AS3956 datasheet(16/109 Pages) AMSCO

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[v1-10] 2018-Apr-27

AS3956 − Detailed Description

VP_IO is connected to VDD only when AS3956 is not in the RF

field (rectified supply voltage is below HF_PON threshold) and

the SPI / I²C interface is activated by pulling /SS signal low. The

switch to VP_IO is controlled by /SS signal. The VP_IO

deactivation is delayed by TVP_IO_DD, so that the switch shall

stay closed in case of shorter times between successive SPI / I²C

activations. The switch is also closed during EEPROM writes

activated over SPI / I²C.

At activation of the switch, the time between the falling edge

of the /SS signal and rising edge of SCLK shall be at least TNCSL

minimum to allow charging of internal VDD buffer capacitor,

expiration of POR signal and to perform a complete IC

initialization. Please note that the only SPI / I²C operations,

which are allowed in this mode, are read and write of EEPROM

and registers.

If the RF field is lost during operation and the external system

(MCU) is supplied over battery and /SS is low then power

manager will automatically connect the VDD to VP_IO.

To enable low power mode where tag consumes ISB_SPI , the

following conditions must be met:

• SPI interface configured

• All SPI interface input lines (including /SS) must be set to

high

• All SPI output lines must be open

Power Mode 3 - External Supply Used to Power

EEPROM and Logic

In this case the external supply shall be used to provide power

to digital blocks, EEPROM, SPI / I²C pads and MCU. External

supply VP_REG is not used. Since this mode can be enabled only

after initialization of the chip, the /SS line must be either

permanently set to low or pulled low for short time to complete

the initialization. When reading from or writing to the EEPROM

via I2C/SPI it is recommended that the MCU should first check

whether there is enough energy available, then switch to Power

mode 3 and execute the SPI/I2C read or write and finally switch

back to the original power mode if required.

In this mode, the HF_PON threshold of the chip is set so that it

will operate at VHF_PON.

If this mode is enabled, AS3956 will not be turned off as long

as there is an external power present.

Power Mode 3 requires external power to be provided on

VP_IO. It is therefore very important to always power up in

Power Mode 0 or 1 (power mode set to PM 0 or 1 in EEPROM)

and then switch to Power Mode 3 by SPI/ I²C command only

when external power is available by writing to IC Configuration

off VP_IO, it is recommended to switch the chip back into Power

Mode 0 again by writing to IC Configuration Register 2.

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