Swan MCU disabling 3v3 pin

Hardware
1

Hey, I have a question about the Swan. I am working on a project where the 3v3 pin on the Swan will power an accessory while the MCU runs on some batteries. Is there any way I can control the 3v3 pin through code? I only want it to be on (powering the external accessory) while the MCU is “awake” and then turn off once the Swan goes into low-power mode. Any help would be appreciated!

1 Like
2

Hi @akadloo - welcome to the Blues community!

The 3V3 pin on the Swan is always on when power is applied, so it can’t be controlled through software. The best approach is to use a GPIO pin instead. That way, you have full control over when the accessory is powered, and you can even drive multiple accessories with different GPIOs if needed.

Here’s a quick example using the Arduino framework that shows how to shut off the GPIO when entering deep sleep:

#include <Arduino.h>
#include "STM32LowPower.h"

#define SWAN_BAUD_RATE                115200

// App starts here
void setup()
{
    // Initial setup
    Serial.begin(SWAN_BAUD_RATE);
    delay(5000); // Wait for 5 seconds to see the serial output
    Serial.println("Low power deep sleep example");

    // initialize A0 as an output.
    pinMode(A0, OUTPUT);
}

// App loops here forever
void loop()
{

    // Wait for 10 seconds before going to sleep
    delay(10000);

    // Set A0 to LOW
    digitalWrite(A0, LOW);

    // Sleep forever
    LowPower.deepSleep();
}

Let me know if this answers your question or if we can help with anything else.

Thanks
Youssif

1 Like
3

Thank you for the help. One clarifying question: would the GPIO pin alone be able to power the accessory? I am using a c1001 radar, and it operates at 3.3 V. Are you saying to use the GPIO to control a power switch or as the VIN for the radar?

Thanks.

4

Rather than low power mode, between radar sessions, an option is to use the “Sleepy Swan” mechanics to completely unpower the swan (and the radar).

{
“req”: “card.attn”,
“mode”: “sleep”,
“seconds”: 3600
}

The only major difference that STM low power sleep and “sleepy swan” via card.attn is mantain state info. But maintaining state using this method is also possible.

5

Hey @akadloo – I wanted to make a correction to my earlier post. After reviewing the Swan schematic more closely and checking a few previous discussions, it turns out you can control the 3V3 OUT rail by disabling the onboard regulator. Programmatically, you can do this using:

digitalWrite(ENABLE_3V3, LOW);
digitalWrite(DISCHARGE_3V3, ENABLE_DISCHARGING);

This effectively turns off the 3.3V output, which sounds like exactly what you’re looking for. That said, I’d still recommend testing this in your setup to ensure your accessory behaves as expected and that low-power mode functions properly alongside this control.

To follow up on your latest question: I checked the C1001 radar datasheet and saw that it can draw up to 100mA. Since the Swan’s GPIO pins (from the STM32L4 series) are only rated to safely source up to ~20mA, they aren’t suitable for powering the radar directly, and doing so would be unreliable and can cause undefined behaviour. If you ever need to control power to an accessory independently of the Swan’s 3V3 rail, the recommended approach is to use a GPIO to control a transistor or logic-level MOSFET. When the GPIO (e.g. A0) is set HIGH, the transistor acts like a switch to allow power through; when LOW, it cuts it off. This gives you safe and flexible control, which is especially useful when powering multiple sensors. But in your case, if this is the only accessory being powered and you’re okay controlling it via the 3V3 output rail, the approach above using ENABLE_3V3 should work nicely.

Here are a couple of reference links that may help:

Let me know how it goes or if you’d like help with anything else.

Thanks
Youssif