Bidirectional SR: Mode Encoding for 74HC194 Pin-Compatible Design
Standard logic ICs like the 74HC194 universal shift register use mode control pins to dictate internal multiplexer routing. Every stage in the register requires precise data selection to execute shifting, loading, or holding operations synchronously.
The circuit provided represents a single stage n of a 4-bit universal shift register. The input to the D flip-flop D is driven by a 4-to-1 multiplexer. This multiplexer is controlled by two mode select pins: S1 (MSB) and S0 (LSB). The multiplexer has four data inputs labeled 0 through 3. These inputs are wired to the current stage output Q, the previous stage output Q_{n-1}, the next stage output Q_{n+1}, and a parallel data input P_n.
Determine the specific operation assigned to each S1 and S0 combination based on standard 74HC194 behavior. Identify the active serial input pins and internal feedback paths for each mode.
Constraints
- Assume positive edge-triggered D flip-flops.
- Assume the 4-to-1 multiplexer selects input 0 when
S1=0andS0=0, input 1 whenS1=0andS0=1, input 2 whenS1=1andS0=0, and input 3 whenS1=1andS0=1. - The register is 4 bits wide, with stage 0 being the leftmost bit and stage 3 being the rightmost bit.
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