Powering Nucleo-F042K6 from external +5V

Jerome Chapdelaine · ‎2017-09-07

Posted on September 07, 2017 at 20:28

Hi,

I am trying to power a Nucleo-F042K6 from the external +5V only, i.e. without USB connected since my code is completed and fully functionnal. According to the user manual this shoud be possible and the only limitation is that the ST-LINK must be connected after.

However the F042K6 will not start because its NRST is kept low (at arount 1.2V), probably clamped by the protection diodes of the ST-LINK which does not have any power.

I suppose I could remove SB9 but then I'm afraid of losing some ST-LINK capabilities if I want to re-flash my code.

Am I missing something in the user manual? SB9 is only mentioned if we want to power the board with 3.3V.

Thanks to all.

Jerome

Tesla DeLorean · ‎2017-09-08

Posted on September 08, 2017 at 14:01

Check ST-LINK firmware version. Report what you have.

Update to current.

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Jerome Chapdelaine · ‎2017-09-08

Posted on September 08, 2017 at 16:48

ST-LINK was V2J25M14, now updated to V2J28M18. It has no effect which was expected because the ST-LINK is not powered if +5V is injected directly on the board. My goal is to power only the

F042K6 and keep the USB + ST-LINK unplugged.

As a workaround I've injected the +5V on the VIN pin, which supplies power to the ST-LINK and allows it to release NRST. It is not perfect because Vin should be >=7V but the drops in the linear regulator are low enough to get a stable 3.3V, and I'm wasting a few milliamps on the ST-LINK... but at least it works.