The HTRC110 datasheet provides a detailed overview of the HTRC110 specifications. The HTRC110 Read/Write Devices Application Note provides very thorough information on how to implement a HTRC110 based circuit.
The application note can get very technical in places and its easy to become quite offput by the sections covering antennas etc if you are not one of the rare bread of engineers who is fully conversant with the black art that is RF design. However there is no need to worry if parts of it go over your head – the application note was of course written by RF engineers for RF engineers. The 125kHz being dealt with is not at all complex from a RF point of view and designs will generally work fine using the default component values and just by taking care with your antennas PCB routing to give it reasonably thick tracks and keep it away from sources of noise. At the end of the day you can make a fully functioning RFID reader using the HTRC110, this driver and a few loops of PCB copper as the antenna!
A great source of design help is to look at one of the many 125kHz RFID reader modules that use the HTRC110, available from many different suppliers (the HTRC110 is a very well known and commonly used RFID read solution). Looking at their antenna implementation notes will often provide the basic information you need to feel comfortable with your design. For example the Micro RWD QT (Quad Tag) manufactured by www.ibtechnology.co.uk uses the HTRC110 with a PIC16F87 microcontroller:
RFID Circuit & PCB Design Notes
It’s important that the voltage supply to the HTRC110 is protected from ripple and interference while a read is being carried out.
Where you have noisy electronics in the proximity that are active while reading a tag:
A simple RC filter of 10R and 1000uF will go a long way to preventing power supply interference. Better still is using a separate power supply from a dedicated voltage regulator if you have the option.
Use a star point for the earthing. Typically this will ideally be the negative pin of a 100 – 1000uF smoothing capacitor, with all the grounds taken back to this single point.
Use thick tracks to high power devices and areas to reduce airborne interference.
If you are able to have +V and 0V power tracks to high power devices run above and below each other on top and bottom layers this will help cancel out the field they generate.
When using long PCB tracks for a connection to an antenna ideally use a decent thickness and route along the edge of the PCB if possible and away from noisy devices / circuitry. Placing ground plane around and below tracking can be useful to protect the signals.
Remember, the enemy of any RFID electronics design is noise! Is there anything else you can think of to reduce it?
Solving Interference Problems
If you have +V and 0V cables running in proximity to the RFID reader twisting them together will help cancel out the field they generate. Also make them as short as possible and if possible angle them away from your reader.
Experiment by turning off the RFID reader and looking at the coil voltage with a sensitive oscilloscope. Rotate the coil for minimum interference. If possible move the coil to a minimum interference position. Even if it is impossible to eventually change the angle or possition of the coil, this experiment may identify ‘Hot Spots’ that will help you zero in on the cause of a problem.
Many RFID readers have interference rejection but the interference may be so great that it is saturating the output of the reader detector. It may be that a low Q coil will give a lower output and the reader will not saturate so it may actually read better. A 1k resistor across the coil is extreme but may still be a good starting point. You can plot the range against several values of resistance from 1k to 10k.
The effects of metalwork are often not predictable. If the RFID reader coil is surrounded by metal in such a way that the steel effectively becomes a shorted turn, then this will appear as negative inductance and you could try to compensate by adding 50pF in parallel with the coil. If it helps try other values. A simple way to see if the metal is having an effect is to isolate the reader and measure its supply both in situ and out of situ. If there is a large difference then you may have a problem.
Multiple Aerials
If your project requires the use of multiple RFID antennas you may consider using some form of aerial switching circuitry. Whilst entirely possible this solution is typically most suited to designs that are making use of expensive off the shelf OEM RFID reader modules. The HTRC110 and its associated components are so cheap that it is usually much more cost effective to provide a HTRC110 for each separate antenna and simply switch the 3 serial connections to your microcontroller using an IC such as the 74HC4053. This also removes the issues of non perfect resistance values for typically used opto isolated MosFet switches (such as the AQY210EHA) which will affect antenna performance.
Writing Tags
Although this driver does not support it the HTRC110 does provide the posibility of writing data to some specific alternative RFID tag types. The datasheet and application note sections covering this can be safely ignored.
HTRC110
The HTRC110 IC was selected as it is a low cost but advanced RFID reader IC which incorporates many features to remove tuning and calibration requirements often found with other RFID reader IC’s. It is widely used in a large number of commerical RFID readers. The HTRC110 may also be used with other RFID protocols and this driver provides an excellent starting point for such development if this is your aim.