3.3v/5.0v selectable IO port expander

Question

I have designed a basic IO port expander that has uses a bi-directional voltage translator in the hope to have a voltage selectable IO to an external device. Voltages are most likely to be 3.3v / 5.0v.

Unforunately, the output from TXB0108PWR are at only at most half of the VCCB voltage, so if set at 5.0v I am getting 2.5v output. The output sometimes doesn't drop to 0 when the IO expander goes to 0v.

• I can confirm the PCF8574 output is correctly going between 0v and 3.3v when off/on.
• 3.3v from a power supply between IO expander and translator works as expected
• I have tried a 10k and 1.2k pullup between the devices to no change.

I would grealty appreciate some pointers, thanks.

Schematic below.

Answer 1

It's easy to be misled by 'voltage translator ICs', which unfortunately sound more convenient and simple than they actually are. Make sure you study the description of their internal circuit in the datasheet and the diagrams usually shown there.

The TXB0108 has 4 k\$\Omega\$ resistors in series with each of its outputs. These ICs are only really intended for driving further logic gates or other high-impedance loads.

If your circuit loads up the TXB0108 output, there will be a voltage drop across that internal series resistor that will cause the lower output voltage you are seeing at the IC pin.

Again, always study the internal description and circuit of these ICs. But the same approach is valuable for many ICs.

(Below diagram is modified from that in Texas Instruments TXB0108 datasheet).

Answer 2

The TXB0108 is a complex chip because it needs to automatically detect the direction which side is the input and which is the output.

The data sheet says if you intend to put pull-ups or pull-downs, they must be 50k or larger, so 10k or 1k2 will not work properly.

The PCF8574 also does not have simple inputs or outputs. Datasheet uses the word quasi-bidirectional. It has internal weak current source as pull-up and it can be less than 50k.

The chips just are not compatible.

Also it makes little sense to use the IO expander at 3.3V supply voltage and then level convert 4 to 8 lines bidirectionally.

It would make far more sense to level shift the I2C bus if required as it has only 2 lines, and set the IO expander voltage to either 3.3V or 5V so it has 8 IOs at correct voltage without a level shifter on the IO. What you are attempting to do might work with the PCF8547 expander directly, but if you change to another and better IO expander, then you can define the pins as inputs or outputs like GPIO on a MCU.