Combinational Circuit 2: Dataflow Verilog Description

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Questions and Answers

What is the purpose of the assign statement in the Verilog code?

To implement the Boolean equation for the output F

What is the meaning of the notation 2'b in the state codes?

It denotes that the code has 2 bits

What is the function of the if-else statement in Verilog?

To make a two-way decision

What is the purpose of the case statement in Verilog?

To make a multiway decision Signup and view all the answers

What is the output of the assign statement f = b & (a |(a_n & c)) | (b_n & c & d_n)?

The output F based on the Boolean equation Signup and view all the answers

What is the meaning of the notation 'b in the state codes?

It denotes that the code is binary Signup and view all the answers

What is the purpose of the parameter statements in the Verilog code?

To give the state codes assigned to each state Signup and view all the answers

What is the name of the module defined in the Verilog code?

comb_ckt_1 Signup and view all the answers

What type of logic function do switches in series represent?

AND function Signup and view all the answers

What is the purpose of using p-channel transistors in switch circuits?

They conduct logic-1 values better than logic-0 values. Signup and view all the answers

What is the characteristic of a static CMOS circuit?

There is a path to the output of the circuit from the power supply or ground. Signup and view all the answers

What is the function of the n-channel transistors in a CMOS circuit?

They connect the circuit output to logic 0. Signup and view all the answers

What is the significance of DeMorgan's law in the given circuit?

It is used to complement the variables. Signup and view all the answers

What is the subfamily of CMOS circuits being discussed?

Fully complementary CMOS circuits Signup and view all the answers

What is the purpose of the switch circuit implementing F?

It connects the circuit output to logic 1. Signup and view all the answers

What is the reason for using n-channel transistors in switch circuits?

They conduct logic-0 values better than logic-1 values. Signup and view all the answers

What is the primary issue when an asynchronous signal is applied directly to a synchronous circuit?

The circuit may enter invalid states Signup and view all the answers

What is the purpose of the Reset signal in the circuit in Figure 4-40?

To set the state of the circuit to S0 (y0, y1, y2 = 1, 0, 0) Signup and view all the answers

What happens when RDY = 1 in the circuit in Figure 4-40?

The circuit cycles through the states S0, S1, and S2 Signup and view all the answers

What is the cause of the circuit failures in Figure 4-41?

The circuit delays, setup and hold times, and flip-flop behavior Signup and view all the answers

What is the result of the circuit in Figure 4-40 entering an invalid state?

The circuit produces an incorrect output sequence Signup and view all the answers

What is the purpose of synchronizing an asynchronous signal with the clock?

To prevent the circuit from entering invalid states Signup and view all the answers

What is the effect of the circuit delays on the behavior of the circuit in Figure 4-40?

The circuit may enter invalid states or produce incorrect output sequences Signup and view all the answers

What is the relationship between the setup and hold times and the behavior of the flip-flops?

The setup and hold times determine the flip-flop behavior Signup and view all the answers

What is the output Z in the sequential circuit with two D flip-flops A and B, one input Y, and one output Z?

A B Signup and view all the answers

What is the type of machine in the sequential circuit with two D flip-flops A and B, one input Y, and one output Z?

Mealy machine Signup and view all the answers

How many D flip-flops are in the sequential circuit specified by the input equations DA = XA + X Y, DB = XB + XA, and Z = XB?

2 Signup and view all the answers

What is the output Z in the sequential circuit with two D flip-flops A and B, two inputs X and Y, and one output Z?

XB Signup and view all the answers

What is the type of machine in the sequential circuit with two D flip-flops A and B, two inputs X and Y, and one output Z?

Mealy machine Signup and view all the answers

How many D flip-flops are in the sequential circuit specified by the logic diagram in Figure 4-49?

2 Signup and view all the answers

What is the purpose of the propagation delay of the storage elements in the sequential circuit?

Negligible in the context of the problem Signup and view all the answers

What is the initial state of the storage elements in the sequential circuit?

All storage elements store 0 Signup and view all the answers

What happens to anti-fuses when a higher-than-normal voltage is applied to them?

They are selectively closed Signup and view all the answers

What is the primary advantage of mask programming over other programming technologies?

It is done by the semiconductor manufacturer Signup and view all the answers

What happens to the devices programmed using the first three programming technologies?

They cannot be reprogrammed Signup and view all the answers

What is the primary function of the storage element in the fourth programming technology?

To drive the gate of an MOS n-channel transistor Signup and view all the answers

What happens to the connection between the source and drain of an MOS transistor when the stored bit value is 1?

It forms a closed path Signup and view all the answers

Why is the storage element technology considered volatile?

Because it requires a power supply to store values Signup and view all the answers

What is the primary function of the floating gate in the fifth programming technology?

To store charge Signup and view all the answers

What is a common characteristic of the first three programming technologies?

They are all permanent Signup and view all the answers

Study Notes

Combinational Circuit Description

A dataflow Verilog description for the circuit in Figure 2-43 is written using the Boolean equation for the output F and using Figure 2-34 as a model.
The module comb_ckt_1 has inputs a, b, c, d, a_n, b_n, c_n, and d_n, and outputs f and g.
The output f is assigned the value b & (a | (a_n & c)) | (b_n & c & d_n).
The output g is assigned the value b & (c | (a_n & c_n) | (c_n & d_n)).

HDL Representation for Sequential Circuits—Verilog

State Codes

In Figure 4-18(d), the states are A, B, C, and D, and are assigned 2-bit binary codes using the notation 2'b.
The if-else statement is used to make a two-way decision based on a condition, while the case statement is used to make a multiway decision based on multiple statements.

Synchronization and Metastability

Asynchronous signals driving synchronous circuits can lead to errors, as shown in Figures 4-39(b) and (c).
The circuit in Figure 4-40 can illustrate erroneous behavior due to an input signal not synchronized with the clock.
The circuit is initialized using the Reset signal, which sets the state of the circuit to S0 (y0, y1, y2 = 1, 0, 0).
The circuit cycles through states S0, S1, and S2, but can enter invalid states (0, 0, 0) or (1, 1, 0) due to metastability.

Sequential Circuit Design

A sequential circuit with two D flip-flops, one input, and one output is designed using the input equations DA = BY + AY and DB = Y, Z = A B.
The circuit's logic diagram, state table, and state diagram are derived.
The circuit is determined to be a Mealy or Moore machine.

CMOS Circuits

Switches in series give an AND function, while switches in parallel give an OR function.
A CMOS circuit has a general structure with a path to the output from the power supply (+V) or ground (0V) during steady-state operation.
p-channel transistors implement the function F, while n-channel transistors implement the function F.
The circuit is static, with paths to both logic 1 and logic 0.

Programming Technologies

Anti-fuses are selectively CLOSED by applying a higher-than-normal voltage to define the logic.
Mask programming is done by the semiconductor manufacturer during chip fabrication.
Both anti-fuse and mask programming technologies are permanent and cannot be reprogrammed.
A single-bit storage element driving the gate of an MOS n-channel transistor is used in large VLSI PLDs.
This technology is volatile, requiring power supply to store values, but can be easily reprogrammed.
The fifth technology uses a floating gate to store charge and control transistor switching.

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