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This paper presents operating margin-oriented design methods for threshold element-based reconfigurable logic circuits (TREC) that can realize any symmetric function. The methods can generate circuits under restrictions that originate in electric and structural properties of physical threshold elements. The threshold element is implemented in a floating-gate circuit that calculates its weighted sum using the charge in the floating gate. The operating margins of the floating-gate circuit are an important factor for its reliable operation because of the use of multiple levels on the charge for the weighted sum. The proposed methods have the three main features. First, the methods guarantee that any symmetric function can always be realized independent of the number of input variables. A designed TREC comprising (k + 2) floating-gate inverters realizes one of 2(k + 1) symmetric functions of k input variables, using binary (k + 1) control variables. Secondly, a unique circuit structure is determined by selecting the proper values for the parameters that characterize the floating-gate inverters. These values maximize the operating margins against variations and fluctuations in both the charge and the floating-gate threshold voltage. Lastly, the operating margins are illustrated with a graphical representation. We can obtain knowledge not only of the relationship between the margins, but also of the design trade-off between circuit performances and the margins for reliable operation. To demonstrate the effectiveness of the methodology, the TREC was designed and evaluated by circuit simulation. The proper operations necessary for realizing or changing any symmetric function were confirmed.