kywwilson11 opened a new pull request, #17416:
URL: https://github.com/apache/nuttx/pull/17416
## Summary
This change adds full RS-485 support to the STM32H5 serial driver, including
runtime configuration via `TIOCSRS485`/`TIOCGRS485`, proper DIR pin handling,
and optional unconfiguration of TX/RX/DIR pins on device close.
Upstream STM32H5 did not yet support the Linux-style `serial_rs485`
interface,
and RS-485 direction control was limited to static polarity and setup during
initialization. My patches introduce:
- A new `rs485_flags` field in the driver, compatible with `struct
serial_rs485`
- Dynamic DIR-pin switching during TX based on `SER_RS485_RTS_ON_SEND` /
`SER_RS485_RTS_AFTER_SEND`
- Runtime enable/disable of RS-485 mode through ioctl
- Optional per-USART *unconfiguration* of RX, TX, and DIR GPIOs on close via
new
Kconfig options (e.g., `CONFIG_USART2_UNCONFIG_DIR_ON_CLOSE`)
These updates bring the STM32H5 serial driver to feature parity with other
STM32 families and finally allow proper, hardware-controlled RS-485
operation.
## Impact
This change is fully backward-compatible:
- Existing UART/USART users are unaffected unless RS-485 mode is explicitly
enabled.
There are no changes to the build system, documentation requirements, or
security model. RS-485 support remains optional and off unless configured.
## Testing
I developed and validated the changes on:
- **Board:** Nucleo-H563ZI
- **Host:** Ubuntu 24.04
- **Toolchain:** `arm-none-eabi-gcc`
`gcc version 13.3.1 20240614 (Arm GNU Toolchain 13.3.Rel1 (Build
arm-13.24))`
- **Transceiver:** TI SN65HVD1781A connected to USART2 (PD5/PD6 + PD4 DIR)
- **PC connection:** USB-to-RS485 adapter
- **Board modifications:**
- Added DIR pin definition to `board.h` (PD4)
- Enabled RS-485 on USART2 in defconfig
- Used a small test application (`rs485test` inside the provided ZIP)
### Test Procedure
1. Built NuttX with the updated serial driver and board configuration.
2. Flashed onto Nucleo-H563ZI.
3. Ran the RS-485 test application (`rs485test`) which:
- Opens `/dev/ttyS1` (USART2)
- Sets RS-485 mode via `TIOCSRS485`
- Reads back mode via `TIOCGRS485`
- Performs multiple TX cycles and listens for host replies
4. Connected bidirectional link to a USB-RS485 dongle on the host PC and
monitored incoming messages.
### Observed Results
**NuttX console output:**
=== RS-485 Test Application ===
Opening device: /dev/ttyS1
Setting RS-485 mode via TIOCSRS485...
Reading back config via TIOCGRS485...
Returned flags: 0x00000003
Starting TX loop...
Wrote 35 bytes itr 1
Wrote 35 bytes itr 2
Wrote 35 bytes itr 3
Received 12 bytes: ... [Sending to n]
Wrote 35 bytes itr 4
Received 5 bytes: ... [ucleo]
Wrote 35 bytes itr 5
Wrote 35 bytes itr 6
Received 11 bytes: ... [Hello there]
Wrote 35 bytes itr 7
Received 1 bytes: 0x43 [C]
Quit command received. Closing port and exiting.
**PC-side serial output:**
Hello from RS-485 test app! Msg 1
Hello from RS-485 test app! Msg 2
Hello from RS-485 test app! Msg 3
Hello from RS-485 test app! Msg 4
Hello from RS-485 test app! Msg 5
Hello from RS-485 test app! Msg 6
Hello from RS-485 test app! Msg 7
### Validation Summary
- DIR pin polarity matched `TIOCSRS485` settings.
- RX-during-TX behaved correctly when enabled.
- GPIO unconfiguration on close worked as expected.
- No regressions observed in non-RS-485 serial operation.
These tests confirm that the new RS-485 implementation is functional,
stable, and integrates cleanly with the STM32H5 UART subsystem.
[rs485test.zip](https://github.com/user-attachments/files/23856726/rs485test.zip)
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