Pi Zero on a digital frame

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One of the ways you can use your PiZero is to transform it into a USB gadget.

There are the things you can turn the PiZero into:

  • Serial (g_serial)
  • Ethernet (g_ether)
  • Mass storage (g_mass_storage)
  • MIDI (g_midi)
  • Audio (g_audio)
  • Keyboard/Mouse (g_hid)
  • Mass storage and Serial (g_acm_ms)
  • Ethernet and Serial (g_cdc)
  • Multi (g_multi) – Allows you to configure 2 from Ethernet, Mass storage and Serial

In addition to the above modules, a few other (less useful) modules are included.

  • Webcam (g_webcam)
  • Printer (g_printer)
  • Gadget tester (g_zero)

and one of the ones I was mostly curious about was the Mass Storage.

So let’s quickly see how to transform your PiZero into a USB storage.

DigitalFrame-PiZero

Andrew and Adafruit worked on this together over Christmas but have  eventually taken two different approaches.

I have followed Andrew Mulholland’s github guidelines with some slight changes.

Get yourself a Rasbian Jessie full edition and resize the root partition via either the GUI or raspi-config.

Note the kernel version and the various details of Jessie with

uname -r

cat /etc/os-release

At this point we want to upgrade the kernel as the most updated one supports natively the USB gadgetry

sudo BRANCH=next rpi-update

And after this is done reboot  (sudo reboot)

As you login you might want to run the two initial commands to see the differences.

Next we need to change the config.txt by appending the dwc2 USB driver at the end of it

echo “dtoverlay=dwc2” | sudo tee -a /boot/config.txt

and enable the general module

echo “dwc2” | sudo tee -a /etc/modules

lastly we need to enable the specific module which in our case is the g_mass_storage one

echo “g_mass_storage” | sudo tee -a /etc/modules

Configuration wise this is it! You should reboot now.

Next we need to create a backing storage used by the Pi to offer some space as USB key

depending on what you choose to do on the next command you will be either creating a floppy, a HDD or a CD-Rom. I went for the HDD 100MB in size.

sudo dd if=/dev/zero of=/piusb.bin bs=1024 count=102400

Andrew opted for a floppy with sudo dd if=/dev/zero of=/piusb.bin bs=512 count=2880

This creates a file at the root level which can be mounted by the module with the following command

sudo modprobe g_mass_storage file=/piusb.bin stall=0

If you now plug your Pi in your computer it should be seen as a RW USB storage device

USB Storage

To start the Pi automatically as USB storage I ended up adding sudo modprobe g_mass_storage file=/piusb.bin stall=0 to /etc/rc.local

USB Storage Contents

At this point my PiZero was ready to be plugged in my Digital Frame, boot up and serve some beautiful pictures. A bit useless if you ask me but fun!

DigitalFrame-PiZero-Blank

Why did I get into this and why it did not quite work as I wanted

I had purchased two or three Digital Frames at my local charity shop for £3 each and have been looking at them sitting quietly in a corner for a while wondering what could I do with those screens. My first thoughts were about turning the screens into a monitor for the Pi. There are quite a few hardware drivers you can buy that promise to do just that. They are £20-30 and many screens aren’t supported. For as much as this seemed a good idea it was not to be.

Then I turned my attention to the USB port that these frames have thinking that if I was to be able to emulate a USB stick from a microcontroller of  some type I should be able to change the pictures of the digital frame as it displayed it. Although the minimum cycle for the digital frame is every 5 seconds it might have still been enough to e.g. periodically

  • Display a picture from a camera
  • Update some information of the OS
  • Game of life

You get the drift. All of course at the fantastic speed of  0.2 frames/s

You can try to mount piusb.bin by looking at its geometry with

fdisk -l /piusb.bin yields

Disk /piusb.bin: 100 MiB, 104857600 bytes, 204800 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xba95698c

Device Boot Start End Sectors Size Id Type
/piusb.bin1 128 198783 198656 97M 6 FAT16

I have a block-size of 512 bytes and the start-block is 128. The offset is 512 * 128 = 65536.
The mount command would be

sudo mount -t msdos -o loop,offset=65536 /piusb.bin usb-key/

It mounts as it should and you can see all there is in piusb.bin shame that nothing I would do had any effects on what was being seen on the USB storage device and in fact was corrupting the filesystem.

🙁

It sort of worked only when the digital frame would stop the slideshow as it would unmount the USB key and read it again when the slideshow would be restarted. So with imagemagick I could produce pictures out of some text like this

OS-RELEA

or the one below, in hope to get at least a rather complicated clock

DATE

but also this method would pretty much corrupt the filesystem.

It turns out that, as often happens, I get into trying things before giving a good read at the documentation and I fall short of my own predicament about RTFM. Had I read it I would have found:

“*BEWARE* that if a file is used as a backing storage, it may not be modified by any other process. This is because the host assumes the data does not change without its knowledge. It may be read, but (if the logical unit is writeable) due to buffering on the host side, the contents are not well defined.”

which stopped all my attempts to do something clever about my PiZero USB OTG Storage Device.

Not every fail is a complete failure though, I have learned a good deal of things, was successful in turning the Pi into a USB gadget and got to know a bit more about ImageMagick along the way.

10 Comments

  • Ok. Now I want a PiZero, and haven’t seen them available yet 🙁

  • I’m not sure if this is possible, but couldn’t you copy the HDD image to a RAM drive with two versions and mount one of them. Then when you want to add pictures, modify the other RAM drive image and mount it in place of the old one, swapping out the older version. Since you would be leveraging the RAM for speed, it might be fast enough for the photo frame to not notice (unless it’s doing a read at that moment…)

    Regardless, very cool idea!

    • Hey Josh,
      AFAIK the trouble seems to be with how the driver handles multiple access or in fact how it doesn’t.
      The rather daunting sentence “*BEWARE* that if a file is used as a backing storage, it may not be modified by any other process …” seems to be quite definitive. I’d love for somebody to come up with a solution somehow but …
      I spoke to Tom Hartley about this as his piShift uses pretty much the same mechanism and he concurs that, at least for the moment, there is no hope 🙁
      Thanks anyway for stopping by Josh

  • Hi Francesco,
    Glad to find this post, I’ve been pondering a similar project. If I understand correctly, you find this system limited because you can’t modify the hdd image contents after it has been mounted by the digital frame?
    In my case, I was gifted a digital frame which is very nice, but unbelievably it doesn’t have a feature to randomize the photos. So every time it wakes up it starts at the beginning with the same sequence of photos, which is very repetitive and defeats the purpose of the thing.
    After reading your post, I’m imagining that I could use a Pi Zero to hold my photos, and randomize the filenames before allowing them to be mounted by the digital frame. And then I could periodically unmount, randomize, and re-mount. I’m curious if you found anything in your own project that would throw a wrench in my plan?

    • Hi Chris,
      the only thing I can think of is that by umounting the drive the digital frame will stop its playback and you will have to hit play again. That’s what I was doing when trying to displaying updated pictures like the ones showing system information at the bottom of the post. One could use the GPIOs to hack into the controls of the digital frame and let the Pi “hit” the play button on its own. Using something like a 4066. There are other ways to achieve the same result but it depends on how the control buttons are wired.
      Let me know what you come up with!

      • Good point. I just ran a test with my digital frame, and if I insert a USB stick with photos it will start the slideshow automatically. I don’t know, is that equivalent to newly mounting an image in software? So hopefully that’s one hurdle cleared. I think I’ll go ahead attempting this, if I can ever find a PiZero in stock. Thanks again, and I’ll let you know if it works out.

        • That is exactly just about the only other option. My frames don’t have autoplay. With that it should work, what brand/model is your frame?

  • It’s the Aluratek 10″ digital frame. Can’t say I recommend you purchasing one, though. Display isn’t great, any dark areas look black, poor viewing angles.

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