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TCA8418设备地址应该为0x68h,而不是0x34. 改0x68h试试能否收到ACK,因为一般7位地址左移一位。
CAD_INT是TCA8418 YFP封装的一个引脚,你的24脚是QFN封装的。
24pin的QFN封装的话,可以看INT引脚,低有效:
按下按键或松开按键INT都应该输出为低电平。