US Patent 7551497 Memory circuits preventing false programming

Memory circuits capable of preventing false programming caused by power-up sequence are provided, in which a programmable unit comprises a plurality of programmable elements, a source bus coupled between an external programming voltage and the programmable elements, a switching unit connected between the external programming voltage and the source bus, comprising a control terminal, and a level shifter, shifting a voltage level of an enabling signal to a first power voltage from a second power voltage lower than the external programming voltage. When the second power voltage is not ready during power up, the level shifter sets the control terminal of the switching unit to a predetermined logic level such that the switching unit is turned off and the source bus is disconnected from the external programming voltage thereby preventing false programming.