Hi all, OK a silly basic question. I'm doing some digital stuff. I wanted a one shot to reset some things, but didn't want to put in an entire chip (74HC123.. or whatever the number is.) So I just used a cap, resistor, diode and inverter.
Thanks, George H.
The output rise time will be slow, due to the limited gain of the gate. Using a 74HC14 Schmitt inverter will help.
It loads down its input very badly on falling edges, and the pulse width will be uncertain by about a factor of 2 due to the threshold voltage tolerance of the inverter.
But for some jobs, a little M-squared-L is just the ticket.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
(weird I can see your reply but not my original post.. google groups is broken again... (yeah I know.))
Check that's what I've got.
Good point. I'll add another resistor in series with the cap. (say 1k in series an 10k to ground.) That will take care of my worries about the diode current too.
Yeah no problem there... I've got plenty of time.
M-squared-L? what's that? A transformer? (googling brings up Mellissa's instagram site... I don't think that was your reference. :^)
George H.
Leave D5 out if you don't mind symmetrical delay. ...Jim Thompson
--
| James E.Thompson | mens |
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| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
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I love to cook with wine. Sometimes I even put it in the food.
Mickey Mouse Logic. (hat tip to Don Lancaster)
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
If you have two inverters you can put two in series and have a little hysteresis back to the first inverter via a R network to give you a nice fast snap.
Of course, if you had a Schmitt inverter that would do the same for you , too. I am sure the common ones are more than quick enough.
Or you can combine a PNP and NPN in a thyristor configuration which will not hold latch but will snap very quickly and give you hysteresis.
Or, you could employ a 555 and just do a one shot there. But in that case, you could simply open the digikey or mouser book and get a power management chip.
Jamie
Thanks Jim... I'll have to look at that a bit more. Is this going to give me a delay or a one shot at the edge? (At the moment I just want a one shot...)
George H.
Thanks Jamie, What i've got looks very simple. Two R's, a C and Diode.
George H.
Got it. (That doesn't sound very good at the design review though.)
Does Don do stuff like this in his CMOS cookbook. (I've never read any of Don's books. I thought it would be obsolete, but it looks to still be selling well on amazon.
George h.
[snip]
It's a delay block, but should serve your purpose as a delayed reset.
I use that architecture in a lot of my chip designs because, without ESD, it's a pretty stable delay, independent of VDD, pretty much just the RC time constant for delays much longer than the inherent inverter delay. ...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
I think it might be the TTL book, but it's certainly one of the two.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Dunno about the TTL book, but in the "CMOS Cookbook" it's on pages 186-189. Great book! A few specific topics might be outdated, but the sections on one-shots and oscillators are particularly handy.
Best regards,
Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis
Got it, thanks Bob. (Figure 4-36 in the second volume.. found on amazon.) Hey I hadn't thought about tying the R and diode to the positive rail. George H. ps anything found in the cmos cookbook must be kosher. :^)
My solution is a cmos 4093 quad two input schmitt trigger. resistor, cap and one gate
Another home-made one-shot is a d-flop that resets itself. That's edge sensitive.
If you connect q-bar to reset with a delay line of some sort, it works fine. An RC is trickier. Original 74xx TTL would usually hang if it tried to reset itself with an RC. Later logic, with more gain, would usually reset clean.
This works well with fast logic, like EclipsLite. Just a bit of trace makes a good delay line, like for a 1 ns one-shot.
-- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
Funny story: Back when this first came out (1977ish) and I was thinking of buying it, my mother asked me what she could get me for Christmas... always a problem for her. So I figured I could hold off on buying it myself, and I told her "the CMOS Cookbook by Don Lancaster".
Christmas came and she gave me something else, saying "I searched the cookbook sections of 3 different bookstores, even asked at the desk, but nobody had heard of the "Sea Moss Cookbook"!
Best regards,
Bob Masta DAQARTA v7.50 Data AcQuisition And Real-Time Analysis
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