Forums

Protecting a CMOS gate input

Started by Pimpom May 15, 2020
On Fri, 15 May 2020 14:26:02 +0100, Piglet <erichpwagner@hotmail.com>
wrote:

>On 15/05/2020 11:53, Pimpom wrote: >> I'm going to use a CMOS bistable chip (not a uP) that's to be manually >> triggered from time to time by a mechanical switch. It has a debouncing >> circuit but since the switch is to be connected by a removable >> jack-and-cable set and operated by non-techno savvy users, I thought I'd >> include a few extra components as a precaution. >> >> The input will have a series resistor followed by a capacitor to ground >> and schottky diodes to Vdd and Vss. Do you think any of this is >> superfluous since the chip already has similar protection built-in? > >The schottky diodes may well be superfluous depending on the value of >series resistance. If R is high enough to keep highest expected surge >voltage well below input ESD diode latch-up trigger current. The >parallel C has to be large enough to limit rise time to least many ns >for the diodes to begin forward conduction. Since the input is from a >push button and the input is not edge rate critical you can afford to >massively overdo the series R and shunt C. You haven't mentioned having >a pullup or pull down resistor, generally pushbutton contacts need at >least 100uA "wetting" current to ensure reliable operation.
, If a long unshielded cable to the pushbutton is used, it is going to pick up RFI, especially since the pushbutton is open circuit most of the time. Make sure you bypass any RF entering rom the cable into your device, before it hits any semiconductors. Do not let the RF current circulate around the PCB, filter it out close to the cable socket. These days the low pass filter should work well for at least from 0,1 MHz to several GHz to get rid of any AM broadcast and smart phone signals. The GSM cellular phone signal is TDMA and if is rectified in an oxidized cable connector or some protection diodes, it will generate a square wave of a few hundred Hertz, which could misfire the flip-flop.
>The GSM cellular phone signal is TDMA and if is rectified in an >oxidized cable connector or some protection diodes, it will generate a >square wave of a few hundred Hertz, which could misfire the flip-flop.
The FF has at least 4V of noise immunity. That would take quite the cell phone. ;) Cheers Phil Hohhs
On Friday, May 15, 2020 at 5:04:57 PM UTC-4, pcdh...@gmail.com wrote:
> >The GSM cellular phone signal is TDMA and if is rectified in an > >oxidized cable connector or some protection diodes, it will generate a > >square wave of a few hundred Hertz, which could misfire the flip-flop. > > The FF has at least 4V of noise immunity. That would take quite the cell phone. ;)
Won't the signal induce a current proportional to the loop size so that the resulting voltage will then be dependent on the input resistance? The devil is always in the details. That's why experts get called in to solve these sorts of problems. Often it is not a matter of black magic, but of being meticulous. The circuit is running from 12 volts, so lots of noise margin, but also the circuits are high impedance. Damned if you do, damned if you don't. -- Rick C. -+- Get 1,000 miles of free Supercharging -+- Tesla referral code - https://ts.la/richard11209
On 5/15/2020 7:28 AM, mpm wrote:
> Not sure I know what a bistable chip is, but I often use optocouplers when connecting microcontroller port pins to the outside world. Of course, that approach limits you to one-way in or out of the chip. (And the de-bounce can usually be done in software.) >
Using an ADC is a valid way to read button states, too. I can take a 8 pin AVR with 3 or 4 ADCs on board and an LM324 to buffer the voltage and protect whatever is on the other side and multiplex 15 or 20 buttons, using the buttons to switch in resistors in a divider to the ADCs and thresholds set in software. A moving-average filter/sample binning + state machine prevents any bounce-glitches it's a very solid arrangement.
On 5/15/2020 9:00 PM, bitrex wrote:
> On 5/15/2020 7:28 AM, mpm wrote: >> Not sure I know what a bistable chip is, but I often use optocouplers >> when connecting microcontroller port pins to the outside world.&nbsp; Of >> course, that approach limits you to one-way in or out of the chip. >> (And the de-bounce can usually be done in software.) >> > > Using an ADC is a valid way to read button states, too. I can take a 8 > pin AVR with 3 or 4 ADCs on board and an LM324 to buffer the voltage and > protect whatever is on the other side
Er, protect the uP inputs, getting my circuits confused :)
On 5/15/2020 8:45 PM, jlarkin@highlandsniptechnology.com wrote:
> On Fri, 15 May 2020 16:23:57 +0530, Pimpom <nobody@nowhere.com> wrote: > >> I'm going to use a CMOS bistable chip (not a uP) that's to be >> manually triggered from time to time by a mechanical switch. It >> has a debouncing circuit but since the switch is to be connected >> by a removable jack-and-cable set and operated by non-techno >> savvy users, I thought I'd include a few extra components as a >> precaution. >> >> The input will have a series resistor followed by a capacitor to >> ground and schottky diodes to Vdd and Vss. Do you think any of >> this is superfluous since the chip already has similar protection >> built-in? > > If you expect/experience static discharges, the network might avoid > failures. Best to not expose any semiconductor the the outside world. > > If you expect ESD zaps to change the state of a cross-coupled gate > flop, put a cap on one output to ground. > >
I thought I'd place a resistor across the switch contacts, high enough not to cause triggering, low enough to prevent static build-up.
On Saturday, May 16, 2020 at 2:32:34 AM UTC-4, Pimpom wrote:
> On 5/15/2020 8:45 PM, jlarkin@highlandsniptechnology.com wrote: > > On Fri, 15 May 2020 16:23:57 +0530, Pimpom <nobody@nowhere.com> wrote: > > > >> I'm going to use a CMOS bistable chip (not a uP) that's to be > >> manually triggered from time to time by a mechanical switch. It > >> has a debouncing circuit but since the switch is to be connected > >> by a removable jack-and-cable set and operated by non-techno > >> savvy users, I thought I'd include a few extra components as a > >> precaution. > >> > >> The input will have a series resistor followed by a capacitor to > >> ground and schottky diodes to Vdd and Vss. Do you think any of > >> this is superfluous since the chip already has similar protection > >> built-in? > > > > If you expect/experience static discharges, the network might avoid > > failures. Best to not expose any semiconductor the the outside world. > > > > If you expect ESD zaps to change the state of a cross-coupled gate > > flop, put a cap on one output to ground. > > > > > I thought I'd place a resistor across the switch contacts, high > enough not to cause triggering, low enough to prevent static > build-up.
I'm more worried about when the cable is plugged in. I recall plugging in audio jacks to music amps and getting the big hum blast. The guy whose equipment it was would yell at me. I guess cabling problems are more of a problem when dealing with cabling different chassis together. Then you have ground differences and lots of things can be a problem. Plugging in a remote switch might be a lot less of an issue if it isn't grounded to anything. -- Rick C. -++ Get 1,000 miles of free Supercharging -++ Tesla referral code - https://ts.la/richard11209
On Fri, 15 May 2020 14:04:53 -0700 (PDT), pcdhobbs@gmail.com wrote:

>>The GSM cellular phone signal is TDMA and if is rectified in an >>oxidized cable connector or some protection diodes, it will generate a >>square wave of a few hundred Hertz, which could misfire the flip-flop. > >The FF has at least 4V of noise immunity. That would take quite the cell phone. ;)
For CD4013, the noise margin at Vdd=5 V is quoted as 1 V and at Vdd=15 V as 2.5 V. Consumer electronics is suppose to withstand a 3 V/m field strength without malfunctioning. A random wire several wavelengths long will capture about similar voltages as a 1/4 wavelength monopole. A 1/4 wave monopole at 900 MHz is about 0.1 m, so 3 V/m will induce 0.3 V into the wire. This is for the far field. When the transmitter is in the near field, the voltage can vary significantly.
On 2020-05-16 08:44, Ricky C wrote:
> On Saturday, May 16, 2020 at 2:32:34 AM UTC-4, Pimpom wrote: >> On 5/15/2020 8:45 PM, jlarkin@highlandsniptechnology.com wrote: >>> On Fri, 15 May 2020 16:23:57 +0530, Pimpom <nobody@nowhere.com> >>> wrote: >>> >>>> I'm going to use a CMOS bistable chip (not a uP) that's to be >>>> manually triggered from time to time by a mechanical switch. >>>> It has a debouncing circuit but since the switch is to be >>>> connected by a removable jack-and-cable set and operated by >>>> non-techno savvy users, I thought I'd include a few extra >>>> components as a precaution. >>>> >>>> The input will have a series resistor followed by a capacitor >>>> to ground and schottky diodes to Vdd and Vss. Do you think any >>>> of this is superfluous since the chip already has similar >>>> protection built-in? >>> >>> If you expect/experience static discharges, the network might >>> avoid failures. Best to not expose any semiconductor the the >>> outside world. >>> >>> If you expect ESD zaps to change the state of a cross-coupled >>> gate flop, put a cap on one output to ground. >>> >>> >> I thought I'd place a resistor across the switch contacts, high >> enough not to cause triggering, low enough to prevent static >> build-up. > > I'm more worried about when the cable is plugged in. I recall > plugging in audio jacks to music amps and getting the big hum blast. > The guy whose equipment it was would yell at me. > [...]
Choose your connectors wisely. Audio jacks have the nasty property that the signal connects before the ground. Stupid, but that's the way it is. Jeroen Belleman
On 2020-05-15, Pimpom <nobody@nowhere.com> wrote:
> I'm going to use a CMOS bistable chip (not a uP) that's to be > manually triggered from time to time by a mechanical switch. It > has a debouncing circuit but since the switch is to be connected > by a removable jack-and-cable set and operated by non-techno > savvy users, I thought I'd include a few extra components as a > precaution. > > The input will have a series resistor followed by a capacitor to > ground and schottky diodes to Vdd and Vss. Do you think any of > this is superfluous since the chip already has similar protection > built-in?
I'd add another resistor from that set-up to the chip itself 1K-100K whatever is convenient -- Jasen.