PCA9545ADGVT datasheet(7/28 Pages) TI

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RESET Input

Power-On Reset

Voltage Translation

Maximum

Typical

Minimum

VCC (V)

4.5

3.5

2.5

5.5

4.5

3.5

2.5

1.5

I2C Interface

PCA9545A

4-CHANNEL I2C AND SMBus SWITCH

WITH INTERRUPT LOGIC AND RESET FUNCTIONS

SCPS147C – OCTOBER 2005 – REVISED OCTOBER 2006

The RESET input can be used to recover the PCA9545A from a bus-fault condition. The registers and the I2C

state machine within this device initialize to their default states if this signal is asserted low for a minimum of tWL.

All channels also are deselected in this case. RESET must be connected to VCC through a pullup resistor.

When power is applied to VCC, an internal power-on reset holds the PCA9545A in a reset condition until VCC has

reached VPOR. At this point, the reset condition is released and the PCA9545A registers and I

2C state machine

are initialized to their default states, all zeroes, causing all the channels to be deselected. Thereafter, VCC must

be lowered below 0.2 V to reset the device.

The pass-gate transistors of the PCA9545A are constructed such that the VCC voltage can be used to limit the

maximum voltage that is passed from one I2C bus to another.

Figure 3 shows the voltage characteristics of the pass-gate transistors (note that the graph was generated using

data specified in the electrical characteristics section of this data sheet). In order for the PCA9545A to act as a

voltage translator, the Vpass voltage must be equal to or lower than the lowest bus voltage. For example, if the

main bus is running at 5 V and the downstream buses are 3.3 V and 2.7 V, Vpass must be equal to or below 2.7

V to effectively clamp the downstream bus voltages. As shown in Figure 3, Vpass (max) is at 2.7 V when the

PCA9545A supply voltage is 3.5 V or lower, so the PCA9545A supply voltage could be set to 3.3 V. Pullup

resistors then can be used to bring the bus voltages to their appropriate levels (see Figure 13).

Figure 3. Vpass Voltage vs VCC

The I2C bus is for two-way two-line communication between different ICs or modules. The two lines are a serial

data line (SDA) and a serial clock line (SCL). Both lines must be connected to a positive supply via a pullup

resistor when connected to the output stages of a device. Data transfer can be initiated only when the bus is not

busy.

One data bit is transferred during each clock pulse. The data on the SDA line must remain stable during the high

period of the clock pulse, as changes in the data line at this time are interpreted as control signals (see

Figure 4).

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