Reset with a minimum activation period

Thread Starter

analog_headache

Joined Mar 8, 2016
19
I was wondering if anyone has come across a chip which only outputs a reset pulse when a controlling input signal is greater than say 40us. I am looking to provide a reset to a chip but with pulse filtering. I don't really care how long the output pulse last for.

I have been looking at POR chips and the MR inputs to them but they all either react instantly or with minimum activation time in the milliseconds.
I need the reset to not react below 40us but activate before 100us. Also looked at smart threshold chips from ST and switch debouncers but they all react too slow :-(. I also don't want an RC filter as the timing isn't repeatable and a second 'glitch' could activate the reset before the 40us if the cap was still charged.

I don't have a lot of space on my PCB so looking for a small chip that satisfy these requirements.

Has anyone come across something similar.

Thanks in advance for any advice you can give.

BR
AH
 

Thread Starter

analog_headache

Joined Mar 8, 2016
19
Thanks for your reply Andrewm

I have been roaming though that genre of chip but the ones I have found (including the one you pointed at) don't reject a pulse on the input. e.g. the TL7705 has a resin pin which allows you to control the output reset but as soon as you activate the resin pin for more than 0.4us, the reset will get activated. I am look for something that will not activate until around 40us

Yes I am look for a more digital solution rather than the RC method

Thanks for taking the time to reply
 

andrewmm

Joined Feb 25, 2011
1,127
How accurate do you need this ?

top of head solutions.


simple solution, an RC integrator network, into a comparator.
small pulses, make a small voltage, bigger pulses make a bigger voltage,
comparator triggers .

Or you make a counter,
counter resets on edge of pulse,
counts, if no pulse comes in in time, output is triggered on a set counter value
( more one if your in an FPGA )

Or trigger a watchdog on each edge,
watchdog period set at say 40 us,
when edge comes in, if watchdog is high, then pulse under 40 us, if low , its longer than 40 us.

something like a good old pair of 555 timers seems about right.

https://www.circuitstoday.com/missi...555 timer IC connected,beat of a sick patient.
 

sparky 1

Joined Nov 3, 2018
482
A flow chart can be made from this set of instructions.
An edge detector is monitoring the on off timing and that signal is fed into a Frequency to voltage converter.
The analog voltage is digitized and fed into I/O port of an inexpensive microprocessor.
A voltage comparator uses 2 voltage references that sets a window for upper and lower threshold.
The period 40 -100 uS equals the frequency range (between 25 kHz and 10 kHz).
The uP controls a ramp voltage which corresponds to the frequency range.
The uP remains in the nested do loop until the comparator signals a reset routine.
The uP outputs to a mosfet that deactivates the process resetting or synchronizing the uP process control.
 
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AnalogKid

Joined Aug 1, 2013
9,039
Yes I am look for a more digital solution rather than the RC method
I don't know what you have against R-C circuits, but it that kind of attitude is a dangerous precursor to future aberrant behavior such as listening to audio through a Class(less)-D amplifier and the downfall of democracy. I STRONGLY recommend admitting yourself to a psychiatric hospital for at least two years of intense counseling.

or

The CD4060 is a combination oscillator and binary counter. With one extra diode it can be configured to wake up on the leading edge of a transition from a switch, count for a while, output one clean pulse, and park itself until the next input edge.

ak
 

AnalogKid

Joined Aug 1, 2013
9,039
Re-thinking my previous post, it might be that your resistance to resistors and capacitors is rooted in a concern for our natural resources. Resistance, in particular, is becoming a rare and valued commodity in regions of decreased cerebral fortitude. But I digress ...

I think the only single-chip solution is an 8-pin PIC programmed to do what you want. Less than 1$ (plus the firmware development time).

Here is an all-digital, non-R-C approach. The oscillator is around $1 at Digi-Key. If you can find one with an active-low enable input, U2 goes away.

The circuit starts when the input goes low. If the input pulse width is 80 us or longer, the output pulse is 40 us wide, delayed for 40 us. If the input is between 40 us and 80 us, the output delay still is 40 us, but the pulse width is the (input - 40 us).

If you don't have a spare inverter available, a variation of this circuit replaces U2 with a diode - but adds a 5 us ambiguity to the delay period.

No resistors were harmed in the production of this design.

ak
Reset-Delay-2-c.gif
 
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Thread Starter

analog_headache

Joined Mar 8, 2016
19
To all you guys, AndrewM, Sparky 1 and AnalogKid that provided your thoughts and time on this, thank you for your suggestions. Your suggestions have helped me form some different approaches. I have a few options now. My favourite is the small PIC or even a small CPLD. I hadn't really considered doing that. However, I have found another chip that should do exactly what I want. It is a NCP308 from onsemi. It has a minimum pulse width on the MR input of 150us which is roughly in the ball park.

Thanks again guys much appreciated.

AH

PS Analog kid, your comments made me laugh :)
 

andrewmm

Joined Feb 25, 2011
1,127
To all you guys, AndrewM, Sparky 1 and AnalogKid that provided your thoughts and time on this, thank you for your suggestions. Your suggestions have helped me form some different approaches. I have a few options now. My favourite is the small PIC or even a small CPLD. I hadn't really considered doing that. However, I have found another chip that should do exactly what I want. It is a NCP308 from onsemi. It has a minimum pulse width on the MR input of 150us which is roughly in the ball park.

Thanks again guys much appreciated.

AH

PS Analog kid, your comments made me laugh :)

Was not aware of the NCP308 , just had a look.

Its a "standard" power on reset chip, with a manual reset input.
I guess you are looking at the MR input ?

The MR input is a fairly standard pin on these chip, designed for a push button to go into it.
so it has a de bounce circuit on it, which I guess is what your looking at using.

Just be careful, As I said, I don't know this chip, but in general.
this denounce width is extremely variable ,
varies over temperature, voltage , and chip to chip.
you notice in the data sheet there is no max or minimum time specified.
it could be 10uS or 500uS , and still be in spec.
 
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