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That will not work. Your signal_0_synch will never be '1', because signal_0_synch will be assigned two times at same clock cycle (one in case statement and in last line). Try to explain what you are trying to do including more details.
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I need a signal (flag) to communicate between two processes. One process set the flag, the other reads it. But the flag should be reset as soon as possible cause the second process will enter a loop always reading the flag='1'. I need a mini pipeline - a sgnal passes its value on and reset. Value lasts for 2 clock, quite enough not to miss it.