Modified Miller Coding
The bit encoding scheme used by NFC-A (ISO 14443-3A) for reader-to-tag communication. Transitions in the carrier signal at specific positions within each bit period encode the data.
What Is Modified Miller Coding?
Modified Miller codingModified Miller codingBit encodingencodingData writing to NFC tags during manufacturing productionView full → for NFC-A reader-to-tag communicationView full → is the bit-level encoding scheme used for reader-to-tag communication in NFC-A (ISO 14443 Type A). It defines how binary values are represented as transitions in the ASK-modulated 13.56 MHz carrier signal.
How It Works
Each bit occupies one period of 9.44 microseconds (128 carrier cycles). The period is divided into two halves. Data is encoded by pause position:
| Data Bit | Rule | Pause Position |
|---|---|---|
| Logic 1 | Pause in middle | Mid-bit transition |
| Logic 0 (after 1) | No pause | Carrier stays on |
| Logic 0 (after 0) | Pause at start | Start-of-bit transition |
A logic 1 always produces a mid-bit pause. A logic 0 depends on the preceding bit: after a 1, no pause; after another 0, the pause shifts to the bit period's start.
Why Modified Miller?
- Self-clocking: Guaranteed transitions allow the tag to recover clock synchronization from the data stream.
- DC balance: Reasonable distribution of carrier-on/off periods maintains stable power to passive tags.
- Noise resilience: Distinct pause positions (mid vs start) provide good bit discrimination.
Comparison with Manchester Coding
| Feature | Modified Miller | Manchester |
|---|---|---|
| Used by | NFC-A (ISO 14443ISO 14443Standard for contactless smart cards at 13.56 MHz (Types A and B)View full →-3A) | NFC-V (ISO 15693) |
| Transition per bit | 0 or 1 | Always 1 |
| Bandwidth efficiency | Higher | Lower |
Manchester guarantees one transition per bit (simpler clock recovery but more bandwidth). Modified Miller is more efficient because not every bit requires a transition.
Position in the Protocol Stack
Modified Miller operates at the lowest NFC-A layer (ISO 14443-2A), applied after the data frame (start-of-frame, data bytes, CRC) is assembled. Each byte is transmitted LSB first with parity bits.
Frame Delimiters
- Start of Frame (SoF): A logic 0 preceded by unmodulated carrier — a unique pattern that cannot appear in normal data.
- End of Frame (EoF): A logic 0 followed by unmodulated carrier.
Hardware Implementation
Modified Miller encoding and decoding are implemented entirely in hardware within the NFC controller (reader) and NFC chip front-end (tag). Application software never interacts with bit-level encoding. Understanding Modified Miller is primarily relevant for hardware engineers designing custom reader circuits and protocol analysts debugging physical-layer communication failures.
Related Terms
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