?

My understanding is the the 5" board is 3v3 - period. The 5V option
just sticks a 3-terminal regulator on the board to make 5V supply
into 3v3 for the internals.

I used the 5V option because I figured that it would be a little
easier on my main board regulators - even though they're switching -
and because I thought the on-board 3v3 3-terminal regulator would
get me a little noise reduction on the main board bus.

It's working fine - except for when I hold my handi-talkie up to it
& transmit.

- Jerry


On 2025-02-06 11:29, K9HZ wrote:

Oh brother, this is starting to get complicated.

The RA8875 is a 3.3V chip… and the inputs, as far as I can tell…
from my research so far… are only 3.3V tolerant.

To make this more confusing, some of the display.com displays have
level shifting hardware onboard that you can enable to use the display
as a 3.3V display or as a 5V display (in line with the power
selections on the MAIN board). However, not all do (particularly the
smaller displays). The boards I made work with 3.3V and 5V display
boards… which is fine for 3.3V operation for all display boards….
The problem comes if you select 5V operation bon the MAIN board… but
either don't have a 5V control line selection on the display board for
level change to 3.3V for the RA8875… or if you DO have the level
change hardware on your display board but have not selected it. In
this case, you will destroy the RA8875 chip.

I can do two things. One is to put together a list of all the
approved Display.com display part numbers (upper middle of the back of
the display) and give guidance on what you need to do so that the
display continues to function with this driver board. The other thing
I can do is make a second board that has a tiny 3.3V regulator on it
to power the three control lines to just 3.3V in the case of running
the display off 5V but no hardware is there for level shifting the
data lines to 3.3V.

Comments?

DR. WILLIAM J. SCHMIDT - K9HZ J68HZ 8P6HK ZF2HZ PJ4/K9HZ VP5/K9HZ
PJ2/K9HZ VP2EHZ

Owner - Operator

Big Signal Ranch - K9ZC

Staunton, Illinois

Owner - Operator

Villa Grand Piton - J68HZ

Soufriere, St. Lucia W.I.

Rent it: [1]

Moderator: North American QRO Group at Groups.IO.

Moderator: Amateur Radio Builders Group at Groups.IO.

email: bill@...



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