Hi, perhaps someone can help straighten out my confusion.
I have some 74HC14N Schmitt-Trigger inverters from TI,
(http://www.ti.com/lit/ds/symlink/sn74hc14.pdf) with my Vcc about 4.5v.
On a breadboard I drive the first inverter with a slow voltage from a potentiometer that
I manually adjust. (25K, outer legs tied to Vcc and Ground, middle point to the input of the
first trigger inverter.) The first inverter output feeds the second inverter input.
The circuit works as expected with a hysteresis gap between ascending and descending trigger levels,
provided I transition the input fast. But if I allow the input voltage to linger in "no-man's-land",
between the two trigger levels, I see the output of both inverters sit somewhere near Vcc/2, with a
fast oscillation (about 100 Mhz, maybe 0.7v p-p), on both the outputs of the first and second gate.
I found something that further deepened my confusion:
http://www.nutsvolts.com/magazine/a...ners-guide-to-reliable-timers-and-oscillators
<quote>
1) The logic gate has no well-defined input thresholds. Near an input voltage of approximately Vcc/2,
this alleged digital device behaves as a high-gain linear amplifier. Normally, this input region is
transitioned very rapidly in pure interconnected logic and is not a problem,
but when passive components are added as loads, this may not be the case.
2) Since passing through the input “no man’s land” near Vcc/2 must be done rapidly,
each CMOS logic family has a minimum input slew rate requirement. For example,
the 74HC family specifies a fast rise time of at least 11 volts/microsecond...
</unquote>
My confused expectations are
a) that the trigger-inverter output should latch close to Vcc or ground with
some kind of reinforcing feedback, without any possibility of outputs lingering elsewhere.
b) that slow signals and slew rates ought to be fine with a Schmitt trigger, e.g. if
I wanted to use this device connected to a temperature sensor, I thought it should work.
Any suggested reading or advice to help me understand what is going on? Obviously the fact
that I'm breadboarding a high speed CMOS device with little concern for layout and stray
capacitances might be a problem. I did tie unused gate inputs down and I did put a
capacitor across Vcc, just in case ...
Thanks
Peter
I have some 74HC14N Schmitt-Trigger inverters from TI,
(http://www.ti.com/lit/ds/symlink/sn74hc14.pdf) with my Vcc about 4.5v.
On a breadboard I drive the first inverter with a slow voltage from a potentiometer that
I manually adjust. (25K, outer legs tied to Vcc and Ground, middle point to the input of the
first trigger inverter.) The first inverter output feeds the second inverter input.
The circuit works as expected with a hysteresis gap between ascending and descending trigger levels,
provided I transition the input fast. But if I allow the input voltage to linger in "no-man's-land",
between the two trigger levels, I see the output of both inverters sit somewhere near Vcc/2, with a
fast oscillation (about 100 Mhz, maybe 0.7v p-p), on both the outputs of the first and second gate.
I found something that further deepened my confusion:
http://www.nutsvolts.com/magazine/a...ners-guide-to-reliable-timers-and-oscillators
<quote>
1) The logic gate has no well-defined input thresholds. Near an input voltage of approximately Vcc/2,
this alleged digital device behaves as a high-gain linear amplifier. Normally, this input region is
transitioned very rapidly in pure interconnected logic and is not a problem,
but when passive components are added as loads, this may not be the case.
2) Since passing through the input “no man’s land” near Vcc/2 must be done rapidly,
each CMOS logic family has a minimum input slew rate requirement. For example,
the 74HC family specifies a fast rise time of at least 11 volts/microsecond...
</unquote>
My confused expectations are
a) that the trigger-inverter output should latch close to Vcc or ground with
some kind of reinforcing feedback, without any possibility of outputs lingering elsewhere.
b) that slow signals and slew rates ought to be fine with a Schmitt trigger, e.g. if
I wanted to use this device connected to a temperature sensor, I thought it should work.
Any suggested reading or advice to help me understand what is going on? Obviously the fact
that I'm breadboarding a high speed CMOS device with little concern for layout and stray
capacitances might be a problem. I did tie unused gate inputs down and I did put a
capacitor across Vcc, just in case ...
Thanks
Peter