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.

[luca-stm32]

Hi fpistm.
Today I made some investigations. In file packages/STMicroelectronics/hardware/stm32/2.11.0/libraries/SrcWrapper/src/stm32/uart.c there are some interesting lines:
Line 141:
/* Pin Tx must not be NP */
if ((uart_tx == NP) && (uart_tx_swap == NP)) {
if (obj != &serial_debug) {
core_debug("ERROR: [U(S)ART] Tx pin has no peripheral!\n");
}
return false;
}

and Line 406:
/* Configure uart */
uart_handlers[obj->index] = huart;
huart->Instance = (USART_TypeDef *)(obj->uart);
huart->Init.BaudRate = baudrate;
huart->Init.WordLength = databits;
huart->Init.StopBits = stopbits;
huart->Init.Parity = parity;
huart->Init.Mode = UART_MODE_TX_RX;
huart->Init.HwFlowCtl = flow_control;
huart->Init.OverSampling = UART_OVERSAMPLING_16;

It seems to me like U(S)ARTS could be only initialized if TX pin is defined and only in RX AND TX mode.

I tried to comment lines near 141 in order to skip TX pin definition control and modify lines near 406 as follow:

/* Configure uart */
uart_handlers[obj->index] = huart;
huart->Instance = (USART_TypeDef *)(obj->uart);
huart->Init.BaudRate = baudrate;
huart->Init.WordLength = databits;
huart->Init.StopBits = stopbits;
huart->Init.Parity = parity;
if (uart_tx == NP) {
huart->Init.Mode = UART_MODE_RX;
}
else if (uart_rx == NP) {
huart->Init.Mode = UART_MODE_TX;
}
else {
huart->Init.Mode = UART_MODE_TX_RX;
}
huart->Init.HwFlowCtl = flow_control;
huart->Init.OverSampling = UART_OVERSAMPLING_16;

I tried to initialize Serial2 forcing TX pin as NC:
Serial2.setRx(PA3);
Serial2.setTx(NC);
Serial2.begin(115200);
(by the way, I think it could be also:
// RX TX
HardwareSerial Serial2(PA3, NC); //USART2
)
but it doesn't work :-(

So my question is: why U(S)ARTs could be initialized ONLY in RX AND TX mode? Is something else to modify in uart.c file in order to be able to initialize U(S)ARTs only in RX OR TX mode?

Thanks again for your time.

Regards.
Luca

[fpistm]

Well it was implemented like this several years ago following Arduino API reference which requires 2 pins.
Anyway several users contribute to extend it: Half duplex, swap, data invert,...

Unfortunately, I don't know if the current driver is able to manage only one mode and don't have time to investigate.

[luca-stm32]

Hi fpistm.
Last night I managed to implement Hardware Serial RX only mode (using only one pin) and I think it works :-)
To make it works, I disabled uart_tx pin not NC control (lines141-147) , enabled RX pin configuration even if uart_tx pin is NP (lines 377-383) and configured UART with UART_MODE_RX if uart_tx pin is NP (lines from 413).
I successfully tried it on a my STM32F4 board.
I don't know if disabling first control (uart_tx not NC) could impact on other configurations.....

To manage RX only mode, I declared UART2 as follow:

// RX TX
HardwareSerial Serial2(PA3, NC); //USART2

or

Serial2.setRx(PA3);
Serial2.setTx(NC);
Serial2.begin(115200);

Both cases work.

In case someone need only TX mode, half duplex mode can be enabled: user must remember to set UART in TX.

Attached, I send my uart.c modified file (I'm not very keen on github...)
uart.c

I hope it could help to improve this fantastic core.

Thanks again.
Best regards.
Luca