Can your own device act as a tag for a phone?
Three routes, and they fail differently. A dynamic tag wired to the microcontroller, which answers even when the device is off. A reader chip that also emulates a card, which needs the device powered. Or a plain tag carrying an address, with another radio doing the work.
Route one, a dynamic tag
The dynamic tag is the route with the fewest conditions. The chip presents a standard tag type to the phone, the microcontroller writes the content over I2C or SPI, and the phone reads it whether the device is running, asleep or unplugged, since the field powers the tag.
Its limit is that the content is whatever the firmware last wrote. A device that has been off for a month answers with the state it had a month ago, which is right for a configuration and wrong for a measurement.
Route two, a chip that emulates
A reader transceiver can play the other role. TI's TRF7970A datasheet lists card emulation beside reader and peer to peer operation, an integrated anticollision state machine for ISO/IEC 14443 A transponder emulation, and a field detector with programmable wake up levels made for passive target operation: the chip sleeps until a phone's field appears, then answers.
Two conditions come with it. The device has to be powered, because the chip draws from the board rather than from the field, and the answers have to be written: emulating a Type 4 tag means answering the commands the Type 4 page lists, the same work an Android service does in software on the card emulation page.
What it buys is a fresh answer at each tap, computed at that moment, which no stored tag can do.
Route three, hand over to another radio
The third route stops trying to carry the exchange over NFC. The tag holds an address, an identifier or the parameters of another link, and the phone moves to Bluetooth or Wi-Fi for anything longer, the division the comparison page draws.
It is the cheapest of the three, and the only one that works when the exchange is more than a few hundred bytes: a tap decides who talks to whom, and a radio built for throughput does the talking.
Choosing between them
Ask what has to be true when the phone arrives. If the device may be off or flat, only the dynamic tag answers. If the answer has to differ at every tap, the emulating chip is the one. If what follows the tap is a file, a stream or a session, plan the handover and let NFC do the introduction.
Then ask what the phone side needs. A standard tag type is read by any phone with no application, while a proprietary exchange means writing an app on both sides, and on iPhone the card emulation page shows how narrow that road is. The phone's constraints usually decide this before the board does.
Frequently asked questions
Can a device answer a phone while it is switched off?
Only through a tag the phone powers, such as a dynamic tag. A chip that emulates draws its power from the board, so the device has to be running.
Does an emulating device need an app on the phone?
Not if it emulates a standard tag type and answers with NDEF. A proprietary exchange needs an app on both sides.
Which route gives a different answer at every tap?
The emulating chip, which computes its answer when the reader asks. A dynamic tag answers with what the firmware last wrote.
What if the exchange is large?
Hand it to another radio. NFC introduces the two devices, and Bluetooth or Wi-Fi carries what follows.