In case you haven’t heard, the Raspberry Pi Zero is the smallest, most low-cost device in the Raspberry Pi family, but it’s also the hardest to find. It has two Micro-B USB ports, one for power and another functions as a dual-role USB OTG port.
One of the more interesting uses for the Raspberry Pi Zero is to get it to behave as a USB device, just like your USB flash drive, for example.
There have been several guides written already, such as the Adafruit one, but most of them were based on the old kernel gadget drivers, like
g_ether. It still works, but not as flexible and likely to be deprecated in future.
A very long time ago, I set up and played around with diskless machines. These are basically PCs can boot up an operating system fully without hard disks. All the operating system files come from a server on the network. It was amazing (well, to me at least)!
Back then, Ethernet cards used to have a DIP/PLCC socket, which allowed you to insert an EEPROM on which you burn a boot ROM. Fortunately I didn’t have to do any of that because the network cards at that time already came with PXE ROMs built-in, just as they do today. To activate this, you just need to select the network card’s option ROM in the boot order, or make it higher up in the boot priority.
As part of the boot process, the network card will request an address from the DHCP server, which also tells the client where it can find the TFTP server with the next boot stage. The ROM will download this file from the TFTP server and start executing it.
That’s how Linux ultimately gets started from the network.
An announcement was made recently on the Raspberry Pi blog that you can achieve total network boot, just like on the PC, without any SD cards.
Last week as I was making my rounds at the supermarket, I came across this digital bathroom scale on sale. With some membership card, the discount was almost 50% and at S$16, I thought that was a pretty good deal. It is “wireless” in that it has a separate display unit that could be detached from the scale itself. This bathroom scale had “HACK ME” written all over it.
It turns out that this bathroom scale is the EB9121 made by a Chinese (OEM?) company called Zhongshan Camry Electronic Co. Ltd (or simply Camry). The box specifically mentions that it uses infrared for transmission, and given that I had some experience looking at IR signals, I thought it would be rather straightforward.
Consumer Electronics Control (CEC) allows control of AV devices that are connected via HDMI. This is the feature of HDMI that enables your TV to automatically turn on and switch to the correct input when you switch on your set-top box, for example. It also allows you to control your set-top box using the TV remote (in some cases).
Electrically, the CEC bus is a single-wire bus that is shared between all HDMI devices, thus any CEC message can be received by all connected devices. Each device then claims one or more logical addresses on which it will receive direct CEC commands.
One interesting feature in the HDMI CEC specifications is Remote Control Pass Through, which allows button presses on the remote control to be passed through to HDMI-connected devices. I thought this feature could be used to unify the various remotes in my living room.
However, not all CEC devices are created equal. As usual, some manufacturers will deviate from the specifications, and/or introduce some quirks in their implementation (as you will see later). They also love to brand CEC with their own funky name, such as SimpLink or Anynet+.
Raspberry Pi as a CEC Bridge
As a quick and dirty way to check out the capabilities of my TV, I used a Raspberry Pi which has a HDMI connection that can be software-controlled. This also meant that I didn’t have to build my own CEC transceiver circuit.