IS USART PIN OF STM32F103RDT6 IS 5V COMPATIBLE?

[Mom.1]

Hi Team,

We are using STM32F103RDT6 MCU, for connecting 5V RS485 transceiver to the MCU. We used (PIN no 16,17) USART2(RX-PA3,TX-PA2)and one GPIO (PIN no 15) pins of MCU. Kindly clarify the below points

1. From the datasheet,it shows all pins are 5V tolerant. Is USART2(PA3,PA2) pins also 5V tolerant?

2. If yes how long it will sustain? Is there any constraints.

Warm regards,

Monisha OM

[berendi]

1. No. Check Table 5. High-density STM32F103xC/D/E pin definitions. 5V tolerant pins are marked as FT, PA2/PA3 aren't.
2. There are no constraints, pins that are actually 5V-tolerant can work indefinitely with 5V applied.

[Mom.1]

Thank you berendi for your response.

Now we are updated our circuit with Pin 29 (PB10-TX1) and Pin 30(PB11-RX1)is used, which is 5V tolerant pin. Please find the image of the circuit.

In the datasheet its shows "For 5V tolerant pin, In order to sustain a voltage higher than VDD+0.3 the internal pull-up/pull-down resistors must be disabled". We are using this UART pins for communicating with RS485 transceiver(5V).

so shall we considered these pins as push pull configuration?

Warm regards,

Monisha OM

[berendi]

Output pins should be either push-pull if the RS485 transceiver would recognize > 2.8 Volts as high level (check its datasheet), or open-drain with external pull-up resistor when it has a higher threshold. Inputs should have external pull-ups if necessary.

BTW, what are those short circuted resistors R48,R49 supposed to do?

[Mom.1]

Hi,

Thanks for response.

The VIH(min)of transceiver is 2. So can I proceed with those 5V tolerant UART pins?

R48 and R49 used for EMI purpose, which is DNP. If there any signal issue,we will cut the trace and mount that resistor.

warm regards,

Monisha om

[berendi]

I don't know how your manufacturing process looks like, but the usual practice is to use a 0 Ω resistor, and replace it with something else when needed, without any traces to cut.

[Mom.1]

I have one doubt ,

While programming I make USART3 pin (29,30) to either push pull or into other default state. Is this USART pin is 5V tolerant on that time, because the datasheet shows "voltage higher than VDD+0.3 the internal pull-up/pull-down resistors must be disabled". Please clarify the same?

Warm regards,

Monisha om

[berendi]

As far as I know the internal pull-up/pull-down resistors are never enabled unless you enable them explicitly in your program. Just don't enable them anywhere.

Check AN2606 though if you are using the system bootloader.