ST25R3911B datasheet(19/116 Pages) STMICROELECTRONICS

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Power-down support block

In the Power-down mode the regulators are disabled in order to save current. In this mode a low power

power

‑down support block that maintains the VSP_D and VSP_A in below 3.6 V is enabled. Typical regulated

voltage in this mode is 3.1 V at 5 V supply and 2.2 V at 3 V supply. When 3.3 V supply mode is set the Power-

down support block is disabled, its output is connected to VDD through 1 kΩ resistor.

Typical consumption of Power-down support block is 600 nA at 5 V supply.

Measurement of supply voltages

Using direct command Measure Power Supply it is possible to measure VDD and regulated voltages VSP_A,

VSP_D, and VSP_RF.

1.2.12

Communication with an external microcontroller

The ST25R3911B is a slave device and the external microcontroller initiates all communication. Communication is

performed by a 4-wire Serial Peripheral Interface (SPI). The ST25R3911B sends an interrupt request (pin IRQ) to

the microcontroller, which can use clock signal available on pin MCU_CLK when the oscillator is running.

Serial Peripheral Interface (SPI)

While signal /SS is high the SPI interface is in reset, while it is low the SPI is enabled. It is recommended to

keep /SS high whenever the SPI is not in use. MOSI is sampled at the falling edge of SCLK. All communication is

done in blocks of 8 bits (bytes). First two bits of first byte transmitted after high to low transition of /SS define SPI

operation mode.

Table 5. Serial data interface (4-wire interface) signal lines

Name

Signal

Signal level

Description

/SS

Digital input

CMOS

SPI Enable (active low)

MOSI

Digital input

Serial data input

MISO

Digital output with tristate

Serial data output

SCLK

Digital input

Clock for serial communication

MSB bit is always transmitted first (valid for address and data).

Read and Write modes support address auto-incrementing. This means that if some additional data bytes are

sent/read after the address and first data byte, they are written to/read from addresses incremented by ‘1’.

Figure 9 defines possible modes.

ST25R3911B

Application information

DS11793 - Rev 6

page 19/116

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