74HC14 Schmitt-trigger inverter
Six inverters, with the 74HC04's pinout, and one difference that is the whole point of the part: each input has two switching thresholds instead of one.
A slow, noisy or bouncing input gives one clean edge, and one resistor and one capacitor turn a gate into an oscillator. Aliases: 7414, SN74HC14, hex Schmitt trigger, Schmitt inverter, hysteresis.
Pins
| Pin | Name | What it does |
|---|---|---|
| 1, 3, 5, 9, 11, 13 | 1A to 6A |
Schmitt-trigger inputs. |
| 2, 4, 6, 8, 10, 12 | 1Y to 6Y |
Outputs, the inverse of their input. |
| 7 | GND |
Ground. |
| 14 | VCC |
Supply, 2 V to 6 V. |
Truth table
| Input | Output |
|---|---|
Rising past VT+ |
Goes low |
Falling past VT- |
Goes high |
Anywhere between VT- and VT+ |
Stays where it was |
The thresholds
From the SN74HC14 data sheet's typical column: at 4.5 V, VT+ is 2.5 V and VT- is
1.6 V, so 0.9 V of hysteresis. The model scales the same two ratios with the supply,
which gives 2.78 V and 1.78 V at 5 V and lands within a tenth of a volt or so of the
data sheet's typicals at 2 V and 6 V too.
The one-gate oscillator
Wire a resistor from an output back to its own input and a capacitor from that input
to ground. The capacitor charges towards the high output until it reaches VT+, the
output falls, the capacitor discharges until it reaches VT-, the output rises, and
round again. With these thresholds the period is about 0.82 R C: 100 k and 1 uF give
about 12 Hz. A test in the engine runs exactly that circuit and holds it to the formula
within 2%.
Properties
None.
What the model gets right
The input is read as a voltage, not a logic level, so a capacitor charging slowly really does cross each threshold once, and the analog solver is asked to land on the crossing so the oscillator keeps time. An input that powers up sitting between the thresholds does not know which side it came from, and shows unknown until it leaves the gap. Propagation delay is the data sheet's typical 12 ns at 4.5 V.
What it does not model
Real chips spread widely around the typical thresholds (the data sheet allows VT+
anywhere from 1.55 to 3.13 V at 4.5 V), so a real RC oscillator can be 20% or more away
from the formula; this one sits on the typical line. No supply current, which on a
real chip rises while an input sits between the rails.
Common mistakes
Using a 74HC04 where you need hysteresis: a plain inverter fed a slow edge can switch
several times on the way through. Leaving unused inputs floating (tie them to GND).
See it in action
Decoder chaser uses one gate of a 74HC14 as the clock oscillator. Open it at /templates.