Pressure calibration for most diamond-anvil cell (DAC) experiments is mainly based on the ruby scale, which is key to implementing this powerful tool for high-pressure study. However, the ruby scale can often hardly be used for programmably controlled DAC devices, especially the piezoelectric-driving cells, where a continuous pressure calibration is required. In this work, we present an effective pressure gauge for DACs made of Manganin metal based on the four-probe resistivity measurements. Pressure dependence of its resistivity is well established and shows excellent linear relations in the 0-30 GPa pressure range with a slope of 23.4 (9) GPa for the first-cycle compression, in contrast to that of multiple-cycle compression and decompression having a nearly identical slope of 33.7 (4) GPa likely due to the strain effect. In addition, the such-established Manganin scale can be used for continuously monitoring the cell pressure of piezoelectric-driving DACs, and the reliability of this method is also verified by the fixed-point method with a Bi pressure standard. Realization of continuous pressure calibration for programmably controlled DACs would offer many opportunities for the study of dynamics, kinetics, and critical behaviors of pressure-induced phase transitions.