What are the differences between the "Read "/Write" and "Fast Read/Fast Write" RF commands?

[Jlevi.1]

For example the custom command `0xAD` is "Read" and `0xCD` is "Fast Read". How should I determine when to use the fast version versus the non-fast version?

How does this relate to the `data_rate_flag` in the `request_flags` byte of the RF request packet?

Note that when I try using the fast versions of commands using the ST25 demo app, I always get "Tag not found" error using several modern iphone and Android devices. This is true with the `data_rate_flasg` both set and clear. Is this expected? Do I need to do something on either the tag or the phone to enable these fast mode versions of commands?

[JL. Lebon · Accepted solution]

Hello,

The main difference betwween Read/Write and Fast Read/Fast Write command is the data rate of the tag's response.

To understand better, you have to understand how ISO15693 data rates work.

• There is 2 different data rates: one for the communication from the reader to the tag, and one for response from the tag to the reader. They are independent and the reader impose both of them.
• For the read to tag communication: the reader chooses between 1/4 or 1/256 data coding, which determine the data rate (1/256 = 1.66kbits/s, 1/4 = 26.48kbits/s).
• For the tag to reader response: the reader imposes the data rate of the response to the tag through the request flags' data_rate_flag bit (low data rate = 6.6kbits/s, high data rate = 26.6kbits/s).

This is for the standard ISO15693 data rates. (please note that NFC Forum uses only 1/4 (26.48kbits/s) for NFC-V readers and high data rate (26.6kbits/s) for Type5 tags).

"Fast" commands are custom ST25DV commands that uses a doubled data rate for tag to reader response. This is done by modifying the modulation of the response by sending half pulses (please see annex A of the datasheet). This gives a data rates for tag's response at 53.2kbits/s.

Please note that the data rate for the communication from reader to tag is not changed in fast commands, only the response is faster.

Those fast commands uses custom modulation for the tag's response. The consequence is that it is not supported by NFC controllers in smartphones. This is the reason why your android phone cannot decode the answer to the Fast Read command. A RFID reader (i.e. ST25R3911, CR95HF...) should be used to have those Fast read commands answers decoded.

Best regards.

[Jlevi.1]

This is an extremely helpful answer- everything I wanted to know. Thank you!