7.9.3 Low Power Card Detection (LPCD)
In the standard mode of operation, the RF field of the reader stays active and the reader is continuously polling for the cards. The basic idea behind the low power card detection
(LPCD) is to provide a functionality allowing to detect cards as soon as they placed in the proximity of the reader antenna, but without the RF field being permanently on. This mode
of operation saves the energy and allows for low power NFC reader designs. The LPCD mode is entered by issuing the LPCD command and comprises two phases: a
standby phase and a detection phase. The standby phase is controlled by the Wake-up Timer4. When the defined time interval elapses, the NCx3320e enters the detection
phase, during which short RF pulse is sent. During this phase, the values of I and Q channels are recorded and stored in the corresponding registers (LPCD_I_Result,
LPCD_Q_Result).
CPU给ncf3320发出LPCD命令让NFC芯片进入低功耗模式，NFC芯片的低功耗模式包括两个阶段：standby / detection，T4 timer控制standby的长短，
T4 timeout进入detection，NFC芯片发出RF脉冲，在这个阶段NFC芯片检测到I/Q两个通道的数值，并保存到两个寄存器LPCD_I_Result / LPCD_Q_Result中，
LPCD_I_Result / LPCD_Q_Result这两个寄存器都是只读的，也就是ncf3320写入的.
The duration of this phase is controlled by Timer3. The measured values are subsequently compared with the min/max limits, which have been stored in the
corresponding threshold registers (LPCD_IMin_IMax, LPCD_QMin_IMax, LPCD_QMax_IMax) during the calibration performed without the card. If the recorded
values exceed the defined limits, an LPCD interrupt is raised, the NCF3320 wakes up and activates normal polling procedure.
T3 timer控制了Detection的长短，detection检测到的I/Q通道值可以从LPCD_I_Result / LPCD_Q_Result读出，芯片自动比较这两个值和4个阀值(保存在3个寄存器中)
如果result值超出阀值就触发中断，然后是正常的polling。3个阀值寄存器可读可写，是在calibration过程中写入的，是谁写入的？
The amount of noise in the receiver of NCF3320 may vary due to environmental factors. This requires flexible solutions being able to combine a high detection sensitivity with a
robustness against false wake up. The presence of the high noise is indicated by LPCD_I_UNSTABLE and LPCD_Q_UNSTABLE bits of the LPCD_Ctrl register, which
are being set during the calibration phase. To address this instabilities, an additional digital filter can be activated by setting LPCD_FILTER bit within the LPCD_Ctrl register.
在calibration阶段，如果有较强的噪音那么LPCD_I_UNSTABLE/LPCD_Q_UNSTABLE这两个只读寄存器会被设置；为了解决不稳定，才设置LPCD_FILTER，
而不是设置了LPCD_FILTER=1以后UNSTABLE才valid?
This filter allows for using more than one sample for card detection, thus making the detection more robust against the noise. This filter allows for narrower detection windows
and for a higher detection range, respectively. In addition to the filter, it is also possible to activate the charge pump by setting LPCD_CHARGEPUMP bit within LPCD_Ctrl register.
The active charge pump allows for full power RF pulse and helps to boost the detection range.
LPCD_FILTER
The LPCD decision is based on the output of the LPCD filter (lower value out of 16 samples) instead of only 1 sample.
设置LPCD_FILTER可以提高检测精度？设置LPCD_CHARGEPUMP增加功耗强度
The LPCD configuration registers have to be configured before the LPCD mode is activated. It is also recommended to set T4AutoWakeUp and T4AutoRestart, start the
timer and then go into the standby. Once the card is detected, the timer stops and the communication will be started. If T4AutoWakeUp was not set, the IC will not enter the
standby mode when no card has been detected.
For a detailed description of the LPCD implementation please refer to the dedicated application note Ref. 2.
7.10.3.2
LPCD command
Command (01h);
This command performs a low power card detection and or an automatic trimming of the LFO. The values of the sampled I and Q channel are stored in the register map. The value
is compared with the min/max values in the register. If it exceeds the limits, an LPCD_IRQ will be raised. After the LPCD, the standby is activated if selected.
The amount of system noise may change due to the environmental condition and specific antenna configuration.
High system noise causes degradation of detection quality. To prevent that an additional digital filter can be activated.
The need for enabling the filter is indicated by two flags: I unstable and Q unstable (see register 3Ah).
LPCD_Ctrl register (address 0x3Ah)
LPCD_Q_UNSTABLE
Indicates that the Q-channel ADC value was changing during the LPCD measuring time.
LPCD_FILTER
The LPCD decision is based on the output of the LPCD filter (lower value out of 16 samples) instead of only 1 sample.
The LPCD calibration derives certain register values which correspond to the “no card” state.
After LPCD calibration the window values which correspond to the “no card” state must be written into the NCx3320,
the timer T3 and T4 must be set, and then the LPCD detection can be started.
During the LPCD detection the NCx3320 disables the host interface and runs the LPCD function as shown in Fig 3.
As soon as a card detunes the antenna from the “no card” state, the NCx3320 wakes up and sends an IRQ to wake up the host μC.
3.2 LPCD calibration
During the LPCD Calibration the NCx3320 defines the I- and Q-channel signal levels for the “no card” state,
and stores it in the Register 0x42 and 0x43:
Register 0x42: LPCD_Result_I register
Register 0x43: LPCD_Result_Q register
这两个寄存器都是只读，是NCF3320在calibration过程中得到的数值？
1.所谓的calibration，实际上就是在没有卡的情况下，执行LPCD，standby阶段结束，在RF扫描阶段，获得没有卡的I/Q通道值，保存到两个只读的result寄存器中；
2.