Alternative to tristate buffer using discretes

[robinaspey]

Hi,
I have a watchdog circuit on a uP that is used to replace the onboard watchdog that is
used for hibernation etc. It tested out most of the time does the right thing.
The external watchdog is an ADM706P, which works fairly well.

The problem is sometimes when this causes a reset the uP isnt able to startup again,
which means the output of the ADM706P watchdog needs to be tristate until the uP
starts up after which an external enable from the uP will un-tristate the output from
the external watchdog.

I've laid out a board but due to the lack of space (and other reasons) I'd prefer to
find another solution perhaps using an open collector transistor to pull the reset line
low. This leads into using an AND gate to switch the NPN open collector / darlington
transistor on.

Again I prefer to do something simple with the transistor so that the the output from
the watchdog (3.3V) and the enable I was planning to use for the tristate can switch
the transistor / darlington on. I'm basically trying to avoid adding any extra IC's.

Is there a nice simple way to do this, e.g. summing the inputs to the darlington -
can test this in LTspice but perhaps someone knows a nice elegant solution that
minimises component count and which they are fairly sure will work.

Regards
Rob

 

[BradtheRad]

A diode is simpler than a transistor, if you place it and orient it right.

It presents high impedance when not conducting.

This is not to say I know if it is suitable for your purpose.

 

[FvM]

The problem is sometimes when this causes a reset the uP isnt able to startup again, which means the output of the ADM706P watchdog needs to be tristate until the uP starts up after which an external enable from the uP will un-tristate the output from the external watchdog.

There are usually better way to achieve this.

 

[joebazoka]

Hi. If you need a brown-out circuit you can use some circuits ideas provided for MIcrochip. I have attached some pictures.