Forums

Problem regarding the Reset Pin of MCU

Started by Huey Fen September 24, 2005
Huey Fen wrote:
> With this, i attach the schematic of my design for your reference.
> The reset circuitry should be okie. Because if MCU is disconnected from it, I would obtain 5V when the reset button is released and 0V when reset button is pressed. As you can see from the schematic as well, I am trying to use a PSD to expand the memory space of my design. If PSD reset pin is the connected to the reset circutry without MCU, then I would still get the correct reading at reset pin of PSD. But once MCU is connected to reset circuitry, everything goes wrong. I suspect the MCU had internally pull the reset pin low. But I don't understand how could this happen. I came across a sentence stated that reset pin of MCU will give an open drain output to indicate internal failure from M68HC11 reference manual. Is it the case that I am facing now? Any solution for that?
> I've been tried to operate the MCU in special bootstrap mode. But I was failed to get the MCU communicate with the host PC. Will the CONFIG of MCU been modified during the bootstrap mode? (But I didn't get the MCU successfully communicate with PC) I didn't do any configuration on the MCU. But the PSD is configured. So I am not sure the COP is being enable or not. I am still trying to figure out this.
> Hopefully that I can get your reply soon. Your help is much appreciated. Thanks a lot. > Huey Fen wrote:
>
>>Hi here. I am doing my final year project on the design of Single Board Computer by using M68HC11. But I am facing a problem on the reset pin of the M68HC11.
>>My reset circuitry worked fine without the connection of MCU. But once the MCU is connected to the reset circuitry, the reset pin (pin 17) of MCU became 0V. I suspected that the MCU had internally pull the reset pin to 0V. But I don't understand how could this be happened. At first, I suspected a failure with the MCU, hence I've replaced it with a new one. But the same problem occured.
>>So may I know what is the possible factor that can cause this phenomenon. Hopefully that you guys out there can give me some hints regarding this matter. Your help is greatly appreciated. Thanks in advance.
> > Please hit return from time to time, ok?
>
> The /RESET pin in an input and an output. There are several possible
> causes for what you describe. Put up a description of your circuit
> and maybe we can help. Do you have COP enabled?
>
> Mike


Hmm... I got an email saying there is a schematic for download at
http://s4.yousendit.com/d.aspx?id=0JXT3KE97Y69F2WCYSU79OY7UZ

That's a very slowly-responding link, but I got it.

Your reset circuitry looks fine. I'd have rearranged it somewhat,
but that should do.

My guess: you have misconfigured the MC68HC11 in some way, and
it's getting an internal reset, which gets sent out over the
/RESET line.

That's why I asked you about COP.

Please don't top-post.

Mike
--
p="p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}
This message made from 100% recycled bits.
You have found the bank of Larn.
I can explain it for you, but I can't understand it for you.
I speak only for myself, and I am unanimous in that!



> I didn't realize that the schematic was stripped. I've upload the schematic to m68hc11 yahoo! groups photo album.u guys may view from there.sorry for the inconvenience.

Reset circuitry looks good. I would decrease -RESET pull-up resistor to
4k7 which is standard recommended value giving better noise immunity and
cleaner transition edges when there's capacitance on -RESET line.
I would also add resistor in series with RESET pushbutton, so that input
voltage for bottom MC34064 doesn't fall below minimum input voltage for
this chip (1.0V), I'm not sure it will surely pull output low with input
grounded.
Does the microcontroller run any software that could issue RESET
internally (watchdog for example) ? What's the NOCOP bit setting in
CONFIG register ?

--
Michal Konieczny
mk@mk@....



I've got my reset circuitry worked. After all the troubleshooting, I found out
that the problem was with the power supply.
I used 9V battery before this and now I have changed to a power supply
provided in the lab of my university, which gives 5V and 3A fixed supply.
At last, I got the correct voltage level for my reset pin.
Thanks for all the helps from you guys. =)

Michal Konieczny <mk@mk@....> wrote:> I didn't realize that the schematic was stripped. I've upload the schematic to m68hc11 yahoo! groups photo album.u guys may view from there.sorry for the inconvenience.

Reset circuitry looks good. I would decrease -RESET pull-up resistor to
4k7 which is standard recommended value giving better noise immunity and
cleaner transition edges when there's capacitance on -RESET line.
I would also add resistor in series with RESET pushbutton, so that input
voltage for bottom MC34064 doesn't fall below minimum input voltage for
this chip (1.0V), I'm not sure it will surely pull output low with input
grounded.
Does the microcontroller run any software that could issue RESET
internally (watchdog for example) ? What's the NOCOP bit setting in
CONFIG register ?

--
Michal Konieczny
mk@mk@.... SPONSORED LINKS
Freescale semiconductor inc Microcontrollers Pic microcontrollers 8051 microprocessor

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Huey Fen wrote:
> I've got my reset circuitry worked. After all the troubleshooting, I found out
> that the problem was with the power supply.


LOL!

Ain't that just the way it is!

Man, no matter how smart we are, the hardware always manages to find
a way to bamboozle us! > I used 9V battery before this and now I have changed to a power supply
> provided in the lab of my university, which gives 5V and 3A fixed supply.
> At last, I got the correct voltage level for my reset pin.
> Thanks for all the helps from you guys. =)
>
> Michal Konieczny <mk@mk@....> wrote:> I didn't realize that the schematic was stripped. I've upload the schematic to m68hc11 yahoo! groups photo album.u guys may view from there.sorry for the inconvenience.
>
> Reset circuitry looks good. I would decrease -RESET pull-up resistor to
> 4k7 which is standard recommended value giving better noise immunity and
> cleaner transition edges when there's capacitance on -RESET line.
> I would also add resistor in series with RESET pushbutton, so that input
> voltage for bottom MC34064 doesn't fall below minimum input voltage for
> this chip (1.0V), I'm not sure it will surely pull output low with input
> grounded.

I had similar thoughts (though I only mentioned that I'd connect things
up differently).

> Does the microcontroller run any software that could issue RESET
> internally (watchdog for example) ? What's the NOCOP bit setting in
> CONFIG register ?

I asked the same!

Man, no matter what you do, there's always another way to screw up!

Mike
--
p="p=%c%s%c;main(){printf(p,34,p,34);}";main(){printf(p,34,p,34);}
This message made from 100% recycled bits.
You have found the bank of Larn.
I can explain it for you, but I can't understand it for you.
I speak only for myself, and I am unanimous in that!