Schottky diode on mez board

[paulbuzzby]

A group member suggeted

To avoid interference between the 5V and USB supplies to the Pico, I recommend inserting a small Schottky diode just before Vsys (pin 39), something like a 1N5819.

[davidhannaford]

The view from Alvaro ( diazgutmann@aol.com ) is that in the mezzanine board the Shottky diode should be placed in the track that goes from the 5V pin of J3 to the Vsys pin 39 of the Pico, as near as possible to the Pico. Maybe the feed to the sensor board should start from J3, from the anode of the proposed diode, and not from pin 39 of the Pico..
As seen on a recent video, Gemini runs OK without this protection, but strange things can occur when connected to a PC or laptop for testing. For example, the voltage from the USB might reverse bias Gemini's voltage regulator, even for intermittent short periods of time, causing hard to understand glitches all over the place.

[IanSuper7]

We need to add this to future board releases and before we order further club stock.

[micromouseonline]

But first, we need to look carefully at what is being suggested, why it is needed and whether it is sufficient.

[IanSuper7]

It is recommended in the Pico data sheet page 20
"The simplest way to safely add a second power source to Pico is to feed it into VSYS via another Schottky diode (see
Figure 16). This will 'OR' the two voltages, allowing the higher of either the external voltage or VBUS to power VSYS, with
the diodes preventing either supply from back-powering the other. For example a single Lithium-Ion cell* (cell voltage
~3.0V to 4.2V) will work well, as will 3×AA series cells (~3.0V to ~4.8V) and any other fixed supply in the range ~2.3V to
5.5V. The downside of this approach is that the second power supply will suffer a diode drop in the same way as VBUS
does, and this may not be desirable from an efficiency perspective or if the source is already close to the lower range of
input voltage allowed for the RT6150."

at 5v regulated, we are not close to the lower voltage limit so should be fine