Le Tuesday 18 August 2015, Graham a écrit :
> You could make the 74HC541 work, since it has a CMOS input and should not
> load the BBB during boot, provided that it has power supplied the entire
> time the BBB is booting.
>
> I was thinking more of something like the 74CBTLV3126 bus switch which
> would disconnect your existing circuits from the BBB, rather than buffer
> them like the 74HC541.
I see. Bus switch is interesting as it allows bi-directional
communication; unused outputs could be configured as inputs...
On another hand, buffer offers a better protection of the BBB pins.
> In both cases, the switching should be done from the 3.3 V bus, but you
> can not power the ICs from the 3.3V bus as you mentioned. As I
> remember, on the BBB, the 3.3V bus rail on the P9 connector does not
> power up until AFTER the BBB has completed booting, so if you power the
> buffer or isolation switch ICs from the 3.3V rail, the ESD protection
> diodes in the IC will clamp the boot lines to ground, preventing the BBB
> from booting. You will need to power the ICs, whichever you choose to
> use, from a separate 3.3V regulator that provides power anytime the BBB
> has power applied.
>
> I think the +5V rail is powered anytime the BBB is powered, so it could
> drive the separate +3.3V regulator.
>
> A few Voltage measurements are appropriate, before you finish your
> re-design.
I plan to add a 5V regulator on my cape, so the servos batteries (6V
Ni-mH) will power the BBB too; I'll also use a 3.7V Lipo battery managed
by the PMIC chip.
So no problem to have the ICs supplied before the boot sequence.
Thanks,
--
Frédéric
--
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