crewID/libfreefare
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Don't keep unsupported tags selected.

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Currently, when the libfreefare finds a NFC target, it checks whether it's a
supported one looking at it's ATQA.

If the target's ATQA is not in the supported ATQA list, the target was not
deselected.  While this is buggy, it allowed to detect 2 tags with colliding
ATQA on a single NFC device and still have the ATQA of each tag (on ATQA
collision, the ATQA is set to 0x0000 by the NFC chip, but as the tag is not
deselected, it will have a second chance to tell it's ATQA when it will be idle
again):

 computer             PCD                   PICC1   PICC2
    |                  |             ATQA:  0x1234  0x6789
    |                  |                      |       |
    |- InListTargets ->|                      |       |
    |                  |------ REQA ----->(===============)
    |                  |<---- ATQA 1234 ------|       |      } Both reply
    |                  |<---- ATQA 6789 --------------|      } simultaneously
    |                  |                      S       |
    |                  | // Collision detected PICC1 only is selected because
    |                  | // only a single tag was wanted.  It's ATQA is set
    |                  | // to 0x0000.
    |<-- ATQA 0x0000 --|                      S       |
    | // Unsuported ATQA, skipped             S       |
    |                  |                      S       |
    | // DESELECT IS MISSING HERE!            S       |
    |                  |                      S       |
    |- InListTargets ->|                      S       |
    |                  |------ REQA ----->(===============)
    |                  |<---- ATQA 6789 --------------S // PICC1 won't reply
    |<-- ATQA 0x6789 --|                      |       S // it is still selected
    | // Supported, deselect it               |       S
    |--- InDeselect -->|                      |       S
    |                  |-------- HALT --------------->X
    |                  |                      |       X
    |- InListTargets ->|                      |       X
    |                  |------ REQA ----->(===============)
    |                  |<---- ATQA 1234 ------S       X // PICC2 won't reply
    |<-- ATQA 0x1234 --|                      S       X // it is deselected
    | // Supported, deselect it               S       X
    |--- InDeselect -->|                      S       X
    |                  |-------- HALT ------->X       X
    |                  |                      X       X
    |--- InDeselect -->|                      X       X
    |                  |------ REQA ----->(===============)
    |                  |                      X       X // No reply
    |                  |                      X       X

                                          Tag timeline legend:
                                            | - Target idle
                                            S - Target selected
                                            X - Target deselected

Since PN53x devices can only known about 2 tags simultaneously, presenting 3
targets to the NFC device caused an infinite loop.

This commit prevents this infinite loop to occur, and will be completed later
by another commit for not relying on the ATQA to determine targets type.
This commit is contained in:
Romain Tartiere 2010-08-04 12:08:38 +00:00
parent 88a4cbbe53
commit bef4ec342f
1 changed files with 2 additions and 1 deletions
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3
libfreefare/freefare.c
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@ -90,7 +90,7 @@ freefare_get_tags (nfc_device_t *device)
}
if (!found)
continue;
goto deselect;
tag_count++;
@ -128,6 +128,7 @@ freefare_get_tags (nfc_device_t *device)
(tags[tag_count-1])->tag_info = tag_info;
tags[tag_count] = NULL;
deselect:
nfc_initiator_deselect_target (device);
}