stm32l4xx standby exit pullup/pulldown

[Geoffrey1]

Posted on October 06, 2017 at 01:45

The stm32l4 family had the very nice feature of programmable pull-up/pull-down on the various GPIO pins to be applied when in shutdown or standby.  In standby mode, these configurations are not lost at wakeup (at least as I read the reference manual).  Is it the case that these pull-ups/pull-downs continue to be applied until the pins are reconfigured ?   Is there a 'moment' when the pins float ?   The reference manual is far from clear on this point, but it makes a difference whether I need an external pull-down.

Geoffrey

[Tilen MAJERLE]

Posted on October 06, 2017 at 09:07

Dear

Brown.Geoffrey

‌,

quoting AN4621, chapter 2.3.1:

Pull-up and pull-down can individually be applied on each I/O during the Standby mode, allowing external devices configuration to be kept.

After you exit standby mode, device reset occurs and in this case, values of pins are set to default state, meaning you don't have undefined behavior, because you can check default pin states in reference manual for specific device. There are different configurations for different pins at startup.

Best regards,

Tilen

[Geoffrey1]

Posted on October 06, 2017 at 13:15

That's what I was worried about.  What's the point of a pull-up or pull-down that floats the pin on exit from standby ?  Another feature that is so irritatingly close to being useful.

Geoffrey

[waclawek.jan]

Posted on October 06, 2017 at 13:35

1. Wouldn't other low-power mode suit?

2.With a bit of work (placing GPIO init directly into startup code), you only need to bridge the time until the processor wakes up, which according to DS is around 20-40us - unless there's any significant leakage, couldn't  the parasitics hold the required level long enough?

JW

[Geoffrey1]

Posted on October 06, 2017 at 13:42

1) No.  I can't afford the 30x power.

– 8 nA Shutdown mode

- 28 nA Standby mode

– 1.0 µA Stop 2 mode

 (the truth is somewhat greater under reasonable operating assumptions). 

2) It's possible, but it's equally possible that this leads to intermittent and undiagnosable problems.  Definitely not consistent with safe design.

[waclawek.jan]

Posted on October 06, 2017 at 14:01

1) can't afford the 30x power

just askin'

2) It's possible, but it's equally possible that this leads to intermittent and undiagnosable problems.  Definitely not consistent with safe design.

This *can* be engineered - at the end of the day, the world is analog. Surely, any backlash hits you not me... 😉

JW