The basic sequential logic building block in which the output follows the data input as long as the ENABLE input is active, is
D latch
Understanding Sequential Logic Blocks
Sequential logic building blocks are fundamental components in digital electronics that have memory. Unlike combinational logic, where the output depends only on the current inputs, sequential logic outputs depend on both current inputs and past inputs (or current state). This memory characteristic allows for the storage of bits and the creation of more complex digital systems like counters, registers, and state machines.
D Latch: The Described Building Block
The question describes a specific characteristic: "the output follows the data input as long as the ENABLE input is active". This behavior is precisely how a D latch operates.
- A D latch is a level-sensitive device. This means its output (Q) directly follows the input (D) as long as its Enable (E) or Clock (CLK) input is active (usually high, or '1').
- When the Enable input is active, the latch is said to be "transparent." Whatever data is present at the D input passes through to the Q output.
- When the Enable input becomes inactive (usually low, or '0'), the latch becomes "latched" or "opaque." At this point, the output Q retains the last data value that was present at D when the Enable input went inactive, regardless of subsequent changes to the D input. It essentially stores that bit of information.
Comparison with Flip-Flops
It is important to differentiate latches from flip-flops, as they are both sequential logic building blocks but differ in their triggering mechanism:
- Latches are level-triggered. Their output can change as long as the enable input is at a certain level (e.g., high).
- Flip-flops are edge-triggered. Their output changes only at a specific transition (edge) of the clock signal (either rising edge or falling edge). They ignore input changes at other times.
Let's examine why the other options do not fit the description:
- J-K flip-flop: This is an edge-triggered device. Its output changes only at the active clock edge, based on the J and K inputs, and it has more complex behavior (toggle, set, reset, no change). It does not have a "transparent" mode where output follows input while an enable is active.
- D flip-flop: While a D flip-flop also deals with data (D) input, it is an edge-triggered device. Its output (Q) updates to the D input's value only at the active clock edge (e.g., rising edge). It does not continuously follow the D input as long as an "enable" is active.
- T flip-flop: This is also an edge-triggered device. Its output toggles (flips its state) at the active clock edge if the T input is high. It is used primarily for counting and frequency division and does not have the "output follows data input" characteristic with an enable.
Therefore, the D latch perfectly matches the description of a sequential logic building block where the output follows the data input as long as the ENABLE input is active.
| Feature | D Latch | D Flip-Flop |
|---|---|---|
| Triggering | Level-triggered (Enable high/low) | Edge-triggered (Rising/Falling clock edge) |
| Transparency | Transparent when Enable is active; output follows input D | No transparency; output updates only at clock edge |
| Output Behavior | Output changes as long as Enable is active | Output changes only at the specific clock edge |
| Memory | Stores 1 bit when Enable is inactive | Stores 1 bit after active clock edge |
Important Questions from Sequential Circuits
A master-slave flip flop has the characteristic that
Number of unused states in a 3-bit Johnson counter are:
In _________ sequential circuits, signals can affect the memory elements only at discrete instants of time.
Shifting the register content to left by two bits can be used to implement
How many Flip-Flops are required for mod-16 counter?
