Scott Myers <[email protected]> writes: > So, what happens when there is a momentary loss of power on an Altus Metrum > product. Say a switch contact bounces under shock and we get a 50 mS drop > in supply.
50ms is a long time...
> Will they reboot
It will reboot if Vcc drops below 3.0V given the reset controller. And
that's "bad" as the flight computer does not attempt to detect that it's
in flight on power up.
> , or is there enough capacitance in the circuit
> to keep it live for that short time. (Say C39 capacitor on the Telemetrum?)
Let's do a bit of math and figure out how long it will last.
The maximum power draw of the circuit is less than 100mA (GPS is 22mA,
radio is 46mA, everything else is in the noise).
At that current and room temp, v_ldo_in is about 200mV below v_lipo due
to the blocking diode, so at 4.1V in, you've got about 3.9V. The TC2185
has around 100mV drop-out, so you need 3.1V on its input to keep the
3.0V reset controller from firing. That gives us 0.8V of margin. 100mA
at 3.0V (the minimum) looks like 30Ω. So, we have our RC time constant of:
τ = 30 * 47e-6 = 1.410e-3 (seconds)
Vt = Vo * exp(-t/τ)
3.1 = 3.9 * exp(-t/τ)
3.1 / 3.9 = exp(-t/τ)
log(3.1 / 3.9) = -t/τ
log(3.9 / 3.1) = t/τ
t = τ * log(3.9/3.1) = 323µs
It will probably last longer than this because if you're not
transmitting and GPS is locked, power consumption will be much lower. At
25mA, you'd last about 1.2ms. That's enough to cover a typical switch
bounce.
Getting to 50ms would require a cap which is 50 times larger, or 2500µF,
which would be about 1" tall and 3/8" in diameter. That would introduce
reliability issues under acceleration, due to the size of the component.
> Of course one does what they can so there is zero loss in power under all
> conditions.
We've thought a bunch about this and really haven't figured out any good
way to recover from power failure; there are lots of possible states you
might come back up in, and firing pyro charges at random based on a
guess that you're in the air seems like a worse safety problem than
not.
--
-keith
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