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Sahithyan's S2
Sahithyan's S2 — Computer Organization And Digital Design

Digital Logic Signals

Logic levels

Ranges of voltages are abstracted away into 0 (Low) and 1 (High). These ranges are called as logic levels.

The ranges would have a gap in-between. This is done to avoid errors by fluctuations.

Example:

  • HIGH - 2V to 3.3V
  • LOW - 0V to 0.8V

Digital waveforms

Voltage levels that are changing back and forth between the digital states.

Pulse

A change between the digital states.

2 types:

  • Positive-going pulse
  • Negative-going pulse

Non-ideal pulse

Even though we abstract the analog states into digital, the analog characteristic can be observed when switching states. An non-ideal pulse will take some time (a very short time) to switch the states.

Pulse width

Pulse width is the time a pulse takes. In non-ideal case, the pulse width is said to be the time between 50% mark of the transition.

Raise and fall time

Ideally a pulse is instantaneous. In non-ideal case, it takes time to transition between states. Raise and fall times are measured between the 10% and 90% time.

Waveforms

A series of pulses. They can either be periodic (pulse train) or non-periodic. A clock is required along with the waveform to convert it into binary.

Duty cycle

In a period waveform, a duty cycle is the ratio of pulse width () to the period ().

Logic family

A collection of different integrated circuit (IC) chips/ chip building blocks that have:

  • Similar input, output and internal circuit characteristics
  • Methods to implement all necessary logic functions

Chips from the same family can be interconnected to perform any desired logic function. Chips from different logic families may not be compatible, so we need to take special steps to interconnect circuits from different logic families.

Electrical behavior

  • Logic voltage level
  • DC noise margins
    Highest LOW voltage of output must be lower than highest LOW voltage of input
  • Fanout
    Number and type of inputs that are connected to a given output
  • Power consumption
  • Speed
  • Noise/interference
  • Electrostatic discharge
  • Three-state outputs

Examples

Examples for Logic Families

The above image is from https://www.jsykora.info/2014/05/logic-voltage-levels/.

  • Light colored strips are for input
  • Dark colored strips are for output

Digital Design

Digital circuits are designed using software. Simulations are ran at logic level. Then the circuit is built using FGPAs. After many verifications, the circuit is fabricated as a chip.

Data transfer

Data can be transferred in either serial or parallel. Serial transfer takes more time while parallel transfer requires more transmission lines.