USB project

This blog is a work in progress, so check back later.

I am making a simple USB device. It looks like this. OK this is 3rd version, after some wiring issues.

What does it do?

Well, it is simple, it is a smart card reader, but also works as a passive smart card in-line monitor. The "card" on the side can be snapped off to just be a reader.

This means I can talk to, for example, SIM cards. We sell SIM cards, and this can allow us to read the ICCID, but some cards we have been able to use ADM1 codes to change operator name and other such things.

We used to do this in the card printer which has a contact station. But the latest batches of SIMs do not work in the printer properly, which is a shame, so working on staff having a reader. I hope we can get nicer cards in future as they are nice when custom printed.

But I can buy a smart card reader? Well yes, but making one is (a) fun and educational, and (b) allows us to make a reader that simply works as a USB keyboard typing the ICCID. Normal card readers won't do that, and you need drivers and code. This will be a neat accessory for staff handling SIM cards.

But I may as well also make it in to a CCID compliant smart card USB reader device whilst I am at it. Why not?

Why a monitor as well

This is simple really - making it a monitor is kind of temporary, hence snap off design. It allows me to see in practice the exact working of card readers. There is a spec, well several versions of specs. Last time I looked at this is was all simple 9600 Baud data each way. Now it is initially roughly 9600 Baud but actually 1/372 of the clock, and then it changes clock divide and Baud rate with a message (which I have yet to find documented). This kind of shows why the monitor was useful, if I cannot find the spec that defines this!

Even just the T=0 protocol is fun. That is well defined, and ETSI document it for free (yay!). But a passive monitor has fun splitting things up cleanly as it does not know which way data is being sent. I also ended up basically clocking in bit streams using ESP32 RMT hardware and then auto Baud rate on that. To my surprise it does work, and I can monitor the card exchanges with a real reader even with the Baud rate changes.

It is a shame the ESP32 UART cannot used an external clock with defined divide rate, but I could not find a way to do that, hence not using UART but using RMT and working out Baud rate from that. RMT basically allows you to DMA clock-in high resolution timings of 0 and 1 on the input, so software UART from there from those timings.

USB host side

This turns out to be simple. I used libusb on macOS (homebrew), and have used it with a printer before on linux. It allows me to find and connect to the device and simply do bulk data out and in messages.

The CCID standard made it easy to talk to the reader, power on the card, and send and receive messages. Very simple to get the ICCID.

This gives me a good test platform for making the CCID USB device side.

Card interface

The card interface as a reader is way simpler than monitoring as I control the clock and dictate the Baud rate. I used the ESP32 PWM to generate the clock (not sure if a better way, but did not find one). I used the UART to talk to the card.

It is not perfect - a card can expect defined extra guard time which is more than usual 2 stop bits, but the cards I am playing with seem happy. Also T=0 has a pull low during guard time if parity error mode, which I have no way we handle. The good news is that the direct connection to a card leaves very little room for bit errors. That said I had to change the recommended 20k pull up to 10k when working at 250,000 Baud.

The T=0 protocol is not that hard to then do, and I am talking to my SIM cards reliably.

Yes, T=1 and others maybe one day, but not for now. All of the cards I am working with use T=0.

USB client side

I don't expect to support all of CCID initially, but should be able to do the basics. I actually want two modes here, a keyboard to type an ICCID, and a CCID USB reader.

TinyUSB is a fucking nightmare, sorry to say, and apologies to those working on it. I am sure there are reasons, but it has been very very hard work just getting started.

My biggest issue is the load of different versions and documentation. Even the very latest ESP IDF documents on tinyusb (saying add dependency esp_tinyusb not tinyusb) have the basic initial examples that simple do not match the library!

There are examples for a USB HID device, but they are confusing to me (yes!), and the code seems to cover HID, CDC, MSD, and so on - some specific device classes. I eventually found some mention of Vendor specific class which may do what I want.

So this is where the blog is a work in progress - I am looking at the vendor class tinyusb stuff now. Stay tuned for next instalment, or follow on Mastodon.

Publishing

The code is likely to be very narrow to our needs, and so this is actually one of the few projects not published open source, yet, but I may sell the hardware at some point as I'll have a few spares. I may do some open source stuff for it eventually.