Friday, July 26, 2013

Using a matrix keypad is easy!

Today I wanted to try out the keypad I have bought for my project. It is a cheap ($2) 4x3 key membrane switch matrix keypad.

The keypad connected to my Iteaduino 2.2.
Using the Keypad library from I was up and running in a few minutes. If I can spare the 7 IO-pins required this is almost too easy ;-)

I will have to find another way to connect it, as this connection uses pin 8 (ICP1), that I intend to use for the manchester decoding of EM4100 RFID tags. However this is no problem, as I think I will end up using a barebone Arduino for the finished product.

I am still waiting for the MIFARE reader kit, so next I will look at my $3 Nokia 5110 84x48 pixel display. It has a Philips PCD8544 controller using SPI for communication. It is well documented, and Adafruit has both a tutorial and a library for it.

Wednesday, July 24, 2013

Decoding/reading 125kHz RFID tags - live at GitHub

Initially I would like to create a system that could use any existing RFID-tag. That way we could use existing tags we already had from our jobs, transportation etc. So I ordered a 125kHZ RFID kit and a 13.56MHz Mifare kit. Both kits included both a reader circuit and a couple of sample cards.

When the 125kHz kit arrived I did not have the courage to connect it to my Arduino, as I was afraid of toasting the inputs with RS232 level signals. I waited for my DSO Quad pocket oscilloscope to arrive (more about that in a later post) and hooked it up. It revealed that the claim about onboard level converters for RS232 was bogus. Pin 3 was only an inverted signal of pin 2. The good thing was that I only needed the TTL level signals and that it worked great as soon as I connected pin 2 (TX) to the Arduino pin 10 and ran this sketch:
 Receives from software serial, sends to hardware serial. 
 RX is digital pin 10 (connect to TX of other device)
#include <SoftwareSerial.h>
SoftwareSerial mySerial(10, 11); // RX, TX

void setup() {
  Serial.println("Scan card...");
  mySerial.begin(9600); // set the data rate for the SoftwareSerial port

void loop() { // run over and over
  if (mySerial.available()) {

Using the serial console in the Arduino IDE I received a steady stream of lines:
Somehow I had started suspecting the cards to be EM4100 compatible, and that is a 64 bit chip.
But each line is only 14 digits - and I expected 16 hexadecimal digits (for 64 bits).
The RFID receiver circuit has a pin labelled "Manchester code output", so I decided to try and decode that signal myself. Not yet comfortable with the Arduino platform I used the tool I knew best: PHP.
My PC could not directly interface with the manchester output pin, so I used the oscilloscope's ability to dump the captured trace to a CSV-file, and then parsed the CSV using PHP.
While working on my parser I found out about two things:

  1. The 64 bits on the EM4100 chip includes 11 header bits, a stop bit and 14 parity bits. This leaves 8 bits for a facility code and 32 bits for the UID.
  2. Another eBay dealer sells the same kit - and he includes a description of the serial output: 2 hex digits facility code, 10 decimal digits UID and 2 digits parity data.  

Not every trace was parsable, but I ended up with parsable CSV-files for each of the cards supplied with the kit. The Awid chip used for access control at my old work place however could not be parsed - it seems not to be EM4100 compatible.
Using "php dump.php DATA-Thick-card-500us.csv 3 70 12 18" I can get this output:
4E 0001943800
- which almost looks like the serial output from the receiver circuit. However I have given up on the 2 parity digits - I cannot fit 14 bits in 2 hex digits ;-) But who cares - the parity data is redundant.

The dump.php program tries both manchester and biphase decoding.

Printed on the card is "0001943800 029,43256". Initially I did not get it - but it is simply the 32 bit UID as a 10 digit decimal number followed by the high and low 16 bits as decimal:  29 * 2^16 + 43256 = 0001943800.

The next step was to implement almost the same functionality on the Arduino. I was inspired by the Arduino Cookbook to use Input Capture on the 16 bit Timer/Counter1. The result was an Arduino library (EM4100) that can capture EM4100 data from a manchester signal. It is available on

About the "any tag" idea: Well - I have become (a little) wiser. There are a lot of RFID standards and de facto standards - and a lot of custom implementations. I will have to settle for less - and EM4100 and MIFARE seems like good candidates.

Now it is getting really late and in the morning my brother and I are going canoing. I should get some sleep ;-)

Thursday, July 11, 2013

Welcome to my journey!

Today I have created the blog and written about the background for the project.

Even though it is not my native language I have decided to write in English. I hope someone outside Denmark can benefit from my work that way.

And thus it begins...