Voltage monitors include a predelay cell having an input responsive to a first clock signal. This cell is configured to generate a predelayed clock signal at an output thereof. A serially-connected string of data delay cells is provided, which has an input responsive to the predelayed clock signal. A serially-connected string of clock delay cells is provided, which has an input responsive to a second clock signal that is synchronized to the first clock signal. A plurality latches are provided. The latches have respective data inputs, which are responsive to first periodic signals generated at respective outputs of the serially-connected string of data delay cells, and respective clock/sync terminals, which are responsive to second periodic signals generated at respective outputs of the serially-connected string of clock delay cells. The latches enable loading of a delay code value, which indicates power supply voltage variation.