S32K328 Interfacing with Full duplex UART RS485

[midhunac]

We are interfacing the S32K328 controller with four RS485 interfaces. According to the datasheet, the driver and receiver need to be enabled using the dedicated RTS and CTS pins of the controller. However, we’ve observed that only LPUART0, LPUART1, and LPUART2 have these dedicated RTS/CTS pins available.

Given this limitation, we would like to know:

1. Is it mandatory to use the dedicated RTS/CTS pins for enabling the RS485 driver?
2. Can we use general-purpose GPIO pins instead to control the driver enable (DE) and receiver enable (RE) signals?
3. Are there any alternative solutions or recommended approaches for interfacing multiple RS485 ports with the S32K328?

Kindly advise.

[PavelL]

Hello @midhunac ,

Please find my answers below:

1. Is it mandatory to use the dedicated RTS/CTS pins for enabling the RS485 driver?
No, it is not mandatory to use the dedicated RTS/CTS pins for RS485 driver enable (DE) or receiver enable (RE) control. The RTS/CTS functionality is primarily intended for hardware flow control in UART communication, but for RS485, the DE/RE signals can be controlled independently.

2. Can we use general-purpose GPIO pins instead to control the driver enable (DE) and receiver enable (RE) signals?
Yes, you can use any available general-purpose GPIO pin to control the DE and/or RE signals of your RS485 transceivers. In many RS485 transceiver designs, the DE (Driver Enable) and /RE (Receiver Enable, active low) signals are often tied together and controlled by a single GPIO pin, simplifying the control logic. However, this depends on the specific RS485 transceiver you are using—please refer to its datasheet for recommended connection schemes.
You will need to ensure that your application firmware sets or clears the GPIO at the correct time (typically before and after UART transmission).

3. Are there any alternative solutions or recommended approaches for interfacing multiple RS485 ports with the S32K328?

• For up to three UARTs, you may use the dedicated RTS pins if you prefer hardware-controlled enable.
• For additional UARTs, or if you require more flexibility, use GPIOs for DE/RE control as described above.
• Ensure that your software manages the timing of the DE/RE signals to avoid bus contention (i.e., enable the driver only during transmission and disable it immediately after).
• If your application is latency-sensitive, consider using UART transmit/receive interrupts or DMA to synchronize DE/RE control with data flow.

Best regards,

Pavel