)

[luca-stm32]

I would like to ask if it could be possible to implement hardware serial peripheral only in RX (or TX) mode.
I searched in Wiki and I found that I could use hardware serial declaring RX and TX pins (both) or in halfduplex mode declaring only TX pin.
I thinked a declaration like this:

HardwareSerial Serial1(PA10, NC);
for only RX mode or:

HardwareSerial Serial1(NC, PA9);
for TX mode.

This could be very useful expecially when you need only RX (for example GPS serial data) using a micro with a little number of pins.
I prefer not to use Softwareserial if micro has hardware serial lines available.
I know that I can use Hardware Serial declaring RX and TX pins withiut conneting TX (or RX) pins, but I think it is a "waste" or resources and sometimes it is not possible expecially if you are using a small micro with a little number of pins.

Thanks for your great work!
Regards.
Luca

[fpistm]

Hi @luca-stm32
which target you used?
You should be able to use Half-Duplex and pin swap together like this only one pin are required.

[luca-stm32]

Hi @fpistm.
I'm using an STM32F437 micro on a custom board.
I'm using PA3 as USART2 RX; PA2 (UASRT2 TX) is used as ANALOG IN.

So, if I understood well, you suggested me to configure USART2 in Half Duplex mode (for this configuration, I need to configure USART2 TX pin PA2 in declaration) and then swap RX to TX pin in order to use PA3, correct?
How can I perfome TX and RX pin swap? Is there an Hardwareserial function to do this? I haven't found it.

I was wondering what happen to PA2 (USART2 TX PIN) when I declare Serial2 in Half Duplex mode as it is connected to an op-amp output: is PA2 in high impedance mode after Serial2 initialization?

Thanks.
Best regards.
Luca

[fpistm]

In Half duplex mode only one pin is initialized,
Unfortunately, F4 does'nt support SWP feature. ( #2601).
So don't you can't do half duplex using RX pin.

[luca-stm32]

Ok, thanks fpistm.
What do you think about my proposal about declare Hardware serial only in RX (or TX) mode?
Reading micro datasheets, it seems to be possible.
I'm not very keen in "low level" programming, and I think I can't be able to make a pull request :-(

[fpistm]

I don't think it is possible.
You can try to init UART then change the Tx pin to use it for an other task. I guess it could works.
while you only use Serial senc command (write, print, println, printf,..)

[luca-stm32]

Excuse me fpistm, I know that my request is out of Arduino API concern, the request could interest few people and unfortunately I can't be of any help for it, and please don't think I have any respect of your hard (really very hard!) work, but I rather think it could be very difficult to implement (and expecially time consuming).
Infact, for example, looking in Drivers/STM32F1xx_HAL_Driver/Inc/stm32f1xx_hal_usart.h I found:

#define USART_MODE_RX ((uint32_t)USART_CR1_RE)
#define USART_MODE_TX ((uint32_t)USART_CR1_TE)
#define USART_MODE_TX_RX ((uint32_t)(USART_CR1_TE | USART_CR1_RE))

This declaration make me think that technically RX (or TX) only mode could be implemented.
And believe me, I know that time is always missing ingredient.

But maybe I missed a detail that makes this truly impossible to implement.

[fpistm]

No worry. Don't know if it is the same for F4. Anyway you can try easily the test I advised before.