Question about NFC library


I'm using Seed Studio NFC library with my PN532 board and it's working fine. The problem is that in example, the function that 'ask' for cards has a timeout (default = 1 second) and this is a problem for my loop.
This is the code that i'm using:

success = nfc.readPassiveTargetID(PN532_MIFARE_ISO14443A, uid, &uidLength, 500); // 500 mS

The problem is that my loop is delayed at least 500mS. I know that I can change this to small value, but even with small value this could be a problem. So my question is: since there's a firmware in the PN532 board, is there any way to check if a card is 'present' without using read function?
Something like 'available()' of Serial interface.

BTW, I'm using in HSU mode.