SLVA720A July   2015  – October 2020 CD14538B , CD14538B-MIL , CD4047B , CD4047B-MIL , CD4098B , CD4098B-MIL , CD54HC123 , CD54HC221 , CD54HC4538 , CD54HCT123 , CD54HCT4538 , CD74HC123 , CD74HC221 , CD74HC423 , CD74HC4538 , CD74HC4538-Q1 , CD74HCT123 , CD74HCT221 , CD74HCT423 , CD74HCT4538 , SN54121 , SN54123 , SN54221 , SN54AHC123A , SN54AHCT123A , SN54LS123 , SN54LS123-SP , SN54LS221 , SN74121 , SN74221 , SN74AHC123A , SN74AHC123A-EP , SN74AHCT123A , SN74LS122 , SN74LS123 , SN74LS221 , SN74LS423 , SN74LV123A , SN74LV123A-EP , SN74LV123A-Q1 , SN74LV221A , SN74LV221A-Q1 , SN74LVC1G123

 

  1. 1Introduction
  2. 2Terminology
  3. 3Theory of Operation
    1. 3.1 Basic Concept
    2. 3.2 Operation
  4. 4Applications
    1. 4.1 General Design Considerations
      1. 4.1.1 Pulse Length Determination
        1. 4.1.1.1 Temperature Stability
    2. 4.2 SN74LVC1G123 Applications
      1. 4.2.1 Switch Debounce
        1. 4.2.1.1 Requirements
        2. 4.2.1.2 Schematic
        3. 4.2.1.3 Component Selection
      2. 4.2.2 Leading and Trailing Edge Detector
        1. 4.2.2.1 Requirements
        2. 4.2.2.2 Schematic
        3. 4.2.2.3 Component Selection
  5. 5Frequently Asked Questions
    1. 5.1 How do I calculate the output pulse length?
    2. 5.2 How do I configure the SN74LVC1G123's inputs for ______ edge triggering?
    3. 5.3 How stable is the output pulse length over VCC changes?
    4. 5.4 How stable is the output pulse length over temperature changes?
    5. 5.5 Which inputs of the SN74LVC1G123 have Schmitt-triggers?
    6. 5.6 Can I connect the Cext pin to ground?
  6. 6Revision History

4.2.2.3 Component Selection

  • R and C are selected by using the datasheet's graphics and the pulse length equation, tw = K × R × C. Because both edge detector circuits are outputting to the same system, the output pulse lengths are the same.
    • The capacitor value is selected first because there are fewer capacitor values available on the market and this simplifies other calculations. By looking at the figures on the datasheet (some of which are in Section 4.1.1), it can be seen that 0.1 µF was a tested capacitor value that will meet our timing requirement.
    • The resistor value is calculated from the pulse length equation, tw = K × R × C. By rearranging terms, R = tw / (K × C). K is found in Figure 4-2 to be 0.925 for C = 0.1 µF and VCC = 5 V. Since the pulse width is a range from 1 ms to 2 ms, a range of R values will be given to match. 10.8 kΩ < R < 21.6 kΩ. 12 kΩ is selected because it is a standard 5% resistor value and yield a pulse length of 1.11 ms.
Table 4-2 Leading and Trailed Edge Detector Component Values
ComponentValue
R12 kΩ
C0.1 µF