Using an AVR as an RFID tag

Experiments in RFID, continued…

Last time, I posted an ultra-simple “from scratch” RFID reader, which uses no application-specific components: just a Propeller microcontroller and a few passive components. This time, I tried the opposite: building an RFID tag using no application-specific parts.

Well, my solution is full of dirty tricks, but the results aren’t half bad. I used an Atmel AVR microcontroller (the ATtiny85) and a coil. That’s it. You can optionally add a couple of capacitors to improve performance with some types of coils, but with this method it’s possible to build a working RFID tag just by soldering a small inductor to an AVR chip:

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The above prototype emulates an EM4102-style tag- a very popular style of low-frequency RFID tag which stores a 40-bit unique ID. I can read my bogus ID value (0x12345678AB) using Parallax’s RFID reader. Below is another prototype, with a larger coil and a couple of capacitors for added range and stability. It is programmed to emulate a HID prox card, a simple FSK-modulated tag with a 44-bit payload. I can read this card successfully with my garage door opener. This one is a little large to conveniently carry around, but a smaller AVR package should help.

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AVRFID Tag Prototype

So, the shiny electrical tape is beautiful, but how does this thing even work? The power pins on the microcontroller aren’t even connected!

As I said, this makes use of several dirty tricks:

  • The coil actually powers the AVR through two of its I/O pins. Nearly every chip out there has clamping diodes on its I/O pins, which prevent voltages on that pin from rising above the chip’s supply voltage or sinking below ground. These diodes are useful for arresting static discharge.When you first hold the RFID tag up to a reader, the chip has no power- the supply voltage is zero. When the coil starts to pick up power from the RFID reader, these two I/O pins are presented with a sine wave, a few volts in amplitude. Anywhere that sine wave exceeds the supply voltage, some energy is diverted from the coil to the chip’s supply rails, via the clamping diode. The end result is that the chip is powered, and the coil’s sine wave is truncated. The top and bottom of the sine have been chopped off, and it looks a lot more like a square wave now.
  • Power filtering using the AVR’s die capacitance. In the smaller prototype, there is no power filtering capacitor at all. In fact, the power is filtered by the internal capacitance of the power planes in the AVR’s silicon die. This isn’t much, but it makes the power supply stable enough that we can execute code even though the supply is pulsing at 125 kHz.
  • Very low voltage operation. This particular ATtiny85 chip is specified for operation at voltages as low as 2.5v. The extended voltage range version (I didn’t have any of these handy) is specified down to 1.8v. But I’m running these AVRs at barely over 1 volt. At these voltages, the normal AVR clock oscillators don’t work- but I can get away with this because of the next hack…
  • The coil is the AVR’s clock source. The inductor isn’t just hooked up to any I/O pin: it’s actually connected to the AVR’s clock input. Remember the square-ish wave we’re left with after the clamping diodes suck away some power? That waveform is now our clock input. The microcontroller is executing code at 125 kHz, in lockstep with the RFID reader’s carrier wave.
  • Firmware? What firmware? At such low speeds, the chip’s firmware looks less like a program, and more like a sequence of I/O operations to perform in sync with each carrier clock cycle. There aren’t a lot of cycles to spare. In the EM4102 protocol, you could potentially do some useful work with the 32 clock cycles you have between each bit. With the HID protocol, though, you need to output an FSK edge as often as once every 4 clock cycles. As a result, the firmware on the RFID tag is extremely dumb. The “source code” is really just a set of fancy assembler macros which convert an RFID tag code into a long sequence of I/O instructions.

The fact that this thing works at all is quite a testament to the robust design of the AVR. The latest AVRFID source is in Subversion, as usual.

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