The NFCT peripheral implements an automatic collision resolution function as defined by the NFC Forum.
Automatic collision resolution is enabled by default, and it is recommended that the feature is used since it is power efficient and reduces the complexity of software handling the collision resolution sequence. This feature can be disabled through the MODE field in the AUTOCOLRESCONFIG register. When the automatic collision resolution is disabled, all commands will be sent over EasyDMA as defined in frame disassembler.
- NFCID1_LAST if NFCID1SIZE=NFCID1Single (ID = 4 bytes);
- NFCID1_2ND_LAST and NFCID1_LAST if NFCID1SIZE=NFCID1Double (ID = 7 bytes);
- NFCID1_3RD_LAST, NFCID1_2ND_LAST and NFCID1_LAST if NFCID1SIZE=NFCID1Triple (ID = 10 bytes);
A pre-defined set of registers, NFC.TAGHEADER0..3, containing a valid NFCID1 value, is available in FICR and can be used by software to populate the NFCID1_3RD_LAST, NFCID1_2ND_LAST, and NFCID1_LAST registers.
NFCID1 byte allocation (top sent first on air) explains the position of the ID bytes in NFCID1_3RD_LAST, NFCID1_2ND_LAST, and NFCID1_LAST, depending on the ID size, and as compared to the definition used in the NFC Forum, NFC Digital Protocol Technical Specification.
| ID = 4 bytes | ID = 7 bytes | ID = 10 bytes | |
|---|---|---|---|
| NFCID1.Q | nfcid10 | ||
| NFCID1.R | nfcid11 | ||
| NFCID1.S | nfcid12 | ||
| NFCID1.T | nfcid10 | nfcid13 | |
| NFCID1.U | nfcid11 | nfcid14 | |
| NFCID1.V | nfcid12 | nfcid15 | |
| NFCID1.W | nfcid10 | nfcid13 | nfcid16 |
| NFCID1.X | nfcid11 | nfcid14 | nfcid17 |
| NFCID1.Y | nfcid12 | nfcid15 | nfcid18 |
| NFCID1.Z | nfcid13 | nfcid16 | nfcid19 |
The hardware implementation can handle the states from IDLE to ACTIVE_A automatically as defined in the NFC Forum, NFC Activity Technical Specification, and the other states are to be handled by software. The software keeps track of the state through events. The collision resolution will trigger an AUTOCOLRESSTARTED event when it has started. Reaching the ACTIVE_A state is indicated by the SELECTED event.
If collision resolution fails, a COLLISION event is triggered. Note that errors occurring during automatic collision resolution may also cause ERROR and/or RXERROR events to be generated. Other events may also get generated. It is recommended that the software ignores any event except COLLISION, SELECTED and FIELDLOST during automatic collision resolution. Software shall also make sure that any unwanted SHORT or PPI shortcut is disabled during automatic collision resolution.
The automatic collision resolution will be restarted, if the packets are received with CRC or parity errors while in ACTIVE_A state. The automatic collision resolution feature can be disabled while in ACTIVE_A state to avoid this.
The SLP_REQ is automatically handled by the NFCT peripheral when the automatic collision resolution is enabled. However, this results in an ERROR event (with FRAMEDELAYTIMEOUT cause in ERRORSTATUS) since the SLP_REQ has no response. This error must be ignored until the SELECTED event is triggered and this error should be cleared by the software when the SELECTED event is triggered.