Looking for a One Shot falling edge triggered with negative pulse IC

Thread Starter

XtaBeat

Joined Mar 18, 2021
7
Hello everyone,

First post here, forum looks good !

Ok so the context:
I am trying to find the simplest way to reset an NRF52 MCU using a DS3231 RTC after a deep sleep period (very low energy application).

So far I managed to do just that with a simple N channel MOSFET between the alarm pin of the RTC and the enable pin of the MCU. But this method is no longer acceptable since suddenly cutting the power supply of the MCU causes various software problems (huge surprise right ?).

So now I'm trying to use a more classic approach: the MCU sets up an RTC alarm, goes to sleep, then some time later, the RTC triggers an MCU reset (RST pin), the MCU wakes up, and so on.

The alarm pin goes from high to low when triggered and the opposite when the alarm is cleared by the MCU. The MCU resets on a rising edge of the RST pin. It is deactivated when RST is at a low level. The voltage supply is 3.3V and the solution must have low energy needs.

I've tested the 555 IC which everyone knows for this application, it's "kind of" working but it's not 3.3V compatible, and the output logic is the opposite of what I need (goes high on rising edge input).

I need something like an LTC6993-3 from Analog:

OneShot.png

This would be perfect if it wasn't only available in a SOT23 package. I am capable of hand soldering SOT23 but it's far from optimal since I'd have to do something like 20 of them...

Do you guys know of any equivalent IC available in DIP ? Or any alternate solution (it must not include too much components).

Thanks and have a good day
 

peterdeco

Joined Oct 8, 2019
327
Welcome. Looks like your LTC part is available only in SMT. The TI brand of LMC555CN is a CMOS 555 and in the datasheet says it will work down to 1.5V. Invert the output with a FET.
 

dl324

Joined Mar 30, 2015
12,668
Welcome to AAC!
I've tested the 555 IC which everyone knows for this application, it's "kind of" working but it's not 3.3V compatible
Signetics states that most devices will operate as low as 3V. Since everyone copied Signetics' design, that should also apply to devices from other manufacturers.
the output logic is the opposite of what I need (goes high on rising edge input).
555 timers are triggered by a falling edge. Use an inverter to invert the output.
 
Last edited:

Thread Starter

XtaBeat

Joined Mar 18, 2021
7
Wow, the support is fast here. Nice to see.

Yeah I also thought about using an inverter/fet to invert the output of the 555 but that's one more component, I would have preferred something entirely packaged into one IC. But if I have to, I'll add one.

Signetics states that most devices will operate as low as 3V. Since everyone copied Signetics' design, that should also apply to devices from other manufacturers.
Good to know ! Then I'm wondering why I get a weird output level of 0V/2V instead of a nice 0V/3.3V on my 555. Maybe I did something wrong, I'll check.

What about a watchdog / supervisor IC? I know some like the MAX706 are available in DIP.
That's definitely interesting. I thought those kind of ICs only came in small SMS packages. I'll have a look.

I used this chip a few times actually but if I remember correctly they only come in SSOP packages.

Okay, so lots of ideas, thank you, I'll keep you guys updated.
 

Papabravo

Joined Feb 24, 2006
15,431
Check the datasheet for the ability of the output to source current. There may be a spec for minimum output high voltage. This is more likely on a bipolar part as opposed to a CMOS part. Which one do you have?

Check the schematic from the TI datasheet to see why the output cannot be pulled up to the supply rail for the bipolar device.
 
Last edited:

dl324

Joined Mar 30, 2015
12,668
Good to know ! Then I'm wondering why I get a weird output level of 0V/2V instead of a nice 0V/3.3V on my 555. Maybe I did something wrong, I'll check.
The datasheet indicates the high level output voltage can't get to the positive supply voltage. Worst case is Vcc-2.25V with Vcc-1.7V typical. What kind of load do you have on the output?

Nat Semi datasheet:
clipimage.jpg
 

dl324

Joined Mar 30, 2015
12,668
You could use LMC555. It can't sink/source as much current, but the output is closer to the supply voltage. If it isn't high enough for your MCU, use a pull up resistor.
 

Thread Starter

XtaBeat

Joined Mar 18, 2021
7
First of all sorry, I was off for a few days.

Check the datasheet for the ability of the output to source current. There may be a spec for minimum output high voltage. This is more likely on a bipolar part as opposed to a CMOS part. Which one do you have?
NE555N so looks like a bipolar part. I've disconnected the 555's output, and I get a nice 0V, but still a 2.82V high level. Anyway, it looks like it's enough of the MCU to consider it a logic 1 level (0.7*VCC => 2.31V).

The 555's output is going to be connected to a particle Xenon board's /RST pin. This is the /RST pin from the board's datasheet (on board reset button):
1616491664979.png

The /RST port is connected to the NRF52840 MCU pin. I think it draws virtually no current.

Since there is a low pass filter for the onboard RST button, I'm guessing the monostable output pulse will have to be longer than the filter's fc. That's not really a problem.

I have still a strange behavior that I fail to understand:
When my trigger is at 3.3V (no alarm), the 555's output is at its low level (0V) : expected behavior
When my trigger is at 0V (alarm), the 555's output stays at it's high level (2.8V) for, it seems, an unlimited amount of time.

I'm using this schematic:
1616493835440.png


If I'm not mistaken, shouldn't the output go back to it's low level after 51.7ms ? I mean I'm pretty sure it should, since this is a monostable. But why doesn't it ? Any idea(s) ?

Bare in mind that at the moment I only have a multimeter and no scope available (but I can if necessary).

PS: I will have also have a look at the LMC555 and CD4528/CD4538 refs thanks.
 

Thread Starter

XtaBeat

Joined Mar 18, 2021
7
The NE555N needs a minimum of 4.5V. Try an LMC555 or raise your voltage.
I think my old NE555N found in my cupboard might have a problem. I changed Vcc to 5V. Disconnected the MCU and RTC:

1616497295598.png

I tried this with "trig" to Vcc, and C1 is never discharching. The 'disch" and "thres" pins are always at VCC.
 

crutschow

Joined Mar 14, 2008
26,721
When my trigger is at 0V (alarm), the 555's output stays at it's high level (2.8V) for, it seems, an unlimited amount of time.
shouldn't the output go back to it's low level after 51.7ms ?
Not as long as the TRIG input is low, since it is a level sensitive input.
TRIG has to go high before the one-shot time-out, otherwise it will immediately trigger a new period, which thus makes the output appear to be continuously high.
To shorten the trigger pulse you can couple it through a series capacitor into two equal value resistors, one to V+ and one to ground, with the junction of the three going to the TRIG input.

The BJT version of the 555's output only goes to about a volt or so below V+.
The CMOS version's output should go very close to V+ with a high impedance load.
 

Thread Starter

XtaBeat

Joined Mar 18, 2021
7
Not as long as the TRIG input is low, since it is a level sensitive input.
TRIG has to go high before the one-shot time-out, otherwise it will immediately trigger a new period, which thus makes the output appear to be continuously high.
Ok this makes a lot of sense then as to why this is not working as intended.

To shorten the trigger pulse you can couple it through a series capacitor into two equal value resistors, one to V+ and one to ground, with the junction of the three going to the TRIG input.
Nice, thank you. This is what I was looking for. This is more components than I anticipated since I still also have to use a FET to invert the output, plus this little trick now on the input.

I think I'm going to order some 555 try this, and also order some LTC6993, see what is faster/easier.

I tried to find some MAX supervisors IC's (like metermannd suggested) but unfortunately they all seem to wait for a release of the input reset to output their pulse which would not work in my case (same problem as 555).
 

crutschow

Joined Mar 14, 2008
26,721
Here's a negative-edge-triggered one-shot circuit which uses three passive components along with one CMOS Schmidt-trigger NAND-gate package that's available in DIP.
The output pulse width is independent of the input pulse width.
The pulse width is approximately equal to 0.85*R2C2.
Out and /Out are complementary outputs. V(/Out) is shown (yellow trace).
Does that do what you want?

1616594770931.png
 
Last edited:

Thread Starter

XtaBeat

Joined Mar 18, 2021
7
That is absolutely perfect. It is cheap, comes in DIP packages, needs very few external components, and it's even available at my local electronic store. Thanks a lot.
 
Top