PCF8574: PCF8574 IO Port Issue

What pins are the PA LED and the reset line connected to? What are the exact values that the SOM is writing to the PCF?

Gu HaiLin,

In your configuration, the RESET on the AT42QT1070 is controlled by the PCF8574 IO expander. How are you controlling the OUTPUT on the PCF8574 in your code? 10 ms is a long time to hold the negative pulse, which is telling me that the PCF8574 is being configured to output LOW on the reset line. 

Regards,

Tyler

PA LED is connected to P0,

Reset is connected to P6.

The negative pulse is not 10ms, it is 0.2ms.

We are sending HIGH (1) or LOW (0) to all port (P0-P7) of PCF8574 to change the output state of that port.

    For example, to keep P6 & P7 HIGH (1) and all other ports to LOW (0), we send value 0xC0 (1100 0000) to PCF8574.

The only explanation for this problem is that some byte with P6 low is sent to the PCF. This is likely to be a software issue.

Please check all the bytes sent to the PCF. If possible, take an oscilloscope trace of the SCL/SDA lines just before the reset line falsely goes low.

Could you give me idea how to set the P6 as input port?

Thanks

The PCF8574 pins are quasi-bidirectional. When a port bit is set to high, then the pin is pulled up weakly, and external devices can override the voltage. In other words, when P6 was written as high, then you can simply read from the PCF to check the actual voltage.