Refresh timer for providing a constant refresh timer regardless of

Abstract

A refresh timer suitable for use in a semiconductor memory device which has a refresh mode of operation which is enabled by a refresh enable clock signal, and which utilizes an operating voltage having a plurality of different operating levels, Vi, where i=1-n, and n is an integer equal to or greater than two. The refresh timer includes a first circuit responsive to the operating voltage for generating a plurality of output voltage signals corresponding to the plurality of different operating voltage levels, respectively. The first circuit includes a plurality of individual voltage detection circuits corresponding to the plurality of different operating voltage levels, respectively, with each ith one of the voltage detection circuits being adapted to detect whether the operating voltage is at the corresponding ith level, and to drive the corresponding ith one of the output voltage signals high when it is detected that the operating voltage is at the corresponding ith level. The refresh timer also includes additional circuitry responsive to the refresh enable clock signal and the ith one of the output voltage signals which was last driven high by the first circuit, for generating a refresh cycle time control signal having a constant period, regardless of the level of the operating voltage.