Seeking NFC chip with GP input; Q's about energy harvesting and i2c

[mcpublic]

1) Does ST make an NFC tag chip that has even one general-purpose, digital input detectable by the tag reader?

2) Can a ST25DV04KC's energy harvesting really power up an 8-bit MPU long enough to take a sensor reading and write back via i2c? The MPU would need to run at 4-8MHz for a very short period of time.

3) Is this i2c interface a "slave" only, or can it read an external i2c sensor under NFC reader control?

4) Does this NFC family "play nice" w.r.t. (avoiding) collisions between, say, 4-5 other devices within the EM field, so the tag reader can read them all within approximately 100ms? (all identical ST NFC tags, of course).

[Ulysses HERNIOSUS]

Hi mcpublic,

ad 4)
ST25R readers support detecting multiple tags. Please beware of the following aspects:

1. Detuning effects - multiple cards may load and detune the matching. So it may happen not all tags are powered sufficiently. You will need to do here experiments
2. Most technologies are supporting anti-collision (aka Collision Resolution activities). Our software implements the procedures prescribed by NFC Forum. In the forum you will find some discussions on deviating for better detecting multiple tags (search for "multiple tags" and look through the posts)
3. Anti-colliding 4-5 tags in 100ms should be achievable in the typical use case. However in bad conditions e.g all NFC-V tags colliding in the same slot may lead to a long anti-collision and a long time way beyond the mentioned 100ms.

I leave the other questions for my tag colleagues to answer.

BR, Ulysses

[JL. Lebon · Accepted solution]

Hello mcpublic, 
1) Does ST make an NFC tag chip that has even one general-purpose, digital input detectable by the tag reader?
=> Currently, ST portofolio only proposes NFC tags with general purpose output. Nevertheless, the ST25TV tag has a tamper detection input that may be used for your purpose: the state of the temper input is read at each boot (and only at boot).
2) Can a ST25DV04KC's energy harvesting really power up an 8-bit MPU long enough to take a sensor reading and write back via i2c? The MPU would need to run at 4-8MHz for a very short period of time.
=> Yes, it is possible, assuming the device is not consuming too much power and RF conditions allow a good transfer of energy. A low power version of the MCU is recommended. The power consumption should be limited under 10W if possible, max 20mW in best conditions (large tag antenna, powerful reader, good antenna coupling). For peak current surge, a small tank capacitor can be considered to absorb.
3) Is this i2c interface a "slave" only, or can it read an external i2c sensor under NFC reader control?
=> ST25DV is a slave only devices and can't read a sensor directly from NFC. A small MCU is required as the i2C master role.
Ulysses has already answered your fourth question.
Best regards.