Detect USB at power-up (not necessarily connected)? (H7 Plus)

OpenMV Products
1

At power-on, I would like to detect whether the OpenMV H7 Plus has been plugged in to a computer. Clearly it can figure this out because the green flashes for the boot loader appear only when connected to USB; they are skipped when powering from battery.

The pyb.USB_VCP().isconnected() method is not what I’m looking for; it reports whether the serial port has been connected… and it never has at the instant of power-on.

Is there a method in some class I missed or a hardware register somewhere I could look at? (I couldn’t see anything useful in pyb.info()… looks like mostly just memory management details.)

Thank you for your thoughts and suggestions!

Context, if helpful:

My application involves saving an image to the H7 Plus’s micro-SD card, then entering standby for 5 minutes (or perhaps longer).

At some point, users will need to collect these images by connecting the H7 Plus to USB and copying them from the micro-SD and then deleting the originals. I’d rather not make them remove the micro-SD card; few computers have readers built-in.

When connected to a computer, it is important not to run my usual code; entering standby would interrupt copying the images. I suspect it could also be problematic to write new images to the filesystem while the micro-SD is being read and written to by the host OS.

While plugged in to USB, I would also like to be able to look for a possible VCP connection to allow optional configuration of device parameters using a simple serial UI.

2

We need to add this to the API.

Here’s how the bootloader sees it:

Um, you can get get the voltage level of the VBUS pin.

Just make a pin object and use USB_VBUS or PA9 as the name and then check if it’s high.

Mmm… this might not be the best though since there’s no pull down. It might randomly work.

3

Thank you. The phrase “might randomly work” isn’t too reassuring, but is there a reason the STM’s built-in pull-down couldn’t help?

usb_vbus = Pin("USB_VBUS", Pin.IN, Pin.PULL_DOWN).value()

…having imported Pin from pyb, of course. With only a couple of tests, putting this at the beginning of the script seems to reliably indicate whether there was USB power at boot. (To test it’s 0 on battery, I connect USB later and check the value of usb_vbus printed in a loop.)

By the way, at first I got an error saying that “PA9” is not defined. But naming it USB_VBUS is more readable for this use anyway.

4

I forgot about the pull down. That definitely fixes the issue.

5

Great; thank you for confirming. I couldn’t perceive anything different about that pin/port from the the STM32 documentation, but I’m new to OpenMV.

6

Such function is very useful to avoid the H7+ to initiate a deep sleep before user had time to connect to the IDE. However, it seems like the current firmware does not include the USB_VBUS name anymore.

import pyb
from pyb import Pin

usb_check = Pin("USB_VBUS", Pin.IN, Pin.PULL_DOWN)

while(True):
    if(usb_check): pyb.LED(3).on()
    else: pyb.LED(1).on()
    
    pyb.delay(1000)

This code gives me an error message “ValueError: Pin(USB_VBUS) doesn’t exist” on line 4.
Any help on this ?

7

Please make a ticket on github to re-add this. It’s just a PIN in the CSV file here: micropython/pins.csv at dc4bf943970558609fa38ecdc1646bd3fa57ee66 · openmv/micropython · GitHub

What’s happening is that the - symbol in front of the pin makes it private.

8

Done. Thank you.

Is there an alternative way to implement this function with the current firmware ?

9

The thresholding method. It can select just the correct pixels.

You can pass multiple thresholds to be used.

10

I am not so sure how this is related to my question. Mistake maybe ?

11

Sorry, was responding to the wrong email thread.

I already did a PR to fix this. Once it’s merged a new development firmware will be available that you can download.