diff options
Diffstat (limited to 'rfid_layer2_iso14443b.c')
| -rw-r--r-- | rfid_layer2_iso14443b.c | 15 |
1 files changed, 14 insertions, 1 deletions
diff --git a/rfid_layer2_iso14443b.c b/rfid_layer2_iso14443b.c index 6207827..d36b5da 100644 --- a/rfid_layer2_iso14443b.c +++ b/rfid_layer2_iso14443b.c @@ -36,10 +36,23 @@ static inline int fwi_to_fwt(struct rfid_layer2_handle *h, unsigned int *fwt, unsigned int fwi) { + unsigned int multiplier; + + /* 15 is RFU */ if (fwi > 14) return -1; - return (256 * 16 / h->rh->ah->asic->fc) * 2 ^ fwi; + /* According to ISO 14443-3:200(E), Chapter 7.9.4.3, the forumala is + * (256 * 16 / fC) * 2^fwi We avoid floating point computations by + * shifting everything into the microsecond range. In integer + * calculations 1000000*256*16/13560000 evaluates to 302 (instead of + * 302.064897), which provides sufficient precision, IMHO. The max + * result is 302 * 16384 (4947968), which fits well within the 31/32 + * bit range of an integer */ + + multiplier = 1 << fwi; /* 2 to the power of fwi */ + + return (1000000 * 256 * 16 / h->rh->ah->asic->fc) * multiplier } static int |