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Why actual pulse width and oscillation frequency unalike from data?

Latest Updated:09/01/2007

Question:

Why is the actual pulse width different from the pulse width given in the data sheet, and why is the actual oscillation frequency different from that given by the theoretical equation?

Answer:

As described in FAQ timer-0201 or timer-0202, the theoretical equation assumes ideal performance by the capacitor and other elements, uses the typical threshold voltage (2/3VDD), and does not take propagation delay into consideration. Therefore, values obtained with this equation are approximate. When greater precision is required for the output pulse width, it is recommended that you determine the values of R and C on the basis of measuring every data (and, when applicable, consider using semi-fixed elements).
The main factors that can cause actual values to differ from the theoretical values are:

  • Effect of IC propagation delay: Especially in the case of high-speed pulses.
  • Variation in threshold voltage: The threshold voltage is set by internal voltage-dividing resistor. Resistance varies within the range of relative error.
  • Effect of capacitor leakage current: Depending on the utilized capacitor, the actual pulse width can be larger than the theoretical value due to the effect of capacitor leakage current, especially for long pulse settings.
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