Chip select nightmare - maybe I need pullup rather than pulldown? Is this safe?

Using Arduino Displays
#1

Having an utter and total nightmare with a Winstar OLED graphical display with two WS0010 controllers.

The chip select is active LOW, and even though none of the docs, diagrams, datasheets or code suggests I need it, the only thing I've not tried yet is pullup resistors (tried pulldown, that did nothing to help).

Yes, I know there are loads of posts and pages about pullups and pulldowns, but I don't want to READ a high or low, I want to drive a signal low.
And unless I've missed something, those pages don't discuss that. Because, as we all know, the Arduino has a perfectly acceptable way of changing the pins to HIGH/LOW.

So, why am I wanting to do this weird thing? I put LEDS on the CS1/CS2 lines (which I did just to make sure the Arduino was actually working) and when the OLED is reset, I noticed they glow about mid-brightness. When I had the pullDOWN resistors, that kept them dark, but of course, they are ACTIVE low, and I want them both to be HIGH at reset time, then initialise each chip in turn pulling one line low.

In other words, if I put a 250R(?) resistor on the same breadboard line as the CS line is connected to, and then connect that resistor to 5v, will this kill a pin on my Arduino or cause a short/overload? And will it even pull it to 0v?

I really don't know what I'm doing at this stage having tried literally everything else from transistors to diodes (as well as endless code of course!).
BTW - The top half (CS1 LOW, CS2 HIGH) works and inits perfectly, so I know that bit works. As soon as I try and initialise the second half, the whole display "shadows" or duplicates below. And I've already had one replacement so I don't think it's the display...

#2

A pull up or pull down resistor will not do you any good on an output. You have something else wrong if it is not working.

#3

Grumpy_Mike:
A pull up or pull down resistor will not do you any good on an output. You have something else wrong if it is not working.

Right, OK, that's what I thought. Thanks for confirming it, I can rule that one out then.

#4

So if you put LED indicators on an active low enable line, you must reference the indicators themselves to Vcc (5V), not to ground. They will then flash - albeit not brightly - at the times the line is being activated. If you have them to ground, they should be glowing fully if the program is functional, except for the brief moments when the enable is active, and there is no way you will be able to to discern the brief time they go off.

Logic probes use pulse stretching circuits in order to flash at a visible rate if the input is changing too fast to be seen.

You do not need a 500 ohm pullup - would only consume 10 mA anyway, quite harmless - 5k or 10k would suit fine, and makes sure the enable does not float down and activate during reset when the pins are "open". So that is a reason you should have a pull-up.

#5

quite harmless - 5k or 10k would suit fine, and makes sure the enable does not float down and activate during reset when the pins are "open". So that is a reason you should have a pull-up.

Yes that is true, I did not want to overcomplicated the answer because in this case that is not where the problem lies. The OP was looking to drive an active low input from an arduino and in those circumstances a pull up or down is not relevant.

#6

Grumpy_Mike:
The OP was looking to drive an active low input from an arduino and in those circumstances a pull up or down is not relevant.

Might be during reset when it is not an output.

#7

Paul__B:

Grumpy_Mike:
The OP was looking to drive an active low input from an arduino and in those circumstances a pull up or down is not relevant.

Might be during reset when it is not an output.

I can’t see that as a problem, at worst the pin will float low and the chip will be active. But as all the other pins will be floating then I can’t see it would make any difference.

#8