Wednesday, November 20, 2013

Simple NFC with 22 MHz crystal


Short update on the Simple NFC: Meanwhile I tested the circuit with a 22 MHz crystal. After some fixes on the code it started to work some kind, but the error rate is too high for a practical use. Without storage oscilloscope it is quite hard to trace down the issue.

Tuesday, November 12, 2013

Simple NFC


Introduction

This article describes a simple approach for emulating NFC tags (to be accurate ISO 14443-3 compatible Type 2 tags). Nothing more than 4 resistors, 3 capacitors and 1 diode are required to connect the RFID antenna to a microcontroller.



On applications where range and high transfer rates aren’t required, NFC can replace WIFI and Bluetooth to exchange data between smartphones and other devices. Compared to other RF interfaces supported by nowadays smartphones, NFC works with 13.56 MHz on a much lower frequency. Combined with the simple hardware layer and the lightweight protocol, it’s ideal for low power and low cost as well as DIY applications.

The inspiration for that project comes from Micahs “software only” 125 kHz RFID tag. Because of the On-Off Keying for ISO 14443-3 tags my design requires a bit more hardware.

Hardware

The LC-circuit consisting of C3 and antenna coil is tuned to 13.56 MHz resonance frequency. The received On-Off Keyed signal is rectified by the schottky diode D1.  The diode combined with C2 and R4 works as envelope detector to recover the transmitted data. This signal is feed into the positive comparator input AIN0. The comparator reference AIN1 is generated by a voltage divider with an additional low pass filter consisting of R2, R3 and C2.


https://www.google.com/search?q=NFC+Data+Exchange+Format+(NDEF)+ext%3Apdf

For sending data from the tag to the reader load modulation is intended. This is realized by internally connecting the counter 0 to its output OC0A. In case of the ATiny85 and some other microcontrollers it’s the same port which is used by the comparator during receiving (AIN0). When OC0A is low more power is drawn from the LC-circuit compared to the condition when OC0A is high.

Probably the design doesn’t match the NFC specifications in a number of aspects. But anyway it worked very well during testing. My test device was a Lumia 620 which uses the very common NFC controller PN544 from NXP.

To be able to generate the subcarrier of approximately 847.5 kHz the clock frequency of the microcontroller must be divisible by the subcarrier with a small or without remainder. The best choice will be a 13.56 MHz crystal as clock source. I used a 13.5925 MHz crystal which worked fine as well. If a 13.56 MHz crystal is not available a 22 MHz crystal should work as well (see the follow up post). Frequencies below 13 MHz will be critical because of computing time shortage.

Deviation in the sub percent region seems to be not critical for the subcarrier frequency. For the average transmitting bit-rate even small deviation has to be considered. That can be done in software (the code is already prepared for 13.56, 13.5925 and 22 MHz).

Software

The software is open source and can be downloaded here.

Further reading

Timo Kasper gives a short introduction on the physical RF-layer. He presents his own RFID emulator design consisting of significant more but only standard components. A nice and short NFC introduction is “Exploring the NFC Attack Surface” by Charlie Miller. All Details on ISO14443-3 tags can be obtained from the ISO/IEC 14443-1, 14443-2 and 14443-3.

The format in which data is stored on the tag is called NDEF. Nokias Wiki gives a short introduction. Everything in details can be obtained from the official specification “NFC Data Exchange Format (NDEF)” from the NFC-Forum.

Friday, November 16, 2012

Graph Touch for Windows 8

I ported Graph Touch to Windows 8. It's a scientific calculator with graph plotter and numeric solver for maxima, minima and zero crossing. The graph can be paned and zoomed by two-fingers. The numeric solver can be used to calculate numeric solutions for equations. Like the Windows Phone version it's free and without ads.


Thursday, May 10, 2012

Simple DIY case for small wares

Here a short description for a rugged DIY case that can be made very easy and requires only simple tooling and easy obtainable materials. Same case type was used for "Data transmission by using the phone display".

The case is made from a middle tube (steal clothes rail) and two end caps with end plates. The end plates are made from 0,7 mm PMMA and hold by the end caps. The end caps are made from thin walled PVC tubing (cable conduit). The PVC tube segments are heated above there glass transition temperature with hot air and pushed then over the ends of the middle tube (thick leather gloves are needed). After that some sanding is required for a nice looking finish.

Monday, April 23, 2012

Update Graph Touch 1.2

Graph Touch 1.2 is ready for download on Marketplace. The scientific calculator has now a new UI and some new features.


Thursday, December 8, 2011

Graph Touch

My first Windows Phone 7 App is published. Graph Touch is a scientific calculator with graph plotter and numeric solver for maxima, minima and zero crossing. The graph can be paned and zoomed by two-fingers. The numeric solver can be used to calculate numeric solutions for equations. It's free and without ads.

Monday, August 29, 2011

Data transmission by using the phone display


Most wares I build up are highly experimental prototypes. Often they have no display and not a full keypad for input the settings. At the PC RS-232 or similar simple cable-based protocols can be used to transfer setting to the ware. But if you want to use your present-day phone there is no established way expect using Bluetooth or wireless LAN, which is quite expensive.

So I came up with the idea to transfer data by flashing the phones display and implemented the corresponding hard and software. The result is a phone-programmable igniter that can be programmed to any time in the future (theoretically several year).

The hardware is kept extremely simple. It can be power by two 1.5 V-batteries. Nevertheless it has a "included" real-time clock and consumes very small amount of power. The receiver is a simple photo transistor. Here you can download the source for the micro controller (Attiny 44 A from ATMEL).


The picture above shows the front window with the photo transistor behind on the left. On the right you can see the 6-pin header for flashing the microcontroller.


The case is made from a steal profile with end cabs thermally formed from a PVC-Pipe. The following image shows the complete circuit.


For sending the time/date to the ware I wrote an App for Windows Phone 7. It sends an 32 bit number and 4 parity bits. With the used Biphase-Mark-Code around 18 Bit/s gross throughput is possible. The C# source for the sending app can be downloaded here.