Re: [N8VEM: 14590] Switch debouncing and related stuff (like CPU reset circuits)

[Paul Anderson <wacky...@gmail.com>]

I have a tube of attiny45's for similar purposes. 8 pins and 4k of flash, cheap enough to drop in almost anywhere. 

--------
Paul Anderson -- VE3HOP

On 2012-09-09, at 4:32 AM, Martin Lukasek <martin....@artax.cz> wrote:

> Hi Sergey,
> 
> I use small ATiny (13 or any other small suitable and cheap) with SW debouncer for ATX power supply, asi it is easy to implement not just debouncing, but also power button delay (long press) for power off. Also you have to keep PS_ON low all the time you want the power on. I can send the schematic and source if interest.
> 
> Best regards
> 
> Martin
> 
> -----Original Message-----
> From: n8...@googlegroups.com [mailto:n8...@googlegroups.com] On Behalf Of Sergey
> Sent: Sunday, September 09, 2012 8:51 AM
> To: N8VEM
> Subject: [N8VEM: 14588] Switch debouncing and related stuff (like CPU reset circuits)
> 
> Hi,
> 
> Electrical engineers among us, please correct me if I wrong :-)
> 
> I was designing an ATX power-on circuit for my next 8088/XT board, and one of the issues was debouncing of the power switch (which is a momentary switch).
> 
> It must be some kind of conspiracy about switch debouncing, it looks like 90% of the related pages on the Internet are referring to this
> "research": http://www.ganssle.com/debouncing-pt2.htm . More exactly to the circuit on Figure 3 (the circuit on Figure 1, which also frequently referred everywhere, is mostly unusable, since nobody will use a double throw switch just to overcome the contact bounce)
> 
> The problematic part there is the purpose of the diode... This diode in the configuration shown on the diagram is useless. It will conduct electricity only then voltage difference on it is bigger than diode forward voltage drop, which happens only during very short time. The given explanation doesn't make any sense. The claim that the voltage on the capacitor Vfinal will be 4.3V, is wrong... after voltage difference on the diode will be less than diode forward voltage drop, the capacitor will continue to charge through R2, and eventually will reach the power supply voltage (5V).
> 
> The only useful purpose that diode might serve in such circuit is the protection of the Schmitt tigger's input. In this case the diode should be connected backwards - with the anode connected to the capacitor and the cathode connected to Vcc.  This is mostly useful then using CMOS gates that have ESD protection diodes on inputs. In this case, when power supply disconnected, capacitor might discharge through these ESD protection diodes, potentially damaging them. If diode is connected as described above, capacitor will discharge through that diode.
> 
> Another problem with debouncing circuits (and CPU reset circuits we use in N8VEM projects) is that capacitor is discharged through switch, potentially passing very high current through that switch, and leading to its failure. To eliminate that we used to add a 10 ohm resistor in series with the switch, but it looks like 10 ohm is too low to limit the current. Most tactile switches are rated for 50 mA. When using 10 ohm resistor, the worst case current will be 5V / 10 ohm = 0.5 A. Ten times more than the switch rating... Now one solution will be just increasing the resistance of that current limiting diode, say to 100 ohm. But replacing resistor in naive way will basically form a voltage divider, and depending on the ratio of R1/R2 (resistor that is used to charge the capacitor to the resistor that used to limit the discharge current), it is very possible that resulting voltage in the "switch closed" position will be higher than Schmitt trigger threshold. So instead of that, I'd recommend using circuit similar to one shown on Figure 3 (or Figure 2) on the page above, but connecting diode in reverse between the capacitor and Vcc.
> 
> Any comments?
> 
> Thanks,
> Sergey
> 
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