Electronics - no place for wimps.   If you don't use it you lose it. - Page 1
Forum section: electronics / projects

author: leonababy
author_rank: Regular Contributor
timestamp: April 18, 2022, 06:09:16 pm
content: I last designed a digital circuit of complexity in 1984. I am designing something that is of modest challenge, with a slow clock (120Hz). In troubleshooting why I have oscillations in my circuit when trying to get a one shot working downstream, I tried decoupling caps, changing routing of wires on my solderless breadboard, adding a LDO regulator to a zero crossing circuit that already had a Schmitt trigger. Flailing about, I finally focused on the inputs and outputs of a 74HCT74. The setup times were met, but when Q went low, it rang like a son of a gun between 0 and 4.5V, at 50 Mhz. This would likely tip off someone who knows what they are doing.A little voice in my brain from way in the past finally spoke up - you have an unused input, you BIG DUMMY! I went to my hand drawn schematic, and sure enough I had 5 pins per FF. 14 pin package, two pins for Vcc and GND. Light bulb getting brighter. Two unneeded pins are missing. Suddenly I realized, a 74 has a preset! DOH! I spent hours on what should have never happened in the first place, or 15 minutes after the symptom was observed.Now I recall why I switched to Systems Engineering. Let the pros do the hardware design.

author: Ian.M
author_rank: Super Contributor
timestamp: April 18, 2022, 06:37:07 pm
content: Yes. Old fashioned bipolar TTL let you get away with stuff like floating inputs as they floated high fairly reliably, as long as there wasn't a trace connected for EMI to wiggle and it wasn't operating in a high EMI environment.However as you have just found this is absolutely not the case with any sort of CMOS logic due to their much higher input impedance, and in most cases except MCUs and programmable logic (with pullups enabled), absence of input pullups. The up-side is, unlike TTL you don't need pullup resistors for unused inputs, you can simply strap them direct to either of the chip's supply rails without an increased risk of latch-up.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: April 18, 2022, 07:59:29 pm
content: It's worse than you think. Sometimes CMOS circuits work with the power supply accidently disconnected.

author: Ian.M
author_rank: Super Contributor
timestamp: April 18, 2022, 08:14:23 pm
content: Quote from: Conrad Hoffman on April 18, 2022, 07:59:29 pmYep. Dave explains parasitic power in this video:EEVblog #831 - Power A Micro With No Power Pin!Forum thread:https://www.eevblog.com/forum/blog/eevblog-831-power-a-micro-with-no-power-pin!/A nasty 'gotya' is that at least one input must be high for the CMOS chip to get parasitic power, so if you get a CMOS chip that *NEARLY* works except it outputs low when all inputs are low suspect a disconnected Vcc pin and similarly if it works except when all inputs are high, and then outputs high, suspect a disconnected Gnd pin.

author: cdev
author_rank: Super Contributor
timestamp: April 18, 2022, 08:24:55 pm
content: A high impedance input picks up what appear to be fairly high voltages in my house. It would probably make a fairly good random number generator, though.

author: mawyatt
author_rank: Super Contributor
timestamp: April 18, 2022, 10:02:57 pm
content: Around 1990 we had a new tech that was a friend of our Dept Head, but he had no business in our lab.Round 1) I had to use him to build a test board for a relatively complex active filter. Testing found the frequency response wasn't even close and traced to all the filter caps values were interchangedHe argued that if the filter design was any good it should work with ANY cap valuesRound 2) He did a digital test board for another colleague Cliff and left all the CMOS unused gate inputs open, he got a ear full from Cliff for thatRound 3) Another colleague Bob, had a complex analog board that had at least 20 op-amps, mostly quad types. Rememeber Bob calling me over to look at this test board which had 20MHz all over the place, but no 20MHz source or clock. A few days later Bob let out a few words from the lab corner when he discovered that this tech had decided since he got reamed for not tying unused CMOS gate inputs to ground that he would tie all the unused op-amp inputs to ground, and since the + and - inputs were ground so the outputs should also be zero volts ground by his thinking, so he tied all the unused op-amp outputs to groundSince this board had many active op-amps scattered all over, and many unused were part of quad op-amps where the unused suffered this fate, all "singing" at the op-amp GBW ProductBest,

author: free_electron
author_rank: Super Contributor
timestamp: April 18, 2022, 10:24:54 pm
content: Quote from: mawyatt on April 18, 2022, 10:02:57 pmNEVER connect both unused opamp inputs to the same potential. That's the sure way to make an oscillator with a frequency purely determined by the speed of the opamp and the parasitic capacitance. You have infinite gain , so a butterfly flapping it's wings on the other side of the world will set it off.Tie one input to ground and set the opamp in unity gain. Better : tie one pin thru a 1k to power, the other through 1k to ground. and another 1k as feedback . that way you have pads to solder a wire on should you have to debug your board. it costs nothing.

author: mawyatt
author_rank: Super Contributor
timestamp: April 19, 2022, 02:48:25 pm
content: Quote from: free_electron on April 18, 2022, 10:24:54 pmWell of course not!!! But that's what this tech decided to do...., then connected the output to groundWe quickly got the techshovedassigned into a new job which involved nothing but paperwork, figuring he could do less damage to the teamBest,

author: TimFox
author_rank: Super Contributor
timestamp: April 19, 2022, 03:47:41 pm
content: Quote from: Conrad Hoffman on April 18, 2022, 07:59:29 pmWhen RCA first introduced their "CosMos" series (4000s) of CMOS logic gates, an RCA engineer visited our local distributor with an interesting presentation.One of his anecdotes: he was testing the prototypes of a counter chip, and finished the setup late on Friday.When he returned to do the actual tests on Monday morning, he found that the maximum toggle frequency was far below spec.Investigating, he found that he had forgotten to apply DC power to the device.The internal input protection diodes were rectifying the square wave input from a low-impedance generator and putting a few volts onto theVddline, but below the minimum voltage requirement.The device still worked, in a limping manner.

author: TimFox
author_rank: Super Contributor
timestamp: April 19, 2022, 04:20:31 pm
content: Quote from: mawyatt on April 18, 2022, 10:02:57 pmI seem to remember another poster here, who shall go nameless, who saw no need to calculate capacitor values for a circuit.

author: mawyatt
author_rank: Super Contributor
timestamp: April 19, 2022, 04:27:21 pm
content: Recall those early RCA CD4000 series. The early 70~80s, we used them a lot and even for analog functions, the inverters made nice limiting amplifiers! They were good to 18V VDD supply, but recall had a version that was good to 20V VDD. There was also an SOS (Silicon on Sapphire) version that was fundamentally RAD hard from ion induced latch-up, the failure mode for most junction isolated CMOS.Best,