If the HLVD or other circuits using the internal voltage reference are disabled to lower the device’s current consumption, the reference voltage circuit will require time to become stable before a low or high-voltage condition can be reliably detected. This start-up time, TFVRST, is an interval that is independent of device clock speed. It is specified in electrical specification.
The HLVD interrupt flag is not enabled until TFVRST has expired and a stable reference voltage is reached. For this reason, brief excursions beyond the set point may not be detected during this interval (see the figures below).
Figure 1. Low-Voltage Detect Operation (INTL =
1)
Figure 2. High-Voltage Detect Operation (INTH =
1)