Latch in Digital Circuits
Latches and edge-triggered registers are fundamental components in digital circuits, each serving a specific role:
Latches
Role: A latch is a level-sensitive circuit used to store and pass data in digital systems. It operates in two modes: transparent mode and hold mode.
Transparent Mode: In transparent mode, the latch allows data from the input (D) to pass to the output (Q) when the clock signal is high. This means that when the clock signal is high, any changes in the input data are reflected immediately at the output.
Hold Mode: When the clock signal is low, the latch is in hold mode. In this mode, the input data is held stable at the output for the entire clock phase. This stability ensures that the inputs remain unchanged around the falling edge of the clock, meeting set-up and hold time requirements.
Types: Latches come in two main types: positive latch (passes data when the clock is high) and negative latch (passes data when the clock is low).
Edge-Triggered Registers
Role: Edge-triggered registers, in contrast to latches, sample and store input data only during a specific clock transition. Positive edge-triggered registers capture data on a 0-to-1 clock transition, while negative edge-triggered registers do so on a 1-to-0 transition.
Construction: Edge-triggered registers are often constructed using latches. A common configuration is the master-slave structure, which combines a positive and a negative latch in series to capture and store data during the clock edge transition.
Variations: Registers can also be built using other techniques, such as one-shot generators that create a “glitch” in the clock signal to trigger the register. Specialized structures can also be used depending on the design requirements.
In summary, latches are level-sensitive components that can operate in transparent or hold modes, while edge-triggered registers capture and store data only during specific clock edge transitions. Both serve crucial roles in digital circuits, with latches often used as building blocks for constructing edge-triggered registers
What distinguishes a positive latch from a negative latch, and how do they operate?
Answer: A positive latch passes the D input to the Q output when the clock signal is high, while a negative latch passes the D input to the Q output when the clock signal is low. Positive latches are sensitive to the rising edge of the clock, and negative latches are sensitive to the falling edge.
Why is it important for inputs to be stable around-the-clock transition in a latch?
Answer: Inputs must be stable for a short period around the clock transition to meet set-up and hold requirements. This ensures that the latch correctly captures and holds the input data during the clock’s edge transition.
What is the fundamental difference between a latch and an edge-triggered register?
Answer: Latches are level-sensitive and pass data continuously when the clock meets specific conditions (high or low). In contrast, edge-triggered registers only sample the input on a clock transition (rising or falling edge), making them more suitable for synchronized data storage.
How does the bistable circuit store information, and how can its stored value be changed?
The bistable circuit stores information by having two stable states, where each state corresponds to a logic value (0 or 1). To change the stored value, a trigger pulse is applied to make one of the stable states temporarily unstable, causing the circuit to switch to the other state.
What is the relationship between the loop gain and stability in a bistable circuit?
In a bistable circuit, stability is achieved when the loop gain (G) is smaller than unity (G < 1). To change the state, the loop gain is temporarily increased to a value greater than 1 by applying a trigger pulse, which makes one of the stable states unstable and causes a transition.