Counters and Registers

Question 1

What are the two types of counters?

Answer 1

Synchronous and asynchronous

Question 2

How does a basic register operate?

Answer 2

• Typically made up of D-type flip flops
• Data on each input is stored in the flip-flop on the rising edge of the clock
• Data can be read from the output of Q
• Each flip flop will have a clock input and clear
• A simple register reloads data after each clock pulse

Question 3

Why do we generally not use gates to control the clock?

Answer 3

We must fully synchronise the systems so that clock signals arrive at the same time at all flip-flops.

Therefore, we do not use gates to control the clock as there are delays associated with gates.

Question 4

What is the purpose of a parallel load/enable?

Answer 4

The load/enable signal allows us to select content to be saved in the register.

If Load = 1, data is loaded. If Load = 0, the register content does not change.

Question 5

What is the purpose of a basic shift register?

Answer 5

A basic shift register is capable of shifting its binary information in one direction, this shift occurs at every clock edge for a basic register.

Question 6

What is a universal shift register?

Answer 6

A shift register that is capable of shifting information in both directions, it also implements parallel load/enable.

Question 7

Describe the ports of a 4-bit universal shift register in VHDL.

Answer 7

• I: in std_logic_vector (3 downto 0)
• A: out std_logic_vector (3 downto 0)
• S: in std_logic_vector (1 downto 0)
• SIL, SIR, clear, clk, reset: in std_logic

I = 4 bit parallel inputs

A = 4 bit parallel outputs

S = 2-bit select input

SIL = Shift Input Left

SIR = Shift Input Right

Question 8

Describe the process statement for a 4-bit universal shift register in VHDL, where:

0. No change
1. Shift Right
2. Shift Left
3. Parallel Load

Answer 8

The process statement depend on the clock and clear.

The process statement evaluates the value of SELECT, S:

When:

• S = “11” = 3. Parallel Load = so register value is equal to input
• S = “10” = 2. Shift Left = 3 least significant values of register + external input SIL
• S = “01” = 1. Shift Right = SIR + 3 most significant values of register

Question 9

What are the two types of counters?

Answer 9

• Ripple (asynchronous)
• Synchronous with clock

Question 10

How is a ripple counter implemented?

Answer 10

Using flip-flops in which the higher order flip-flop is connected to the input of the next higher order flip-flop.