The first step was deciding how to implement this. While the device itself supports quite a wide range of interfaces, including a GPIO one, I decided that since I’m only going to be able to test and use practically the serial interface, I would at least start with just that. So you’ll probably see the first output as a module for pyserial that implements access to CP2110 devices.
The second step was to find an easy way to test this in a more generic way. Thankfully, Martin Holzhauer, who commented on the original post, linked to an adapter by MakerSpot that uses that chip (the link to the product was lost in the migration to WordPress, sigh), which I then ordered and received a number of weeks later, since it had to come to the US and clear customs through Amazon.
All of this was the easy part, the next part was actually implementing enough of the protocol described in the specification, so that I could actually send and receive data — and that also made it clear that despite the protocol being documented, it’s not as obvious as it might sound — for instance, the specification says that the reports 0x01 to 0x3F are used to send and receive data, but it does not say why there are so many reports… except that it turns out they are actually used to specify the length of the buffer: if you send two bytes, you’ll have to use the 0x02 report, for ten bytes 0x0A, and so on, until the maximum of 63 bytes as 0x3F. This became very clear when I tried sending a long string and the output was impossible to decode.
Speaking of decoding, my original intention was to just loop together the CP2110 device with a CH341 I bought a few years ago, and have them loop data among each other to validate that they work. Somehow this plan failed: I can get data from the CH341 into the CP2110 and it decodes fine (using picocom for the CH341, and Silicon Labs own binary for the CP2110), but I can’t seem to get the CH341 to pick up the data sent through the CP2110. I thought it was a bad adapter, but then I connected the output to my Saleae Logic16 and it showed the data fine, so… no idea.
The current status is:
- I know the CH341 sends out a good signal;
- I know the CP2110 can receive a good signal from the CH341, with the Silicon Labs software;
- I know the CP2110 can send a good signal to the Saleae Logic16, both with the Silicon Labs software and my tiny script;
- I can’t get the CH341 to receive data from the CP2110.
Right now the state is still very much up in the air, and since I’ll be travelling quite a bit without a chance to bring with me the devices, there probably won’t be any news about this for another month or two.
Oh and before I forget, Rich Felker gave me another interesting idea: CUSE (Character Devices in User Space) is a kernel-supported way to “emulate” in user space devices that would usually be implemented in the kernel. And that would be another perfect application for this: if you just need to use a CP2110 as an adapter for something that needs to speak with a serial port, then you can just have a userspace daemon that implements CUSE, and provide a ttyUSB-compatible device, while not requiring short-circuiting the HID and USB-Serial subsystems.