74HCT194

The 74HCT194 is high-speed Si-gate CMOS devices and are pin compatible with low power Schottky TTL (LSTTL). They are specified in compliance with JEDEC standard no. 7A.

The functional characteristics of the 74HCT194 4-bit bidirectional universal shift registers are indicated in the logic diagram and function table. The registers are fully synchronous.

The "194" design of the 74HCT194 has special features which increase the range of application. The synchronous operation of the device is determined by the mode select inputs (S₀, S₁). As shown in the mode select table, data can be entered and shifted from left to right (Q₀ Q₁  Q₂, etc.) or, right to left (Q₃   Q₂   Q₁, etc.) or parallel data can be entered, loading all 4 bits of the register simultaneously. When both S0 and S1 are LOW, existing data is retained in a hold ("do nothing") mode. The first and last stages provide D-type serial data inputs (D_SR, D_SL) to allow multistage shift right or shift left data transfers without
interfering with parallel load operation.

Mode select and data inputs of the 74HCT194 are edge-triggered, responding only to the LOW-to-HIGH transition of the clock (CP). Therefore, the only timing restriction is that the mode control and selected data inputs must be stable one set-up time prior to the positive transition of the clock pulse.

The four parallel data inputs (D₀ to D₃) of the 74HCT194 are D-type inputs. Data appearing on the D₀ to D₃ inputs, when S0 and S1 are HIGH, is transferred to the Q₀ to Q₃ outputs respectively,following the next LOW-to-HIGH transition of the clock. When LOW, the asynchronous master reset (MR) overrides all other input conditions and forces the Q outputs LOW.

The "194" is similar in operation to the "195" universal shift register, with added features of shift-left without external connections and hold ("do nothing") modes of operation.