What is a dynamic tag, and when does a device need one?
A chip with two doors into the same memory: an NFC interface for the phone and a wired one, I2C or SPI, for a microcontroller. A device needs one when a phone has to read or change what the device holds, with no pairing, no app on the other side and, often, no power.
Two interfaces, one memory
A dynamic tag is a chip that answers a phone on one side and a microcontroller on the other, over the same bytes. TI's RF430CL330H datasheet describes one plainly: an NFC Type 4 tag with an ISO/IEC 14443 B interface up to 848 kbps, an SPI or I2C interface to read and write the NDEF message in 3 KB of internal memory, and a check that the structure written is valid NDEF.
The part that changes how code is written is the interrupt. The chip has a register and an output pin that report when a read or a write from the phone side has finished, so the firmware reacts to a tap instead of polling its own memory.
What it opens
The datasheet's own applications are the ordinary ones: Bluetooth pairing, Wi-Fi configuration, a diagnostic interface, a sensor interface. What they share is a device with no screen and no keyboard, and a phone that becomes both for the duration of a tap.
The handover pattern is the common one. The tag carries what the other radio needs to connect, the phone reads it and moves the conversation to Bluetooth or Wi-Fi, and NFC never carries the payload. The comparison page explains why that division is the right one.
What a plain tag cannot do
A plain tag holds what was written to it. The device it is glued to cannot change those bytes, cannot read them, and never learns that a phone came by: a sticker is a label, not a channel.
That is enough for a link, a serial number or a manual. It stops being enough as soon as the content depends on the device's state, which is the line between a printed label and an interface.
What to check on the datasheet
Which tag type the chip presents, because that decides which phones read it without an app: a Type 4 tag over ISO/IEC 14443 B is read by anything compliant, as the tag types page sets out. How much memory it exposes, since an NDEF message and its envelope have to fit, which the usable memory page measures.
Then the wired side: which bus, at which voltage, and whether the chip can be read by the microcontroller while a phone is talking to it. And whether it works with the device unpowered, the question the unpowered configuration page takes on, since that is what makes a tag useful in a box on a shelf.
Frequently asked questions
What makes a tag dynamic?
A wired interface into the same memory the phone reads, so a microcontroller can change the content between taps.
Does the phone need a special app?
Not if the chip presents a standard tag type and the content is NDEF. A Type 4 tag is read by any compliant phone.
How does the device know a phone read it?
Through the interrupt the chip raises when a read or a write finishes, on a register and a pin, rather than by polling the memory.
Can the microcontroller and the phone access it at once?
That is an arbitration question the datasheet answers per chip, and it is the detail to check before the design depends on it.