TXB0108: outpuy PortB sometimes can't be pull down.

Hi all,

There is a problem confuse me . Could anyone give me some suggestions? 

The application is as follow:

1. VCCA=1.8V , VCCB=3.3V
2. Use GPIO to control pin10(portA.8) to generate a trigger signal.
3. Pin11(portB.8) is output, connect to module with a 100k ohm pull-up resistor.
4. The GPIO output mode can be set in pull-up mode or pull-down mode by software.

 GPIO configuration: pull down mode, drive current 2mA

 issue:  when the trigger signal become low, the output will be low (0.3V) or 1.5V (can't be pulled down).

             when configure the GPIO driver current 6mA or 8mA, the output can be pulled down to 0.3V all the time.

The datasheet said that input current must >2mA. Could you explain how to select drive current to ensure the output can be pulled down?

thanks a lot!