Design of Ternary Schmitt Triggers Based on Its Sequential Characteristics

By analyzing the working characteristics of binary Schmitt circuits we find their sequential characteristics, which makes us follow the method of sequential circuits to design Schmitt circuits. The sequential design technique is extended to the study of ternary Schmitt circuits in this paper. The designed ternary Schmitt trigger has a similar structure with a ternary flip-flop. PSPICE simulation with TTL technology shows that the circuit has correct Schmitt characteristics. 1. Sequential characteristic analysis of Schmitt circuits In the traditional binary digital circuits, Schmitt circuit is a useful device. One of its important characteristics is that it has different thresholds for the positive-going and negative-going direct current (DC) transmission characteristics, and the difference between them is called hysteresis. Such a trigger scheme makes it capable of receiving slowly varying input signals and effectively suppressing the interference imposed on the input signal. In digital circuits, Schmitt circuit is used as an effective device to reshape input signals. Obviously, in the multiple-valued logic, multiple-valued Schmitt trigger should hold its corresponding position and usage as its binary counterpart. Studying multiple-valued Schmitt circuits is helpful for the completeness of the multiple valued logic study. In recent years, some research progress has been made on the design of multiple-valued Schmitt circuits based on TTL, CMOS, ECL, etc. However, it is still lack of the study of their design principle. Therefore, the research should start from the binary Schmitt circuits. Traditionally, Schmitt circuit is thought of as a special gate, which has threshold-skipping effect. Taking a binary Schmitt circuit for example, it is different from a general gate, which has only one threshold (VTH) to detect the input signal. Schmitt circuit shows that there are two thresholds, VT+ and VT– Figures 1(a) and (b) show the different responses to a triangular wave input signal for a general inverter and a Schmitt inverter, respectively. Fig. 1 (a) General ternary reshaping circuit (b) Ternary Schmitt reshaping circuit From Fig.1 (b), it is noted that when the input signal increases, the threshold of the circuit is VT+, however, when the input signal exceeds VT+ and enters the high level region S1, the threshold immediately skips from VT+ to VT–; vice versa, when the input signal decreases to VT– and enters low level region S0, the detecting threshold of the circuit skips from VT– to VT+ again. Traditionally, Schmitt circuit is classified as a special combinational gate. However, we believe that Schmitt circuits are of an obvious sequential characteristic, because when the input signal Vi is between VT+ and VT–, the circuit level completely depends on whether the circuit is in high level or low level before entering the region, i.e., it is related to "the input history". Therefore, the sequential characteristic VTH