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worlickwerx / pi-parport

retro parallel port for raspberry pi
GNU General Public License v2.0
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handle standby mode #66

Open garlick opened 3 weeks ago

garlick commented 3 weeks ago

Problem: the HAT+ spec describes standby mode, where 5V is powered and 3V3 is not.

https://datasheets.raspberrypi.com/hat/hat-plus-specification.pdf

Since the 74LVC16128 is powered by both 3V3 and 5V, having it only partially powered up when the pi is in standby might be bad? I didn't spot anything about this in a quick skim of the data sheet.

If we need to switch off 5V when 3V3 is off, we can add a load switch such as the TPS27081

quorten commented 3 weeks ago

When all power is removed from the 74LVC16128, this chip does seem to provide ESD protection. A low-current continuity test on the chip outputs when powered off makes them appear as if they are bridged together (probably bridged with ground).

I would assume pi-parport can handle going into standby mode fine. It's just that there wouldn't be ESD protection from the transmission line equivalent to that of power-off. The load switch would make for a better design, at a somewhat higher complexity and cost.