On 21/04/2011 21:47, V G wrote:
> The Nexys 2 board comes with a 4 digit 7-segment display. Ok so as
> soon as I opened the packaging, I carefully powered on the board. The
> preloaded board self test loaded and some digits on the 7-segment
> started to flash, indicating that everything was okay. I immediately
> noticed that segment c (bottom) right segment of each digit lit up
> dimly when it was activated. All other segments worked fine. When the
> segment turned off, it turns off as usual.
>
> Each digit is connected to it's own common anode line. A logic 0 turns
> on the digit via a PNP transistor. There are 4 digits and therefore 4
> lines. Each segment has it's own line. So 8 lines for controlling the
> segments, including decimal point. Since it takes only one line to
> control each segment C, a possibility is that there is a pullup
> resistor somewhere that is dimming segment C. Note that a logic 0
> activates the segment. Another possibility is a faulty segment on each
> of the digits. Or something else.
>
> I've googled tirelessly for this issue and got no results. Help anyone?

Can you get a probe onto the line(s) to see if the base is dropping=20
fully to 0V? Also whether the C segments anode is at the correct voltage.
I think it's unlikely that all the C segments are faulty, so something=20
up with the power to them is probable, like a weak pullup stopping the=20
base of the PNP falling to 0V.
Of course, the easy way to figure if it's the pullup is to turn it off -=20
this should be pretty easy, there is likely a setting during the=20
layout/synthesis/pin assignment phase if you don't want to figure out=20
how to do it in verilog. Someone else should know the specifics - I am=20
going off what I know about the Actel software, which has a combo box to=20
select pullups on/off during pin assigment (along with drive strength=20
and other things)

--=20
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