Digital Electronics: Timing Diagrams

Timing Diagrams are graphs of digital signals as a function of time.

Square Wave featuring Rise-Time and Fall-Time.

Using the 50% Mark to measure the delays:

  • In the above image, X-Axis: Time, Y-Axis: Voltage or the Signal.
  • For example, let us ask ourselves what is the delay from S1 to S2?
    • So, the frame of reference, in general, is the 50% mark of the rising-edge or the falling-edge to measure the time difference Δt = t2 - t1.
    • Have a common frame-of-reference.

Conventions (Shown Below):

*Different companies have a slightly different timing-diagram convention.
  • Tri-State means the impedance is high (using Mega-ohms values).

Asynchronous and Synchronous Timing Diagrams:

Asynchronous Timing Diagram.
  • Asynchronous TDs use combinatorial logic like Gates and Circuits without a clock. Output (Z) showing the propagation delay Tpd.
Synchronous Timing Diagram.

  • Devices and ICs like Microcontrollers, Microprocessors, Flip-Flops (single-bit memory circuits), RAMs, Memories, and clocked-circuits in general, are synchronous when a clock is used.
    • Flip-Flops: T = Toggle, D = Data, SR = Set-Reset, etc are like imagine a box with gates inside it, which provide a single-bit memory.
      • Example: D-Type FF => Gets the input (D) memorizes it and gives it to Q as per the sampling and it is a single bit memory (meaning, 1 FF has 1-Bit, 8 FFs connected would have 8-Bits).

An example timing diagram of a D Flip-Flop shown below or above (Synchronous Timing Diagram).

*The timings get violated if the Setup-Time or Hold-Time gets violated which causes the meta-stability condition. Also, generally speaking, there are around 200s of timing parameters that mean different things depending upon the kind of IC.

So, let us see a Static Memory / SRAM timing diagram (shown below):

  • In this, Read is HIGH and Write is LOW (refer Rd/Wr signal above).
  • Other signals are: /CS = Chip Select, Data, Address (which could be either 0 or 1).
*Static memories aren't cheap.


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