unpredictable led output on nexys2 board

[iamonion]

Hi everyone!
I'm learning to design PWM with nexys2 board and i've encountered very
strange problem,
I have one PWM that drives two LED-s, i program synthetised design by
JTAG and everytime i load it, i observe one of two possible results:
first... the expected one led 7 and led 4 lights up slowly, leds 6 and
5 are off as in code below:
led(3 downto 1) <= pwm_out & pwm_out & pwm_out ; -- these leds work
always ok
led(0) <= slowclk; -- this led is ok too
led(7 downto 4) <= pwm_out2 & '0' & '0' & pwm_out2;
But sometimes led 7 has inverted driving so it is on at beginning and
dims out and what is more weird led 6 is on, but it should be off,
there's no special signal just '0' specified. There's also some self-
test flashed to board, it's LED's work always as intended.
Clock frequency of PWM is 50MHz.
What could cause this problem?
Thanks.
Juraj Galbavy

 

[iamonion]

Hello,
I think i've solved my problem by moving one jumper pin from position
slave serial to JTAG.
Now after reset the default test is not run and i can transfer
configuration many times with
no defects on LED outputs, yet.
As i realised before changing jumper, any design i tried to run had
some chance to have led 7 and 6
outputs to be inverted both at the same time.

Juraj