Feedback Circuits

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Feedback Circuits - PowerPoint PPT Presentation

Feedback Circuits. Two inverters, with feedback. If the first input is 0 , a 0 gets fed back into it. 0. 0. 1. If the first input is 1, a 1 gets fed back into it. 1. 0. 1. This circuit will hold its state forever - stable. write. in. out. Changing the value.

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Presentation Transcript
Feedback Circuits

Two inverters, with feedback

If the first input is 0, a 0 gets

fed back into it

0

0

1

If the first input is 1, a 1 gets

fed back into it

1

0

1

This circuit will hold its state forever - stable

write

in

out

Changing the value

Replace one inverter with a tri-state inverter

How can we force this to be either ‘1’ or ‘0’?

Add a tri-state inverter for input

When write=‘1’, straight connection from input to output – We can write values in

This is a simple one-bit memory cell.

When write=‘0’, feedback connection – holds state

A

B

C

D

E

F

Ring Oscillators

1

0

0

1

0

1

1

0

0

1

0

1

B

C

D

E

F

A

Odd # of stages leads to ring oscillator

Timing Waveform:

A and F arethe same wire

R

Q

R S Q Qb

0 1

1 0

0 0

1 1

Qb

S

1

0

1

0

?

0

0

1

0

1

?

0

1

1

0

0

Cross coupled NORs

NOR:

‘0’ if either of its inputs are ‘1’

S = set

R = reset

Q = output

1

0

0

1

hold

hold

When a NOR has one input at ‘0’, it inverts the other input

0

0

WhenR and S are 0

This is called an R-S Latch

R

Q

Qb

S

R

S

Q

Qb

Nors with R=S=1

Re-examine the inputs R=1 and S=1:

What does it mean to both set and reset at the same time?R-S latch says “both lose!”  Q and Qb both are 0

• What happens when R=S=1 (Q=Qb=0) and R or S changes to 0?
• S changes to zero first Reset wins (Q=0, Qb=1)
• R changes to zero first  Set wins (Q=1, Qb=0)
• Both change at the same time to R=S=0…

When R=S=1 changes to R=S=0, we get an uncontrolled oscillation (unstable)

1

0

0

0

1

1

0

1

1

0

1

0

Moral of the story: R=S=1 is bad  Make it an illegal input

S’R’+SR’ == R’

S’R+S’R’ == S’

SR’

R S Q Q

0 1 1 0

1 0 0 1

0 0 hold hold

1 1 0 0

Q=1

Q=0

Q=0

Q=1

S’R

SR

SR

SR

Q=0

Q=0

SR’

S’R

S’R’

S’R

SR’

SR+S’R’

Q=1

Q=1

R-S Latch states

State: The current status of all memory elements.

Changes to states occur only when inputs change

All possible inputs must be “covered” by an arc out of each state

Stop

GND

R

Run

+5

Start

GND

S

+5

Using an R-S latch

R-S Latches are useful whenever a system should “remember” the last input

• Design a system with two pushbuttons: Startand Stop
• Whenever Start is pushed, the signal Run will be asserted.
• Run should remain asserted until Stop is pushed.

Input

GND

Out

+5

OutIdeal

OutActual

R

Input

Out

GND

+5

S

GND

Switch Debouncing

Switch Depressed

Switch Released

R

Q

Q

R

S

Q

Q

S

R

Q

R

En

S

Q

S

Q

R

S

Q

En

An enabled (gated) R-S latch

Unclocked R-S Latch

Output changes only wheninput changes, and enabled

Level-sensitive R-S Latch

Clock Signal

R

R

Q(enabled)

Q(regular)

S

S

En

Set

Set

Set

S

R

Reset

Reset

En

Q(regular)

Q(enabled)

Q

Q

R

R

S

S

Q

Q

En

Regular vs. Enabled R-S latches

Assume that latches have no time delay (ideal)

Enabled latch only changes when enable is asserted

R

Q

S

Clock

Clock(En)

Set

Set

Set

S

Reset

R

Reset

Q

Q

R

S

Q

En

Clocking an enabled R-S Latch

A clocked R-S latch follows the R/S inputs when the clock is asserted.

A clocked R-S latch stores the value when the clock goes low whenthe clock is not asserted.

D

Q

C

clk

D

Q

R

S

Q

En

clk

Clock

D

Q

D-Latches (gated)

In a D-latch, the output follows the input when the clock is high.

When the clock is low, the output remains what it was on the falling edge of the clock.

D

Q

en

VHDL for D Latches (gated)

LIBRARY ieee;USE ieee.std_logic_1164.all;ENTITY Dlatch ISPORT( D : INSTD_LOGIC; en : INSTD_LOGIC; Q : INOUT STD_LOGIC); END Dlatch;

Inputs: D and EnOutput: Q

INOUT instead of OUT: Can be used as an input within the ENTITY

ARCHITECTURE behavior OF Dlatch ISBEGIN PROCESS(D,en) BEGINIF (en = ‘1’) THEN Q <= D;ELSE Q <= Q;END IF;END PROCESS; END behavior;

PROCESSlist  State of latch can change due to a change in any of these values

Continuously monitors D and en looking for changes

If enabled, Q D

If not enabled, Q keeps its old value