TRF7960 - cannot read or detect tag

Garry - 

lets start with the basics - you already took one step in this direction by testing the antenna - good move

so then - first - when using ISO14443A tags - you need to check that you are setting ISO control register to 0x88 for first commands -will go through that in a moment, turning the transmitter on (either 0x21 or 0x20, depending on your VIN) which you must be since you said you measured it and setting the mod depth register correctly (should be 0x01 for 100% mod depth, which ISO14443A requires) - in this development phase you are in, i would also recommend you read the registers back to ensure you have written them and that your SPI is correct. With the TRF7960, -60A, 61, 62A and 63A, you need to change your clock polarity between writes and reads.

then please verify your first command string out is either: 

0x8F 0x90 0x3D 0x00 0x0F 0x26 ==> REQA

or

0x8F 0x90 0x3D 0x00 0x0F 0x52 ==> WupA

and any other mods you made to the FW to get it onto the CC2541

thanks.

here are some slides that will walk you through the steps - let us know if you are using a real ISO14443A card (also called NFC Type 4A) instead of a Type 2 (which also uses ISO14443 (but only to -3 part of the standard and is what these slides are about)

0525.NFC Forum Type 2 Tag Platform Operations with TRF7970A_02_18_2014.pdf

Josh,

Thanks for your response. I have verified that the tag is 14443A using the development kit as the correct LED lights. I have verified that the modulation and some other registers have the correct values but I will read them all as you suggested and will let you know the result

Our antenna board is on quite long wires (maybe 5cm) do you think this could cause problems? As I said, it did work on the development board with the same length wire

5cm is not too long. i would venture you could even go further (like 3m) with 50 ohm coax with just a little line loss...the antenna is to be matched to 50 Ohms as the output of the TRF79xx devices reference circuit is 50 Ohms, therefore transmission line theory/concepts/rules can be used.  

after you check your register settings, also have a look at the layout and double check it against the TRF7960AEVM or the TRF7960ATB designs. Would be looking for that you separated the TX from the RX and the TX from the digital, as those designs illustrate.

http://www.ti.com/lit/zip/sloc221 (TRF7960ATB design files)

http://www.ti.com/lit/zip/sloc086 (TRF7960EVM design files)

Josh,

All the registers seem to be configured fine. I am also sending the byte sequence you told me to check against

I also checked our schematic against yours and I cannot see any differences apart from the fact we are using 3V and your schematic is based on 5V - but I don't think this should make any differences - and I am programming the correct voltage in the register in the TRF chip

Any other suggestions to try? 

Garry - 

maybe its better if you send me some screen shots of what you are doing and the schematic and layout just to triple check for you. 

if you are not comfortable posting on open forum, just send to josh.wyatt@ti.com 

Sorry for the late reply...I had been away.

I will try to send you the schematic when I can get hold of it from our hardware engineer

I noticed that the TRF7960A EVM board has around a 26V peak to peak signal at the start of the antenna track

On our board that voltage is around 7V peak to peak. Is this anything worth investigating? We are working with 3.3V instead of 5 but I cant think that could explain such a difference in this voltage. 

Garry - 

this means your antenna is most likely not tuned up correctly. 

on a +5VDC system, with register 0x00 set for 0x21, you will see ~9Vpp at the 50Ohm point and ~25-30Vpp on the coil itself. 

on a +3VDC system, with register 0x00 set for 0x20, you might see around 6-7Vpp at the 50Ohm point and 20-25Vpp on the coil - 

something in that range.

so what is the j + XL of your coil at 13.56MHz? (you can use a network analyzer or MFJ SWR/Inductance meter to measure that)