ELEC1303 - Lecture 2

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ELEC1303 - Lecture 1
Digital Abstraction - Slides

Voltage Thresholds and Logic Values

• Senders and receivers need an agreed-upon mapping between voltage levels and logical signals so that they can communicate with each other

• We have a voltage range specifically defined as 0 and 1

  • e.g. zero-2.5 V → 0, 2.5-5 V → 1

• However what if the receiver sees the voltage is around 2.5 V? Does it interpret the signal value as a logical 0 or 1?

  • We don’t want a 0 to be mistaken for a 1 or vice versa
    We need to account for noise which can distort electrical signals
  • To eliminate such confusion, we prescribe a forbidden region that separates the two valid regions and outlaw “close calls”

• Thus, a voltage in this forbidden region is not a valid voltage

The highest input voltage value that a digital device must recognize as a logical 0 is called the low voltage threshold ${\text{V}}_{\text{IL}}$

The lowest input voltage value that a digital device must recognize as a logical 1 is called the high voltage threshold ${\text{V}}_{\text{IH}}$

Noise Margins

• Noise margins act as a safety buffer to prevent an incorrect interpretation of a signal due to noise
  • We want as high a noise margin as possible

Key Voltages Used by Digital Circuits

• ${\text{V}}_{\text{OH}}$ $-$ The lowest output voltage value that a digital device can produce when it outputs a logical 1
• ${\text{V}}_{\text{OL}}$ $-$ The highest output voltage value that a digital device can produce when it outputs a logical 0
• ${\text{V}}_{\text{IH}}$ $-$ The lowest input voltage value that a digital device must recognize as a logical 1
• ${\text{V}}_{\text{IL}}$ $-$ The highest input voltage value that a digital device must recognize as a logical 0

If the noise on a line is less than the noise margin, the circuit will still correctly detect the signal. However, if the noise is larger than the margin, the circuit may misread a 0 as a 1, or vice versa — leading to logic errors.

Noise Margin Low $-$ the amount of noise a low signal can tolerate before becoming undefined
${\text{NM}}_{\text{L}}={\text{V}}_{\text{IL}}-{\text{V}}_{\text{OL}}$

• ${\text{V}}_{\text{IL}}$ is the low voltage threshold of the receiver
• ${\text{V}}_{\text{OL}}$ is the voltage output low of the sender

Noise Margin High $-$ the amount of noise a high signal can tolerate before becoming undefined
${\text{NM}}_{\text{H}}={\text{V}}_{\text{OH}}-{\text{V}}_{\text{IH}}$

• ${\text{V}}_{\text{OH}}$ is the voltage output high of the sender
• ${\text{V}}_{\text{IH}}$ is the high voltage threshold of the receiver

Static Discipline

• Static discipline is the concept that if you feed a valid logic level into a digital circuit, the output will also be a valid logic level

That means:

• If you give a valid low (0) or valid high (1) to a gate,
• It will output a valid low or high,
• Even if there is some noise in the signal

To satisfy static discipline, you must meet two requirements:

1. Noise margins must be non-negative: ${\text{V}}_{\text{OL}}<{\text{V}}_{\text{IL}}<{\text{V}}_{\text{IH}}<{\text{V}}_{\text{OH}}$
2. If inputs meet valid input thresholds, then the system must guarantee its outputs will meet valid output thresholds:

A valid low input should always produce a valid output voltage; and a valid high input should always produce a valid output voltage (see slides for full explanation)

Note: that a device that receives an invalid input does NOT have to produce a valid output

Signal Regeneration

• Digital systems are restorative
  • i.e. they remove noise at each stage due to the noise margin
• Analog systems accumulate noise over time

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ELEC1303 - Lecture 3