FPGA quiz: what can be wrong

vhdl code

LED1 <= some_signal;
LED2 <= blink_one_second;

the LED1,2 are connected to LED's
blink_one_second is simple wire that drives LED2 and nothing else.

now the code works in real FPGA with following behaviour:

when some_signal = 0, then LED2 blinks.
when some_signal = 1, then both LEDs blink as full sync to each other.

what can cause this?

some wrong answers: faulty FPGA fabric, bad PCB, bad power supply.

eh, the solution to the problem was weird. but it does exist. sure I
assumed FPGA fabric faulty first as I had overstressed the FPGA with
5V supply, and reversed 3.3V supply too. but FPGA is alive and fully
working, yet the weird behaviour exist. and this is actually correct
behaviour in the particular case.

I wonder if somebody is able to quess the answer to the issue. happy
thinking!

Antti

> 
> I wonder if somebody is able to quess the answer to the issue. happy
> thinking!
> 
> Antti
> 

incorrect ucf file had some other signals driving the pins?

LEDs are installed backwards and there is also maybe a short?

On 15 Okt., 17:59, Jeff Cunningham <j...@sover.net> wrote:
> > I wonder if somebody is able to quess the answer to the issue. happy
> > thinking!
>
> > Antti
>
> incorrect ucf file had some other signals driving the pins?
>
> LEDs are installed backwards and there is also maybe a short?

no.
no pin mapping problem, no LED connection problems.
if LED1 and LED 2 are driven from other signal they work properly.
keep thinking;)

I solved the issue, but to avoig going nuts I called my wife to look
the code too.
then id take some break and slept over it. and finally solved.

Antti

Bad Power Supply?
Ground loop or supply instability generating continuous powerup of you FPGA.

Bad VHDL coding?
Description of asynchronous logic (latches).

Laurent

Antti wrote:
> vhdl code
> 
> LED1 <= some_signal;
> LED2 <= blink_one_second;
> 
> the LED1,2 are connected to LED's
> blink_one_second is simple wire that drives LED2 and nothing else.
> 
> now the code works in real FPGA with following behaviour:
> 
> when some_signal = 0, then LED2 blinks.
> when some_signal = 1, then both LEDs blink as full sync to each other.
> 
> what can cause this?
> 
> some wrong answers: faulty FPGA fabric, bad PCB, bad power supply.
> 
> eh, the solution to the problem was weird. but it does exist. sure I
> assumed FPGA fabric faulty first as I had overstressed the FPGA with
> 5V supply, and reversed 3.3V supply too. but FPGA is alive and fully
> working, yet the weird behaviour exist. and this is actually correct
> behaviour in the particular case.
> 
> I wonder if somebody is able to quess the answer to the issue. happy
> thinking!
> 
> Antti
> 

On 15 Okt., 19:38, "Amontec, Larry" <laurent.ga...@ANTI-
SPAMamontec.com> wrote:
> Bad Power Supply?
> Ground loop or supply instability generating continuous powerup of you FPGA.
>
> Bad VHDL coding?
> Description of asynchronous logic (latches).
>
> Laurent

NO
NO
NO
NO

waiting for more suggestions :)

Antti

Antti wrote:

> On 15 Okt., 19:38, "Amontec, Larry" <laurent.ga...@ANTI-
> SPAMamontec.com> wrote:
> 
>>Bad Power Supply?
>>Ground loop or supply instability generating continuous powerup of you FPGA.
>>
>>Bad VHDL coding?
>>Description of asynchronous logic (latches).
>>
>>Laurent
> 
> 
> NO
> NO
> NO
> NO
> 
> waiting for more suggestions :)
> 
> Antti
> 

if this is not the VHDL code, let us know the VHDL generating the 
some_signal and the blink_one_second.

On 15 Okt., 19:58, "Amontec, Larry" <laurent.ga...@ANTI-
SPAMamontec.com> wrote:
> Antti wrote:
> > On 15 Okt., 19:38, "Amontec, Larry" <laurent.ga...@ANTI-
> > SPAMamontec.com> wrote:
>
> >>Bad Power Supply?
> >>Ground loop or supply instability generating continuous powerup of you FPGA.
>
> >>Bad VHDL coding?
> >>Description of asynchronous logic (latches).
>
> >>Laurent
>
> > NO
> > NO
> > NO
> > NO
>
> > waiting for more suggestions :)
>
> > Antti
>
> if this is not the VHDL code, let us know the VHDL generating the
> some_signal and the blink_one_second.- Zitierten Text ausblenden -
>
> - Zitierten Text anzeigen -

the signal driving LED2 goes only to LED2, also no signals used to
generate blink_one_second are in any way used in the part that
generates some_signal, let say some_signal comes from external switch
directly, and has no FPGA logic except routing.

Antti

I had a somewhat similar puzzle quite a few years ago. The code had a 
flip-flop generating  out1 and an async assignment out2 = not out1. The two 
outputs were driving external buffers. Somehow the outputs got enabled 
together and the buffers smoked. I traced the problem to the synthesizer 
setting, which created another flop for the out2 and moved negation to its 
input. I don't remember all the details now, but IIRC the clock was missing 
for whatever reason when I first powered the board... Apparently debugging 
that board was my first assignment at a new job. Imagine how I felt when it 
smoked :)


/Mikhail 

On 15 Okt., 20:26, "MM" <mb...@yahoo.com> wrote:
> I had a somewhat similar puzzle quite a few years ago. The code had a
> flip-flop generating  out1 and an async assignment out2 = not out1. The two
> outputs were driving external buffers. Somehow the outputs got enabled
> together and the buffers smoked. I traced the problem to the synthesizer
> setting, which created another flop for the out2 and moved negation to its
> input. I don't remember all the details now, but IIRC the clock was missing
> for whatever reason when I first powered the board... Apparently debugging
> that board was my first assignment at a new job. Imagine how I felt when it
> smoked :)
>
> /Mikhail

eh, but no one is getting closer to my issue.
another hint, no flip flip or any sync logic thing is causing this
problem.
its also not in any way some compiler or P&R weirdness at all.

Antti

On Oct 15, 11:33 am, Antti <Antti.Luk...@googlemail.com> wrote:
> On 15 Okt., 20:26, "MM" <mb...@yahoo.com> wrote:
> eh, but no one is getting closer to my issue.
> another hint, no flip flip or any sync logic thing is causing this
> problem.
> its also not in any way some compiler or P&R weirdness at all.
>
> Antti

Did someone get too cheap in the board design and only use a single
current limit resistor for both LEDs? If the blink LED has a
significantly lower forward drop than the some_signal LED, then when
blink LED turns on, it turns off some_signal LED, and when it goes
off, some_signal LED comes back on. (You said they blink at the same
time, but didn't mention polarity).