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White boarding a RoadTest Review of The Microchip PoE to USB-C®︎ adapter

colporteur

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What are your expectations at connection point C?

 

I am considering pitching an application for the Microchip PoE to USB-CRegistered Power and Data Adapter RoadTest review. I completed a review of a PoE device, Raspberry Pi4B (4GB) plus POE Hat - Review and figured why not leverage the knowledge into another PoE review.

 

The drawing shows my preferred setup for the RoadTest, a Linux laptop interfacing through a PoE Injector communicating with a Raspberry Pi4B. The injector is a device I acquired to resolve an issue I discovered during the RoadTest Review The Road to Raspberry Pi4B/ PoE Hat RoadTest Review (PoE insight needed) . I want to avoid more purchases.

 

After reviewing the documentation I'm confused about my expectations at point C in the diagram. I was hoping a white board session with others might clarify my understanding. One of the opportunities I miss in my retirement is the napkin discussion with technical minds during a coffee break at work. Using a drawing scribbled on a napkin, like minds worked to offer insight into solving a problem presented at the table.

 

The Microchip PoE to USB-CRegistered Power and Data Adapter provides power and data communication path. Can someone confirm my understanding that the Microchip adapter is acting as a Ethernet to USB adapter for communication? I found reference to a LAN7800 USB driver chip while drilling down through one of the documents. I assume that means the end device (i.e. Pi would require the driver support).

 

I'm not familiar with many devices that combine USB port functions of power and communication. The brochure for the product shows a tablet connected as a device. I have an Apple tablet that can act as device but that makes it rather difficult to take measurements under varying loads. In my test scenario I'm thinking I will need a breakout cable to separate Microchip adapter power from the USB signal. As an additional test in my review, I was hoping to use a Android laptop I have as an additional device to test the endpoint. But again I would need to separate power and USB signals.

 

Questions:

What are your expectations at connection point C?

Parents

  • I love this site! I miss the coffee break napkin discussions at work but this site provides the ideal replacement. My thanks to & for making posts to my discussion. Your input enabled me to discover a feature of the the Raspberry Pi. I had never heard of USB OTG (On The Go) or that such a features was available on the Pi.

     

    I followed up on your posts and set out to test the Raspberry Pi USB OTG interface. I assembled a cable that would allow me to connect my laptop USB port to the Pi4B USB-C. I disconnected all peripherals from the Pi4B hoping the laptop USB port could sufficiently power the Pi.  I followed the instructions listed in the links and changed the /boot/config.txt & /boot/cmdline,txt files to enable OTG.

     

    The moment of truth, I plugged the Pi4B into the laptop and booting started. I then attempted to connect to the Pi (previously configured wireless connection) and was rewarded with success. The Pi4B booted! A quick ip a command and I discovered a usb0 interface on the Pi. I then made /etc/dhcpcd.conf configuration to determine if I could assign an IP address to the interface. That was a success also.

     

    I then got lost thinking I could connect to the interface over my network. After some head scratching I realized the errors of my ways. It is not on my network but rather a end to end connection from the laptop to the Pi.  I discovered the laptop had created an addition network interface for the usb port but had no IP configuration. I'm assuming if that interface was configured correctly I would be able to talk. I will leave that for possible RoadTest Review details.

     

    Thanks to your input I have at least one RoadTest Review scenarios to explore. I also think physical separation of data and power on a USB-C is another good investigation scenario.

Reply

  • I love this site! I miss the coffee break napkin discussions at work but this site provides the ideal replacement. My thanks to & for making posts to my discussion. Your input enabled me to discover a feature of the the Raspberry Pi. I had never heard of USB OTG (On The Go) or that such a features was available on the Pi.

     

    I followed up on your posts and set out to test the Raspberry Pi USB OTG interface. I assembled a cable that would allow me to connect my laptop USB port to the Pi4B USB-C. I disconnected all peripherals from the Pi4B hoping the laptop USB port could sufficiently power the Pi.  I followed the instructions listed in the links and changed the /boot/config.txt & /boot/cmdline,txt files to enable OTG.

     

    The moment of truth, I plugged the Pi4B into the laptop and booting started. I then attempted to connect to the Pi (previously configured wireless connection) and was rewarded with success. The Pi4B booted! A quick ip a command and I discovered a usb0 interface on the Pi. I then made /etc/dhcpcd.conf configuration to determine if I could assign an IP address to the interface. That was a success also.

     

    I then got lost thinking I could connect to the interface over my network. After some head scratching I realized the errors of my ways. It is not on my network but rather a end to end connection from the laptop to the Pi.  I discovered the laptop had created an addition network interface for the usb port but had no IP configuration. I'm assuming if that interface was configured correctly I would be able to talk. I will leave that for possible RoadTest Review details.

     

    Thanks to your input I have at least one RoadTest Review scenarios to explore. I also think physical separation of data and power on a USB-C is another good investigation scenario.

Children
  • in reply to colporteur

    Wow, I also didn't know. Only knew the Pi 0 had such features. Thanks for sharing!

  • in reply to hugohu

    Say again?

     

    I assumed the USB OTG was USB-C dependent. USB OTG is supported on the Pi Zero mini-sub connector? Hummm.... I will add that to the RoadTest Review scenario investigation.

  • in reply to colporteur

    Actually, USB-OTG has been a thing since the mini-USB-B connector came out (pre-dating microUSB) and is a way for mobile devices to assume a host role in addition to the (usual) device role so they could run things like USB memory sticks, card readers and other accessories. That being said, the OTG on earlier versions of the Pi B/B+ was used in the host role to supply USB to the USB Ethernet + Hub chip to produce the two/four downstream ports.

     

    But the Raspberry Pi Zero (and Model A/A+, although missing an ID pin for dynamic role switching) have a trick up their sleeve because they don't use this USB Ethernet + Hub chip, so they can act like a device as well using USB-OTG as the board has been designed with the necessary direct pin connections - https://gist.github.com/gbaman/50b6cca61dd1c3f88f41 . Since the Pi4 has a PCIe USB 3.0 controller, the USB ports are now supplied from this and the USB-C power in port now uses the OTG hardware for device role as default.

     

    But since the Microchip adapter is a device role, you really only want the port for the host mode. I would suspect that it might take some cable sleuthing to get the right combination to get a Pi Zero up with the adapter - perhaps a USB-OTG cable + A to C cable. But it should be achievable in some way.

     

    - Gough

  • in reply to Gough Lui

    I'm thinking, (would like to test), that they Microchip is analogous to a USB network dongle. I have such a beast in my kit bag for when I encounter a laptop without an RJ45 Ethernet port. The Microchip documentation make reference to the LAN7000 chip. I assume the USB-C data connection establishes an Ethernet connection using this.

  • in reply to colporteur

    Well, it is a USB to Ethernet adapter, except it does PoE so it can backfeed power to the connected device using USB PD.

     

    Whether it can work as one when not connected to a PoE port (i.e. a regular port) may be worth testing and will depend on its design.

     

    - Gough

  • in reply to colporteur

      wrote:

     

    .... analogous to a USB network dongle ....

    I believe this is the best summary.

    Yes, with the additional function to power the device that's plugged in to it - if that's a device that can be powered via that USB port.