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Please have a look: cold and warm reset
And it is not necessary that the ATR is always be different when cold to warm. It will different when you changed the card contents or some application inside card do it.
many thanks for your attention, by the way it's better that I categorize my question to clear the air:
1- I found that there are many cards which in cold reset and warm reset return same ATR, and also there are many cards which return different ATR in cold reset and warm reset, so why does it happen? why some cards have 2 ATR? what are the benefits when we get 2 different ATR in cold reset and warm reset?
2- I found that after warm reset Bit Duration Clock Cycles and Work Waiting Time will change, why is that? does the card work slower after warm reset?
To start with ATR (Answer To Reset) mainly contains information for the host for further communication with the card after the first reset. This includes the maximum communication speed, supported contact based protocol. It also consists of 'Historical Bytes' which has some OS specific information. For example an OS provider may provide information like the current card state, Chip Type used, the IC Fabricator etc.
Mostly the warm and cold reset will have the same value.
As per ISO7816 there are 2 supported modes of communication:
1. Negotiable mode: Here the ATR indicates the maximum communication speed. Hence the host can communicate with the card using speed not exceeding the mentioned limit
2. Specific Mode: Here the ATR indicates the only communication speed to be used. Hence the host should use this speed to communicate with the card.
This mode of communication might be a requirement for few domain like Banking.
There are smartcard available that support these 2 modes and on COLD reset the specific mode gets activated and this is indicated in the ATR bytes. (TA2)
Warm reset will always be available and the byte TA2 will not be present in it.
This is one difference as per my knowledge.