Query on ALE pin of 8051.

hi

i was working on a circuit based on an 8051 microcontroller. 89C52 to
be precise. till date my program wasn't executing at all. the program
counter wasn't moving ahead. i had connected the ALE pin (pin 30) to
ground and after floating it, the circuit was running fine.

ALE is a output active high pin and i wasn't aware that it shouldn't
be forcefully grounded. i'm still not sure why you shouldn't tie
output pins to vcc or ground. is there some logic behind this?

thanks 

ali

"Aliasger" <aliasger_18@hotmail.com> wrote in message
news:d0817e19.0404260114.d55ba6d@posting.google.com...
> hi
>
> i was working on a circuit based on an 8051 microcontroller. 89C52 to
> be precise. till date my program wasn't executing at all. the program
> counter wasn't moving ahead. i had connected the ALE pin (pin 30) to
> ground and after floating it, the circuit was running fine.
>
> ALE is a output active high pin and i wasn't aware that it shouldn't
> be forcefully grounded. i'm still not sure why you shouldn't tie
> output pins to vcc or ground. is there some logic behind this?

Yes: if you tie an output to VCC or ground, you create a short circuit,
possibly leading to destruction the component. Only unused INPUTS should be
tied to VCC or ground.

Meindert

Aliasger wrote:
> hi
>
> i was working on a circuit based on an 8051 microcontroller. 89C52 to
> be precise. till date my program wasn't executing at all. the program
> counter wasn't moving ahead. i had connected the ALE pin (pin 30) to
> ground and after floating it, the circuit was running fine.
>
> ALE is a output active high pin and i wasn't aware that it shouldn't
> be forcefully grounded. i'm still not sure why you shouldn't tie
> output pins to vcc or ground. is there some logic behind this?
>
> thanks
>
> ali
For the same reason that you wouldn't short the two pins of a battery
together.  If you short an output that is sourcing a logic 1 to ground the
transistor in the IC's output buffer is connected directly across the power
rails.  It will then, in all probability, be destroyed or at least get
dangerously hot, by the excessive current.

ALE (Address Latch Enable) is used to de-mux the data and address lines 
when accessing external memory.

Mark


Aliasger wrote:
> hi
> 
> i was working on a circuit based on an 8051 microcontroller. 89C52 to
> be precise. till date my program wasn't executing at all. the program
> counter wasn't moving ahead. i had connected the ALE pin (pin 30) to
> ground and after floating it, the circuit was running fine.
> 
> ALE is a output active high pin and i wasn't aware that it shouldn't
> be forcefully grounded. i'm still not sure why you shouldn't tie
> output pins to vcc or ground. is there some logic behind this?
> 
> thanks 
> 
> ali

"Aliasger" <aliasger_18@hotmail.com> wrote in message
news:d0817e19.0404260114.d55ba6d@posting.google.com...
> ALE is a output active high pin and i wasn't aware that it shouldn't
> be forcefully grounded. i'm still not sure why you shouldn't tie
> output pins to vcc or ground. is there some logic behind this?

You have much to learn, grasshopper ;).

I suggest you get hold of a copy of "The Art of Electronics" by Horowitz and
Hill. It's all in there. If you only ever buy one techy book, this should be
it.

Steve
http://www.sfdesign.co.uk
http://www.fivetrees.com

"Steve at fivetrees" <steve@NOSPAMTAfivetrees.com> wrote in message news:<408d831f$0$95327$65c69314@mercury.nildram.net>...
> "Aliasger" <aliasger_18@hotmail.com> wrote in message
> news:d0817e19.0404260114.d55ba6d@posting.google.com...
> > ALE is a output active high pin and i wasn't aware that it shouldn't
> > be forcefully grounded. i'm still not sure why you shouldn't tie
> > output pins to vcc or ground. is there some logic behind this?
> 
> You have much to learn, grasshopper ;).
> 
> I suggest you get hold of a copy of "The Art of Electronics" by Horowitz and
> Hill. It's all in there. If you only ever buy one techy book, this should be
> it.
> 
> Steve
> http://www.sfdesign.co.uk
> http://www.fivetrees.com

why thank you kind sir, i will have to get myself a copy :-)