STM32F723 USART TX and RTS drive capability

Bags · ‎2026-02-25

Couldn't see a definitive answer in the datasheet - What is the source \ sink capability of a USART TX \ RTS pin?

The nearest answer I could find was page 128 of DS13313 Rev 5 -

"GPIOs (general purpose input/outputs) can sink or source up to ±8 mA, and sink or source up to ±20 mA"

but there isn't anything specifically that mentions alternate function pin capability.

Can TX and RTS pins directly source or sink an LED via a 220 Ohm resistor (that would be ~15mA each) ?

I'm thinking I may have to add external drives like high side MOSFETs otherwise.

Thanks.

TDK · ‎2026-02-25

AF functionality uses the same GPIO circuitry to source/sink current. The drive strength is the same as what you've found in the datasheet.

> Can TX and RTS pins directly source or sink an LED via a 220 Ohm resistor (that would be ~15mA each) ?

Yes, provided the sum of all sources/sink remains within limits. Something like 200 mA.

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View solution in original post

TDK · ‎2026-02-25

AF functionality uses the same GPIO circuitry to source/sink current. The drive strength is the same as what you've found in the datasheet.

> Can TX and RTS pins directly source or sink an LED via a 220 Ohm resistor (that would be ~15mA each) ?

Yes, provided the sum of all sources/sink remains within limits. Something like 200 mA.

If you feel a post has answered your question, please click "Accept as Solution".

Bags · ‎2026-02-26

Excellent.

Was running out of PCB space so thats good news. 6 LEDs makes 180mA, so just got under that too :o)

Andrew Neil · ‎2026-02-26

Basically the same question (and answer) here: STM32L Series LPUART Voltage Level.

As @TDK said, it's the GPIO specifications - not specific to the AF in use.

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.

TDK · ‎2026-02-26

Why do you keep marking your own replies as the solution?

If you feel a post has answered your question, please click "Accept as Solution".

Bags · ‎2026-02-26

Why does your platform allow marking of own replies to solutions?

If its a platform bug you should raise an engineering change request to fix it.

Chris21 · ‎2026-02-26

"6 LEDs makes 180mA, so just got under that too"

Did you though? Limit for F723 looks to be 120 mA.

Also note: "This current consumption must be correctly distributed over all I/Os and control pins. The total output current must not be sunk/sourced between two consecutive power supply pins referring to high pin count LQFP packages."

TDK · ‎2026-02-26

It's not a bug. If your own answer was the solution, you should mark it as such. But that's not what is happening.

If you feel a post has answered your question, please click "Accept as Solution".

Andrew Neil · ‎2026-02-26

@Bags wrote:
Why does your platform allow marking of own replies

because sometimes you do answer your own question!

But the posts you've marked are not the answers to the questions - they're just your comments on the answer.

See here for how to change the marked post.

@Bags wrote:
If its a platform bug

No, it's not a bug.

PS:

Here is an example where I did answer my own question.

A complex system that works is invariably found to have evolved from a simple system that worked.
A complex system designed from scratch never works and cannot be patched up to make it work.

Chris21 · ‎2026-02-26

Although if LEDs are 15 mA each 6 * 15 = 90 mA...