Why is there a diode parallel with resistor in Reset circuit for microcontroller

[vantusaonho]

Hi everybody!
I checked a available board and found this circuit. I wonder that what is function of diode in that circuit? Please tell me if you know it already. Thanks!!!


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Answer from Metal: "The diode helps discharging the capacitor quickly when VDD is turned off, as simple as this."

My next question: Yes! I found out several case which a diode in parallel with resistor like that. But I think in this case that the diode has another function. Do you think like me?



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[metal]

The diode helps discharging the capacitor quickly when VDD is turned off, as simple as this.

[TucoRamirez]

It's a free-wheeling diode!!

check this:


http://www.embeddedadventures.com/pages/p/News/id/46

[reksbg]

As metal said it helps the capacitor to discharge during VDD glitches. But keep in mind that this is an old design - most (if not all) of the new MCUs have internal Low-Voltage Detection and Reset circuits that monitor the VDD and also they have ESD protection diodes on almost all the pins to VDD and GND. So there is an internal diode in the chip that will do the same thing when the VDD drops below the capacitor voltage.

[metal]

Guys, look at figure 3-2 in PIC18F452 datasheet, page 26, all is explained there. I came across this hint  about info in the datasheet from another board where someone mentioned that this datasheet contains necessary info about this reset circuit, moments ago, I figured I should mention it here too.

[Old_but_Alive]

I have never ever bothered with a diode, and never had a problem, also, the capacitor is not really needed with the power-up timer facility in almost all PIC's these days

[wellnerson1]

I hope that the diode is acting as a clamping diode - not to let the voltage at the RESET pin to go beyond VDD so that to protect the RESET pin due to any contingency happened. i.e. is if the voltage at RESET pin tries to become greater than VDD the diode starts conducting to clamp the voltage to VDD.

[hate]

16f series PICs are programmed by applying ~13V to their reset pins, 18f series should be the same (not sure). So keeping the VDD at 5v will sure prevent putting the pic in programming mode and doesn't really damage the reset pin. If the diode in your first schematic was inverted, I could say it's a protection for the 5V power supply while in programming mode but it's not. Maybe you have misinterpreted the pcb?

[TucoRamirez]

yep i think the same as hate but in the sch there's no PROGRAM label so i think is a preprogrammed mcu or whatever ...

[hate]

Well it's sure preprogrammed as the ISP programming is disabled by design. What I meant was, if both ISP programming and an external reset circuit is required, the simplest approach would be the exact same design except the diode would be inverted.

[Catcatcat]

I have almost all designs using such a scheme, it does not prevent in-circuit programming

[oxygen007m]

The diode helps discharging the capacitor quickly when VDD is turned off, as simple as this.

Exactly Metal is right , but other function of this diode is preventing clear pin from exceeding voltage above VDD which may be necessary in some conditions .

[hate]

I've seen the datasheet, the purpose of the diode seems to be so. But what makes one think that when the VDD is disconnected, it's connected to GND or any similar effect so the cap discharges. What's the mechanism behind this? From my point of view VDD will be open circuit when disconnected so the cap won't discharge. ;/

[metal]

think of the diode as a short circuit for the capacitor when there is no VDD.

[pickit2]

think of the diode as a short circuit for the capacitor when there is no VDD.

And other comments the supply for the circuit is via the capacitor, so capacitor acting like a battery is discharged very quickly.

[hate]

think of the diode as a short circuit for the capacitor when there is no VDD.

Maybe I couldn't express myself clearly. To think the diode as the short circuit for the cap, the disconnected VCC must act like GND, right? So the anode of the diode would then be connected to one pin of the cap and cathode to GND whereby the other pin of the cap is also connected. How is that possible?

And other comments the supply for the circuit is via the capacitor, so capacitor acting like a battery is discharged very quickly.

Do you mean VCC decreases until it reaches GND level?

[pickit2]

in the circuit of the first post, there in no reset button, so reset is by removing the power.
without the diode there could be a quick power off/on and the state of reset pin, would not go low due to voltage still in the capacitor, not falling to reset value.

[zab]

Diode is to ensure reset too go low with supply off.so that controller may reset with next time supply restores. The way is used to avoid brown out hanging of controller.