ST25R3920B datasheet(49/158 Pages) STMICROELECTRONICS

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DS13890 Rev 4

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ST25R3920B

Application information

157

4.3.2

Communication interface selection

The active communication interface is selected via the I2C_EN pin. If this pin is pulled to

GND, the ST25R3920B operates in SPI mode. If this pin is pulled to VDD_D, the

ST25R3920B operate in I2C mode.

4.3.3

Serial peripheral interface (SPI)

The ST25R3920B hasve a standard serial peripheral interface with clock polarity of 0, a

clock phase of 1, and an active low slave select signal. Communication starts with the MCU

pulling BSS low. The MOSI pin is samples on the falling edge of SCLK, and the state of the

MISO pin is updated on the rising edge of the SCLK signal. Data is transferred byte-wise,

most significant bit first. Read and Write commands support an address auto increment to

reduce communication time. Table 9 provides an overview of the SPI signals.

The MISO output is in tristate as long as no output data is available. Due to this the MOSI

and the MISO can be externally shorted to create a three-wire SPI. During the time the

MISO output is in tristate, it is also possible to switch on a 10 kΩ pull down by activating

option bits miso_pd1 and miso_pd2 in the IO configuration register 2.

Figure 12. Exchange of signals with a microcontroller

Table 9. Serial data interface (4-wire interface) signal lines

Name

Signal

Signal level

Description

I2C_EN

Digital input

CMOS

Pull to GND for SPI operation

BSS

Digital input

Active low - Slave select

MOSI

Digital input

Master out - Slave in (MCU → ST25R3920B)

MISO

Digital output with tristate

Master in - Slave out (ST25R3920B → MCU)

SCLK

Digital input

Serial clock

IRQ

Digital output

Active high - Interrupt output pin

MS69290V1

MOSI

MISO

I/O

MOSI

MISO

MOSI

Separate SPI input and

output signals to MCU

Bidirectional data

IO signal to MCU

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