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stm32f4zg failing to connect with st link after some programming attempt

Question asked by mishra.arun on Feb 24, 2014
Hi there!
I have designed a board around stm32f4zg controllers. In my first attempt the board worked fine and everything went good. But suddenly one of my board stop connecting with st- link (one that comes embedded with discovery boards). I've made no pull-ups and pull down on swd pins as suggested in reference manual. after a few times my another board also failed to connect. The board which failed to connect shows permanent high logic on swdio and swclk pins. I also noticed that, the board which failed have little different programs than my good board. In my code I do not alter any gpios related top swd pins (I am a victim of this already :) ). Cant understand why boards are failing so frequently. However the program they contains works fine. Any suggestion will be appreciated.
Thanks  

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