Is 3.3V enough for the VGA sense pin? (pin 9)

Bill Henning

I'm wondering if I don't need a 5V rail on a prototype that otherwise does not need 5V...

Does anyone know?

Thanks,

Bill

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Ale

I do not connect the "sense" pin, at all and I tested in several CRTs and LCD and all of them work. Not that I tested with one but EIZO monitors (widescreen LCDs) seem not to use that pin at all. I do not know the rest though.

Cluso99

I am curious also. But you need 5V for keyboard and mouse if you have them.

The 5V regulator does share some of the power reduction/dissipation on the way to 3v, but of course you could always use a lower input voltage providing you dont require the 5V.

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Phil Pilgrim (PhiPi)

Bill,

This is an excellent question. Unfortunately, I fear the answer may be monitor-dependent. It will depend in large part on which logic family is used to input the sense line. In the case of 74LS and 74HCT logic, no problem. OTOH, 74HC logic could be a problem, since its high input threshold can be as much as 3.5V (but almost never is). On the whole, you probably won't encounter any difficulties at 3.3V.

-Phil

Toby Seckshund

A while back it was normal to not have the nineth pin installed into the cable assembly and so there ater lots of those leads still kicking around. They always give normal VGA pics but do not allow the OS to spot "new hardware installed". My homemade Demmoboard does not have a pin 9 connection and has worked ok on LG, Philips,Sanyo .... W/screen or not.
I have one keyboard that seems to run ok on 3.3V only but more that do not, I have yet to find a mouse that was happy.

Bill Henning

Thanks for the info guys!

The PCB I'm working on does not have a keyboard or mouse, and uses all 3.3V logic, and I wanted to save the 7805 and +5 rail if possible - looks like its pretty safe!

I may put in an optional input that goes to pin9 to feed it 5v if needed (or jumper to 3.3v) in case of non-working monitor.

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