Input Paper nfc-a collision Resolution Activity
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- Background of this Input Paper
- Requested Action
- Proposed Correction
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Title: |
Correction to NFC-A Collision Resolution Activity |
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To: |
Mode Switching Task Force | |
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Submitting Companies: |
ST-Ericsson Asia Pacific Pte Ltd | |
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Contact Details: |
Jafer Hussain: jafer.hussain@stericsson.com | |
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Date: |
4th September, 2012 | |
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Replaces: |
N.A. |
This input paper discusses an error found in the NFC-A Collision Resolution Activity and proposes a correction.
Background of this Input Paper
[ACTIVITY 1.0] section 9.3.4 describes the Flow Chart and Requirements for NFC-A Collision Resolution Activity. It can be shown that it results in an incorrect operation when collision occurs at the last bit of NFCID1 CLn (where n=1, 2 or 3) in an SDD_RES Response. The text “incorrect operation” means “No device is resolved”.
IPR
Input in this document is subject to the Intellectual Property Rights Policy of NFC Forum, Inc which may be found at http://www.nfc-forum.org/members/legaldocs/NFC_Forum_IPR_Policy.pdf. A member who is submitting technology for inclusion in a Specification must confirm that all controlled necessary claims included in this submission will be licensed on RAND conditions.
Requested Action
The submitter requests the Mode Switching Task Force to consider the proposed correction to the NFC-A Collision Resolution Activity for inclusion in the next version of [ACTIVITY].
Submission Details
Assume that there is an NFC-A polling device and two NFC-A listening devices with bit-frame anti-collision support (e.g. T2T, T4AT).
Assume that the value of CON_DEVICES_LIMIT is set to 1 (for simplicity).
Assume that both the NFC-A listening devices have a unique single-size NFCID1 with same value of the first 31 bits except the LAST bit. i.e. last bit of NFCID13. The last bit of NFCID1 of NFC-A Tag 1 is 1b, and that of NFC-A Tag 2 is 0b.
During the NFC-A Collision Resolution Activity, NFC-A polling device transmits SDD_REQ CL1 Command with value of SEL_PAR set to 0x20. As both the NFC-A listening devices respond with SDD_RES Response, NFC-A polling device receives SDD_RES Response with collision at the last bit of NFCID1 as per symbol 7 of the NFC-A Collision Resolution Activity.
As a result, the value of INT_COLL_PEND will be set to 1 as per symbol 8 and the NFC Forum Device will proceed to symbol 10 in order to specify the number of valid bits for SEL_PAR.
It is important to note that the maximum value of SEL_PAR is 55 (bits) as follows:
1 byte : SEL_CMD
1 byte : SEL_PAR
3 bytes : NFCID10, NFCID11 and NFCID12
7 bits : NFCID13
1 byte : BCC
Since the collision has occurred at the last bit of NFCID1, it is NOT possible to use the SDD_REQ CL1 Command because the number of valid bits is now 56. SDD_REQ CL1 Command with 56 valid bits (i.e. SEL_PAR = 0x70) actually represents the SEL_REQ CL1 Command. But as per symbol 12, SEL_REQ Command is sent ONLY if there is NO collision in the SDD_RES Response which is a wrong assumption, and is not valid for this scenario when collision occurs at the last bit. In this situation, it will generate an error condition for the NFC-A polling device and thus results in an incorrect operation. i.e. No device is resolved.
This scenario is captured in the following figure.
NFC-A Tag 2
NFC-A Tag 1
Polling Device
Technology Detection Activity
Generate RF field
IDLE
IDLE
SENS_REQ
Collision resolution supported.
Assume similar SENS_RES
SENS_RES Tag 2
SENS_RES Tag 1
Collision Resolution Activity
y
CON_ANTICOLL := 1
INT_COLL_PEND := 0
CON_DEVICES_LIMIT := 1
READY_A
READY_A
SDD_REQ CL1 (SEL_PAR = 0x20)
Collision at the LAST bit of NFCID1 CL1
SDD_RES Tag 2
SDD_RES Tag 1
Number of valid bits: 56
CANNOT send SDD_REQ CL1 Command
SEL_REQ CL1 Command is NOT sent
ERROR condition
NO device resolved
Proposed Correction
CON_ANTICOLL := 1
INT_COLL_PEND := 0
CON_DEVICES_LIMIT := 1
Collision resolution supported.
Assume similar SENS_RES
Collision at the LAST bit of NFCID1 CL1
IDLE
ACTIVE_A
NFC-A Tag 1 successfully resolved
NFC-A Tag 1 identified.
device_counter := 1
Continue …
SEL_RES Tag 1 (NFCID1 complete)
SEL_REQ CL1
Number of valid bits: 56
Send SEL_REQ CL1 Command
Technology Detection Activity
NFC-A Tag 2
IDLE
NFC-A Tag 1
Polling Device
SENS_REQ
SENS_RES Tag 1
SENS_RES Tag 2
SDD_REQ CL1 (SEL_PAR = 0x20)
READY_A
READY_A
IDLE
SDD_RES Tag 1
SDD_RES Tag 2
Collision Resolution Activity
y
Generate RF field
The logic behind the proposed correction is that when the NFC-A polling device receives SDD_RES Response with collision at the LAST bit of NFCID1 CLn (where n=1, 2, or 3), it MUST send the SEL_REQ CLn Command (where n=1, 2, or 3) with last bit of NFCID1 CLn (where n=1, 2, or 3) set to 1b.
Below are the changes to be made in the ACTIVITY Specification in order to implement the proposed correction:
9.3.4 Flow Chart and Requirements for NFC-A
Changes to Figure 7 and Requirements 25 are highlighted in red below.
Requirements 25: Collision Resolution Activity – NFC-A
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The sequence number of Symbol 11 and onward must be incremented by 1 such that Symbol 11 is changed to Symbol 12, Symbol 12 is changed to Symbol 13, and so on. -
Addition of New Symbol 11.
New Symbol 11:
The NFC Forum Device MUST proceed to Symbol 15 if the number of valid bits is equal to 56.
Otherwise, the NFC Forum Device MUST proceed to Symbol 12.
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