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Re: gEDA-user: another I/O port idea

Hi DJ,

Is having -10V and +10V supply hard in your design?

If so, I think your design with diodes will work if you find transistors that have a tight VGSth spec range. That's hard to get for sure... Tri state anything is going to need tighter analog level control for all parts, which goes away from low tolerance design and gets you into noise susceptibility for any length of wire run...




I like complementary FETs (as an inverter) with 10Megohm R's biasing the tristate out and the gates so they don't receive radio myself. In other words, I think the availability of 0.9V VGS for getting RDSon = 100 ohms or so is easy to shop for in tiny packages.

Then your next stage can use a 10V supply to hard turn on some heater and solenoid handling FETs.


If 10V is off the drawing board, I think it IS going to be hard to shop for the 10A FETs with RDSon of .001 Ohm that switch from 0.66V or 2.7V.



For motor and power control signal runs, I am liking the idea of plastic optical fiber, master-slave, daisy-chained-repeater function, fail-safe-cut-cable, one-control-channel-per-wavelength, not tristate.

John G

DJ Delorie wrote:
Been thinking about what it would take to convert to all 3.3v (outside
the last stage I/O drivers) in the furnace project. 




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.



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