TagTechnology定义了所有标签的共有接口类
BasicTagTechnology 实现了TagTechnology的一些接口
再有具体的标签协议继承BasicTagTechnology
NFC-A 遵循ISO 14443-3A协议。
关键字
ATQA Answer To Request acc. to ISO/IEC 14443-4
ATS Answer To Select acc. to ISO/IEC 14443-4
DIF Dual Interface (cards)
COS Card Operating System
CL Cascade Level acc. to ISO/IEC 14443-3
CT Cascade Tag, Type A
n.a. not applicable
NFC Near Field Communication
PCD Proximity Coupling Device (“Contactless Reader”)
PICC Proximity Integrated Circuit (“Contactless Card”)
PKE Public Key Encryption (like RSA or ECC)
REQA Request Command, Type A
SAK Select Acknowledge, Type A
Select Select Command, Type A
RID Random ID, typically dynamically generated at Power-on Reset
RFU Reserved for future use
UID Unique Identifier, Type A
NUID Non-Unique Identifier
当读卡器检测到了卡片就会发送REQA请求命令，在感应区内的卡片接收到REQA，将会反馈ATQA消息。
SAK的第六位表明PICC卡是否符合ISO/IEC14443-4 标准。
The bit numbering in the ISO/IEC 14443 starts with LSBit = bit 1, but not LSBit =bit 0. So one byte counts bit 1...8 instead of bit 0...7.
The ISO/IEC 14443 transfers LSByte first. So e.g. 0x 00 44 (ATQA of the MFUL) is often received as 0x 44 00.
ATQA ISO/IEC 14443-3 协议标准
ATQA Coding of NXP Contactless Card ICs
SAK coding of NXP Contactless Card ICs
如下图可以根据ATQA，SAK，ATS判断厂家和产品的类型
举例
<string name="tag_000409">MIFARE Mini, NXP</string>
<string name="tag_000408">MIFARE Classic 1k, NXP</string>
<string name="tag_000218">MIFARE Classic 4k, NXP</string>
<string name="tag_004218">MIFARE Classic 4k, NXP</string>
<string name="tag_004400">MIFARE Ultralight, NXP</string>
<string name="tag_000488">MIFARE Classic 1K, Infineon</string>
getResources().getIdentifier(prefix + atqa + sak + ats, "string", getPackageName())
NfcA get(Tag tag)
获取NFCA一个实例，首先判断
tag.hasTech(TagTechnology.NFC_A)
这个TAG是否是NFC_A TAG。
然后才生成NFC A TAG实例
getAtqa()
读写器呼叫磁场内的卡片，卡片对呼叫做出应答。收到卡呼叫命令后，卡片将对命令做出应答(Answer To Request,ATQA),
告诉读写器自己是否遵守面向比特的防冲突机制，
ATQA/SENS_RES
这里获取卡片应答的消息。
举例：用来判断是否是Mifare Classic tag
NfcA nfca = NfcA.get(tag);
byte[] atqa = nfca.getAtqa();
if (atqa[1] == 0 &&(atqa[0] == 4 || atqa[0] == (byte)0x44 ||atqa[0] == 2 || atqa[0] == (byte)0x42))
getSak()
SAK/SEL_RES
返回选择命令的信息
举例：
byte sak = (byte)nfca.getSak();
if (sak == 8 || sak == 9 || sak == (byte)0x18 ||sak == (byte)0x88)
Transceive()
Send raw NFC-A commands to the tag and receive the response.
发送命令到TAG，并接收返回值。
注意：
1：发送命令时不用增加CRC，因为系统会自动加。
2：发送命令是完整的字节。SENS_REQ3：因为是I/O操作，会发生阻塞，不用在主线程调用这个函数。
getMaxTransceiveLength()
获取最大的发送字节长度
参考文档
NfcA.java
http://www.nxp.com/documents/application_note/AN10833.pdf
http://nfc-tools.org/index.php?title=ISO14443A
时间
起草与2015-10-15