The power-on reset voltage is confidential, and Cypress doesn’t guarantee the actual POR level. So please keep spec of Power up cycle.
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The following is described in the device data sheet.
"If during operation the power supply drops below the specifiedVDD minimum,
the system should issue a START condition prior to performing another operation."
It can be read when the reset condition occurs when the voltage falls below the minimum VDD voltage ( =4.5V ).