PN7642 RF Design-in Tips and Tricks
Prerequities:
PN7642 design-in recommendations
1// Impedance tuning
PN76 family antenna design guide
The target impedance is chosen based on the target application. If full power is required (e.g., POS terminals). The target impedance of 15-17 Ω is recommended.
For lower power applications using ULPCD, the higher impedance is typically preferred, 30-50 Ω (symmetrical tuning).
2// Dynamic power control
PN7642 - Basic RF power limitation using DPC
3// H-Field check
There are given limits, especially for the maximum H-field radiated by the reader. Exceeding these limits might lead to destroying the NFC Card/NFC Tag.
The H-Field can be measured with the help of test equipment, as
ISO 10373-6 Test PICC
EMVCo 3.0 Test PICC
For indication only, the customers can use "smart" Field Strength Probes as shown below :
Tomas_Parizek_0-1764148815270.jpeg
Note: The most critical position occurs when the card is placed directly on the NFC antenna . In this case, if the H-field exceeds the maximum allowed level, the output power must be reduced using DPC settings.
4// HF Attenuator value
Turn on the RF Field with the DPC set and enabled from the previous step
Read the CLIF_RXCTRL_STATUS register and check the HF_ATT_VAL as shown below.
Tomas_Parizek_1-1764147827990.png
The value for the "unloaded" condition with full power shall be approximately 35-45dec.
If the value is out of this range, the customer is required to adjust the Rx resistors to reach this value.
5// Receiver settings
Check the "Power" range and Communication Range with the default settings provided by NXP.
Power Range -> The distance at which the NFC Tag can still generate its answer, but the NFC Reader does not see it
Communication Range -> The distance at which the NFC Tag can still communitate with the NFC Reader
Ideally, Power Range ≈ Communication Range
Also, the NFC Reader should not generate any false communications as e.g., "HAL COLLISION ERROR".
The optimisation of the receiver can be done in the following way:
Enter DPC Calibration
Go to the "ARC" menu and "disable" the ARC algorithm
Tomas_Parizek_0-1764153827503.png
This will force the IC to use the RX settings from the following Register/EEPROM
SIGPRO_RM_TECH_REG
DGRM_RSSI_REG
5.1// SIGPRO_RM_TECH_REG (RM_MF_GAIN parameter)
This parameter basically defines the gain of the input amplifier.
Select SIGPRO_RM_TECH_REG
Switch "operation" to EEPROM and choose the required technology
Increase the RM_MF_GAIN to 0x02 (it depends on the setup).
Tomas_Parizek_0-1764154133976.png
5.2// DGRM_RSSI_REG (DGRM_SIGNAL_DETECT_TH_OVR_VAL parameter)
This parameter defines a threshold from which the internal logic starts to decode the incoming signal.
If the threshold is too low or very close to the noise floor, the system can detect the noise as an NFC Communication.
It is therefore, Threshold + margin > noise floor
The best routine is to perform "Signal Detection Threshold" analysis. This can be done with the help of the NFC Cockpit (described in PN7642 design-in recommendations)
As a result, the user can obtain the mean value of the "Noise," and suggested "DGRM_SIGNAL_DETECT_TH_OVR_VAL" threshold based on the inserted "Margin."
Maring (m) + Noise mean value (μ) = Threshold
6+16=23
Then this value shall be written in "DGRM_RSSI_REG" EEPROM as shown below.
Tomas_Parizek_0-1764155707250.png
6// ULPCD Settings
We recommend the following ULPCD Settings as a starting point.
ULPCD VDDPA should be chosen in such a way that the HF Attenuator value is not 0x00! The typical value for HF Attenuator in ULPCD is around 0x05-0x0B.
6.1// RSSI Threshold evaluation
For a proper RSSI Threshold selection, it is recommended to perform the ULPCD Calibration, e.g., 20 times, and check the "jitter" of the RSSI signal for your device.
Tomas_Parizek_1-1782466350613.png
If you see that the RSSI value is jittering, e.g., 1 unit as shown above. The absolute minimum threshold for this case is 2.
However, it is always recommended to include adequate margin (To prevent false wake-ups).
Generally, the margin of 2 units is sufficient. So in this case, the optimum threshold will be 4.
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