Re: gEDA-user: furnace controller II: I/O driver

["Phil Taylor" <phil@xxxxxxxxxxxxxxxxxxxxxxxx>]

Just scratching the surface here, but, there doesn't appear to be a bias path
for your BJT's in this ckt.  The mosfets depend on leakage (in the bjt and in
their gates) to turn off.  The gate drive BJT's, being single ended, can't
pull the gates to "off" actively.  The circuit depends on leakage to do it. 
It's an analog to a software race condition, will both devices turn off at the
same time?  Chances are it'll work okay, and chances are, the little mos
devices will tolerate some shorting of the rails if that's the way the
switching speeds align.  However.

Given that you have a noisy environment.  What's the driving impedance when
the mos devices are tristated?  ... it's real high, meaning that weird noise
on the board, in the air, or in the ground path can cause those un-driven
gates to start to turn-on, which is a bummer.  It would seem that something
needs to pull those gates to source when not driven.

Have you fired up a demo of this ckt?

phil


DJ Delorie <dj@xxxxxxxxxxx> wrote:

> 
> I've talked about this before, and this is probably the weirdest part
> of the new furnace controller, so I'm open for input.
> 
> The furnace controller will have eight of these I/O drivers, two per
> zone.  Nominally one for input and one for output, but in my case it's
> one for the dallas 1wire interface and one 9600 baud for the LCD
> panels.  I'm trying to stay a bit generic to make the board useful for
> other applications, or future changes to the thermostats.
> 
> The goal is to, essentially, create a high current filtered tri-state
> I/O port, using two GPIO pins.  Details on the web page:
> 
> http://www.delorie.com/house/furnace/pcb2/io_port.html
> 
> If Maxim has a chip for this, let me know ;-)
>