The TXS02326 is a complete dual-supply standby Smart Identity Module (SIM) card solution for interfacing wireless baseband processors with two individual SIM subscriber cards to store data for mobile handset applications. It is a custom device which is used to extend a single SIM/UICC interface to be able to support two SIMs/UICCs.

This IC is an gift for the programmers as the device complies with :

  • ISO/IEC Smart-Card Interface requirements as well as GSM and 3G mobile standards.
  • A high-speed level translator capable of supporting Class-B (2.95 V) and Class-C (1.8 V) interfaces
  • Two low-dropout (LDO) voltage regulators
  • An integrated “fast-mode” 400 kb/s “slave” I2C control register interface
  • A 32-kHz clock input for internal timing generation
  • Battery pack removal for safe power-down the two SIM cards with two programmable debounce counter

The connection diagram is shown below:

The TXS02326 is controlled through a standard I2C interface reference to VDDIO. It is connected between the two SIM card slots and the SIM/UICC interface of the baseband. It has two basic states, the reset and operation state. The baseband utilizes information in the status registers to determine how to manipulate the control registers to properly switch between two SIM cards.

RESET STATE summary:

In the reset state the device settings are brought back to their default values and any SIM card that has been active is deactivated.

• Any pending interrupts are cleared
• I2C registers are in the default state
• BSI and SDN counter value in the registers are set to four clock cycles or “0000 0100”
• Both on chip regulators are set to 1.8V and disabled
• All SIM1 and SIM2 signals are pulled to GND

The interfacing diagram of baseband processor is shown below:


The TXS02326 supports both Class C (1.8V) or Class B (2.95V) SIM cards. In order to support these cards types, the interface on the SIM side needs to be properly setup. After power up, the system should default to SIM1 card. The following sequence outlines a rudimentary sequence of preparing the SIM1 card interface:
• Configure the SIM1 regulator to 1.8V by asserting B1 = 0 in the SIM Interface Control Register (08h). The system by default should start in 1.8V mode.
• Configure the OE signal by asserting B0 = 0 in the Device Control Register (0Eh). The default value essentially disables the OE pin and the device is configured as an auto direction translator.
• The baseband SIM interface is set to a LOW state.
• Disable the SIM1 interface by asserting B2 = 0 and B3 = 0 in the SIM Interface Control Register.
• Disable the SIM2 interface by asserting B6 = 0 and B7 = 0 in the SIM Interface Control Register.
• VSIM1 voltage regulator should now be activated by asserting B0 = 1 in the SIM Interface Control Register.
• Enable the SIM1 interface by asserting B2 = 1 and B3 = 1 in the SIM Interface Control Register.
• The SIM1 interface (VSIM1, SIM1CLK, SIM1I/O) is now active. The TXS02326 relies on the baseband to perform the power up sequencing of the SIM card. If there is lack of communication between the baseband and the SIM card, the SIM1 interface must be powered-down and then powered up again through the regulator by configuring it to 2.95V by asserting B1 = 1 in the SIM Interface Control Register.


The following sequence outlines a rudimentary sequence of switching between the SIM1 card and SIM2 card:

• Put the SIM1 card interface into “clock stop” mode then assert B2 = 1 and B3 = 0 in the SIM Interface Control Register (08h). This will latch the state of the SIM1 interface (SIM1CLK, SIM1I/O, SIM1RST).
• There can be two scenarios when switching to SIM2 card:

– SIM2 may be in the power off mode, B6 = 0 and B7 = 0 in the Status Register (04h). If SIM2 is in power off mode, the SIM/UICC interface will need to be set to the power off state. In this case the baseband will most likely need to go through a power up sequence iteration
– SIM2 may already be in the “clock stop” mode, B6 = 1 and B7 = 0 in the Status Register (04h). If SIM2 is in “clock stop” mode, the interface between the baseband and the device is set to the clock stop mode levels that correspond to the SIM2 card interface.
• After determining whether the SIM2 card is either in power off mode or clock stop mode, the SIM2 card interface is then activated by asserting B6 = 1 and B7 = 1 in the SIM Interface Control Register (08h) and the negotiation between the baseband and card can continue.
• Switching from SIM2 to SIM1 done in the same manner.


Both SIM card interfaces can be configured to automatically shut down upon disconnecting the battery. The shutdown threshold BSIThreshold is configured in B3 of the Device Control Register (0Eh). Two threshold levels are available for this configuration. When the BSI input level exceeds the BSIThreshold level that caused this power-down, both SIM card interfaces will automatically be shut down. If the battery removal interrupt is enabled through B2 of the Device Control Register, then an interrupt will be issued to the baseband on IRQ. This case may happen if the user decides to remove the battery. There are two scenarios for shutting down each SIM: SIMx is “active”, or in “clock stop” mode. In clock stop mode, when the debounce timer expires, the SIMx signals all go low immediately, then the regulator is disabled one 32KHz cycle later. If SIMx is active, the signals go low and the regulator is disabled in a particular sequence to be described in the next section.
The SIM2 interface can also be configured to automatically shut down via the SDN pin.