STM32F723 USART TX and RTS drive capability

[Bags]

 

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]

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.

[Bags]

Excellent.

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

[Andrew Neil]

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.

[TDK]

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

[Bags]

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.

[TDK]

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.

 

[Chris21]

"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."

[Chris21]

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

 

[Bags]

Sorry, 180mA was my estimated max for the entire PCB with a little bit of added margin, but I can crank down the 6 LEDs a bit (they are Optos) before edges become an issue.

I forgot to check the total current sunk for the alternative H723 MCU - luckily its 20mA higher than a F723 and not less.

I'll breadboard an Opto to a UART TX\RTS to verify that area before I finalize the PCB.

[Hoorain753]

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[gbm]

Do you realize that any modern LED supplied with 15 mA current would make you room very bright? For modern LEDs used as status lights, the typical reasonable current is 0.5..3 mA. In most of my boards I use 1k5 or 2k2 series resistors for driving LEDs.

[Bags]

Optos don't light up rooms unless you made a serious wiring mistake.

15mA is the upper limit but I probably can crank it down to 10mA without impacting max baud rate...