AnsweredAssumed Answered

swd programming and 3.3v power ???

Question asked by apuf.mac on Jan 26, 2013
Latest reply on Oct 4, 2013 by Clive One
Hi all,
I've had great difficulties setting up my own STM32F405 board and having it actually run, but finally I've succeeded booting it and programming it from my stm32f4Discovery.

However I have a very strange behaviour :
If I power my board using USB+LM1117 3.3v regulator (seems mostly clean/accurate  on scope), if I try to program it with STLinkv2 interface (from discovery board) the connexion fails. a scope on swd clock seems to indicate that the clock starts to oscillate and then stops.
It won't program the chip but once programmed, the program will run on that power source with the right clocks, VBats & all.

If I power my board using batteries (3x1.2v before regulator) and connect it to st-link from discovery, it fails, then if I try again I can program it. Then everything runs OK.

If I leave the probe on the Vcc pin of swd, it seems that power goes from 3.3v to 3.6v when powered from usb, and from 3 to 3.3 when powered from batteries : what gives ? Strangest thing, if I leave the probe, the device seems to run OK and program OK even with USB ! If I remove the (passive, tektro, 1Mohm) probe, I cannot program until I go to battery power.

I recall having see a thread (by bil.til?) where the conclusion of swd link ability to communicate  was to step down the power of the chip from 3.3v to 3v but I can't find it (bad forum software).

Can anyone enlighten me ?

Besides I have a somewhat related question. If you pull up Boot0 to 3.3v, does the chip consider that VPP is pulled high (albeit not at 9v) - because it's the same pin ? does the fact that VPP must only be applied for 1 hour of the life of the chip means that Boot0 must be pulled up for 1 hour for the life of the chip ?