ELEC1303 - Lecture 11

|← Back to Home

ELEC1303 - Lecture 10
Decoders & Encoders - Slides

BCD to 7-Segment Display Decoder

• A 7-segment display is made up of seven individual LEDs
  • Each LED labelled from a-g
  • Commonly found in digital clocks
• The output of a decoder determines which LED segments are on

Encoders

• Encoders are combinational circuits that perform opposite function of a decoder
  • Generates an output binary code for the active input

• For correct operation, requires that only a single input line is asserted at any one time
• Simple encoders can be made with just OR gates:
  
  

Limitations

• We assume only a single input line can be active at any time
  • Having more than one input can create undesired outputs
• If no inputs are active, then the output is 000… which should usually only occur for when $d_0=1$

Priority Encoder

• A priority encoder can be used to solve the above two issues

A priority encoder works through:

• Assigning priority to inputs
• Only encoding the highest priority active input
• Introducing an extra output to indicate whether all inputs are inactive

Decoder Applications

• Used when an output or group of outputs are to be activated only on the occurrence of a specific combination of input levels
• Used widely in memory
  • They respond to input address code to active particular memory location

Encoder Applications

• Encoder converts information into some coded form
  • e.g. convert calculator number input into binary code

---

Building Memory - Slides

Memory

• Three properties to be considered memory:
  • Holds values
  • Able to read saved values
  • Able to change saved values
• One-bit memory $-$ holds a single bit, 0 or 1

Storage

• Using loops, circuit outputs become inputs

• Still extra steps needed to be considered memory
  • We cannot change the value in the loop!

RS Latch

• Using NOR gates instead of inverters allows us to make a RS latch
  • RS → Reset-Set

• The two inputs R and S let us control outputs Q and Q’
  • Q and Q’ feed back into the circuit
    

Set Operation: RS = 01

• S → 1 $⟹$ ${\text{Q}}_{next}^{\prime }$ → 0 $⟹$ ${\text{Q}}_{next}$ → 1
  i.e. set Q to 1
• Outputs then stop changing
  • Circuit becomes stable

Reset Operation: RS = 10

• R → 1 $⟹$ ${\text{Q}}_{next}$ → 0 $⟹$ ${\text{Q}}_{next}^{\prime }$ → 1
  i.e. reset Q to 0
• Outputs then stop changing
  • Circuit becomes stable

Remember, there is propagation delay for all these changes

• R=S=1 should never be used

RS Latches are Memory

• RS latch meets all of our criteria above to be considered memory
• The output Q is considered the state of the latch
  • Represents the data stores

---

ELEC1303 - Lecture 12