gEDA-user: another I/O port idea

[DJ Delorie <dj@xxxxxxxxxxx>]

Been thinking about what it would take to convert to all 3.3v (outside
the last stage I/O drivers) in the furnace project.  I was thinking
how hard it would be to get mosfets with a small enough Vgs for my
last I/O port plan (Vgs would need to be less than 1v) when it
occurred to me that I can use the high Vgs to my advantage.

Checking a few random mosfets, it looks like turn-on starts around
1.5v and really kicks in around 2v.  2v is more than half of 3.3v, we
just need a little more drop to keep the 1.5v above it also.  Consider
this schematic:

http://www.delorie.com/pcb/io_port3.html

When OUT (output from the micro) is tri-stated, the gate voltages
should be 2.15v and 1.05v (half of 3.3v, offset by the 0.5v diode
drops), which is below the Vgs threshold.  When OUT is high (min
2.64v) then Q2's gate sees min 2.14v Vgs, enough to turn it on
(there's a second mosfet stage for power).  When OUT is low (max
0.66v), Q1 sees 2.14v Vgs, enough to turn *it* on.

It could probably be done with resistors replacing the diodes, too.
Hmmm... four equal resistors gives 0.825v Vgs during tri-state, but
2.97v Vgs when active.

Comments?


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