Tool/software:
The following is stated in section 8.4 of the document SLLSEN2B.
A strongly driven input signal can weakly power the floating VCC through an internal protection diode and cause undetermined output.
What exactly is a strongly driven input signal?
Is it a voltage (signal) higher than VCC or a high frequency signal?
Hi Wada-san,
Thank you for reaching out and for showing interest in ISO7840.
This referring to a situation where VCC is floating, that is VCC is unpowered. In such a situation, if INx pin is supplied with a signal which can source sufficient amount of current required by VCC, then it can power the device through an internal ESD bypass diode. While the device is powered through the input, the device might not operate reliably due to the unconventional operation of device.
e.g., please consider the below I/O schematic from ISO7840 datasheet. If VCCI is floating and INx is applied with a signal that also support the current required by VCCI, then the INX pin can power VCCI through the diode highlighted in below picture. This can make the device power-up even through VCCI is not supplied with external power supply while the operation might not be reliable, therefore, the output can be undetermined.
Let me know if you have any further questions, thanks.
Regards,
Koteshwar Rao
