TCA9534A: a regular update of configuration register

Hello Takao-san,

"After I set the direction setting of I/O port,"

-You mean register 1, correct? Register 0 is a read only.

"if I write the same direction setting to Register 3 (Configuration Register) again during output or input control,

are there influences on the output signal or input signal?"

-Any bits which are Zero in register 3 will turn the port pin to an output. A 1 will result in an input. This is not the same as setting an output off or on. If you are trying to set the pins to all outputs, you should be writing 0000 0000 or 0x00h not the same values you put for register 1.

"I concern that the configuration register is inluenced by noise and changed its register setting.

To address this concern over, I think to update the configuration register regularly."

-The noise you are likely seeing is the port set as an input. This particular device does not use an internal pull up resistor and therefore will is just a high impedance floating node. The voltage on the port pins defined as inputs will be any value it picks up from external noise.

If you properly set the port to an output, you should see either the Vcc value for an 'on' or GND for 'off' output. Inputs will not behave like this and will be random.

Thanks,

-Bobby

Hello Takao-san,

1)  Assuming you are writing the same setting you should not see any changes in the output or input signal. 

2)  It is a good practice to periodically do reads/writes to critical registers.  I2C is a pretty robust protocol but it is possible to have noise corrupt bits and thus at the system level doing a periodic read and writes is a good idea.

Just curious, what is the environment of the device and what is the application? 

-Francis Houde