hi Phil,
I have found that 10mA or 15mA is sufficient segment current for good 'readability'.
Have you finalised the Driver IC type.?
E

 

hi Phil,
I have found that 10mA or 15mA is sufficient segment current for good 'readability'.
Have you finalised the Driver IC type.?
E

I am using a MAX7219 to drive a UDN2981A

 

You did say you have common anode displays, right?

The MAX7219 is designed to directly drive common cathode displays.

In order to interface the MAX7219 with a common anode display you will need inverting high side drivers at the digit side, and inverting low side drivers at the segment side.

So the ULN2003 should work on the segment side, but the UDN2981 does not invert so you will need to invert the digit side before the drivers, or find inverting high side drivers.

If I’m wrong please feel free to correct me, because it’s kind of hard to get a complete picture without a diagram, and I don’t want to mislead you.

 

You did say you have common anode displays, right?

The MAX7219 is designed to directly drive common cathode displays.
.

You got it! The TS has already identified the displays as common anode in response to an earlier post of mine. And Sensacell also identified the ULN2003 as the chip to drive the segments.

 

You did say you have common anode displays, right?

The MAX7219 is designed to directly drive common cathode displays.

In order to interface the MAX7219 with a common anode display you will need inverting high side drivers at the digit side, and inverting low side drivers at the segment side.

So the ULN2003 should work on the segment side, but the UDN2981 does not invert so you will need to invert the digit side before the drivers, or find inverting high side drivers.

OOPs!! The high side is OK now i think but we need a need a positive signal from the MAX7219 for each segment. Maybe I can find a version of the MAX7219 with designed for common cathode or as you say an inverter could be found.
Thanks
Phil

 

You did say you have common anode displays, right?

The MAX7219 is designed to directly drive common cathode displays.

In order to interface the MAX7219 with a common anode display you will need inverting high side drivers at the digit side, and inverting low side drivers at the segment side.

So the ULN2003 should work on the segment side, but the UDN2981 does not invert so you will need to invert the digit side before the drivers, or find inverting high side drivers.

What about using a second transistor array to invert the signal all on the low side
Phil

 

“What about using a second transistor array to invert the signal all on the low side”

I don’t see how.

I think you may be stuck using discrete PNPs at the digit side.

 

“What about using a second transistor array to invert the signal all on the low side”

I don’t see how.

I think you may be stuck using discrete PNPs at the digit side.[/QUOTE
The MAX7219 is producing a positive signal for each segment to the UND2981 which is directing each segment cathode to ground so it should work unchanged or there is something I am not seeing.

 

hi Phil,
Are you have a problem when using Quotes.?
Eric

 

hi Phil,
Are you have a problem when using Quotes.?
Eric

Don't know what you mean?

 

Post #28 is the query.?, its difficult to follow,

 

Post #28 is the query.?, its difficult to follow,

Sorry if I am using the right terminology. I was advised that the MAX7291 or UDN2981A the would need the signals inverting. My post was to explain that it seems OK to me.

 

“The MAX7219 is producing a positive signal for each segment to the UND2981 which is directing each segment cathode to ground so it should work unchanged or there is something I am not seeing.”

Yes, what you are not seeing here is that the UDN2981 cannot direct anything to ground, because it is a high side driver. (non-inverting high side driver) (positive in-positive out)

The ULN2003 will direct the segments to ground. (inverting low side driver) (positive in-negative out)

 

“The MAX7219 is producing a positive signal for each segment to the UND2981 which is directing each segment cathode to ground so it should work unchanged or there is something I am not seeing.”

Yes, what you are not seeing here is that the UDN2981 cannot direct anything to ground, because it is a high side driver. (non-inverting high side driver) (positive in-positive out)

The ULN2003 will direct the segments to ground. (inverting low side driver) (positive in-negative out)

Thank you . I was misinterpreting the UNL2003 design