Help needed again ! Taming a PCA9534A port extender :)

Thread Starter


Joined Jul 30, 2013
Hello again,

I recently started a new thread about interfacing a PCF8574 with transistors to control solenoid valves (see here:

So far so good. Throughout the thread I explained that I chose to use a PCA9534A portextender (

Now, I hooked up the device and without doing anything to it (meaning writing any commands over the I2C bus to the device) all output pins should be pulled to 5 V, correct (datasheet states on page 8 that power on reset all pins will be pulled to 1 meaning 5 V if I interpreted the datasheet correctly) ?

What I observe now is that the pins have around 2 V and I was wondering if somebody could explain what I am doing wrong, if I am doing something wrong ?

Some more infos about the connections of the chip:

- Pin 8 is connected to ground
- Pin 16 is connected to 5 V
- all address pins are pulled to ground and the output pins are connected to various devices (4050 level shifter and transistors)
- INT pin is not connected

On the TI website one can find an errata to the datasheet of the chip ( which talks about the power on reset and that if the specifications are not met then the chip goes into some kind of locked state. I was wondering if that could be the issue that I couldn't read the 5 V on the output pins on power up ?

Or could it be the issue that in the control register all bits are pulled to 1 which defines the ouput pins as inputs (compare page 8 on datasheet, table 5, power up default for the control register) ?

As always any help is greatly appreciated.



PS: If I have forgotten to supply any infos necessary to wrap ones head around the issue then I would be more than happy to supply it.


Joined Jul 22, 2007
Which microcontroller are you using to talk to it? Do you have an I2C bus analyzer or any way to watch the bus?

From the 9534A data sheet
The PCA9534A is pin-to-pin and I2C address compatible with the PCF8574A. However, software changes are
required due to the enhancements in the PCA9534A over the PCF8574A.

Thread Starter


Joined Jul 30, 2013
I solved the problem myself.

The configuration register of the chip has all bits set to high on power up which configures them as input pins. simply by puling those register bits all to low made the chip work.