DocID029132 Rev 3 7/25
UM2042 Hardware
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2.2.1 USB connectivity and power supply
The ST25R3911B-DISCO board can be connected via USB to a host device.
Right behind the Micro-USB plug, an ECMF02 common mode filter with ESD protection for
USB
2.0 interface is placed. The common mode filter is connected to VBUS and to the
D+/D- data lines of the USB. All the USB terminals are ESD-protected by the built-in ESD
diodes of the ECMF02.
Additionally, the power supply filter for the 5 V supplies the 3.3 V LDO, which, in turn,
supplies the microcontroller.
A green LED is connected to the USB VBUS supply and shows the USB-power status.
2.2.2 STM32L4 MCU
This schematic includes the microcontroller needed to operate the NFC reader IC.
The MCU is clocked by a 32.768 kHz crystal. The MCU uses an internal PLL to generate the
48 MHz USB clock and the 80 MHz core clock from the provided 32.768 kHz.
Connector P201 is a JTAG/SWD interface to assist in software development. It can be
connected to an external ST-LINK/V2 device using a 9-pin Cortex
®
-M adapter.
The six blue LEDs (LED201 to LED206) show the current mode of communication. If
multiple protocols are active at the same time, then all corresponding LEDs are turned on.
The five resistors (R218 to R222) can be used to disconnect the MCU from SPI interface
header P204. This SPI cross-connect mode can be used to operate the ST25R3911B with a
different MCU PCB by cross connecting the ST25R3911B SPI to the second MCU PCB.
Connectors P202 and P203 can be used to connect an external UART or I2C host device to
the MCU.
2.2.3 ST25R3911B
The ST25R3911B is directly connected to the filtered 5 V USB supply, and there are
additional supply filtering components placed close to it. The jumper JP301 can be used to
measure the supply current of the chip. If this measurement is performed, the ferrite bead
L301 has to be removed.
Capacitors C301 to C304 provide additional filtering. All decoupling capacitors have been
placed as close as possible to the ST25R3911B. Special care has been taken for C310,
C302, C314 and C301, as they are the decoupling capacitors for the high power driver
stage.
In order to demonstrate the capacitive wake-up feature, two capacitive electrodes are
placed on the PCB. Additional electrodes can be connected using UFL connectors on P301
and P302.
For SPI cross-connecting to another reader PCB, the resistors R301 to R305 can be
removed. Then the STM32L4 MCU SPI can be connected to the ST25R3911B on any
customer PCB via header P204.
Thanks to J303 it is possible to bypass the internal VSP_RF regulator, this is required in
case output currents higher than 200 mA need to be supplied.
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