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microcontroller waking up immediately when powered by battery

Question asked by p.r on Oct 5, 2012

when i power the stm8l1512c6 on the stm8l discovery board the microcontroller wakes up immediately after entering sleep mode. this does not happen when i power the microcontroller by the power supply. In my program the mc has to stay in halt mode for 50 seconds but when i connect the battery to the 3.3V pin on the stm8l discovery board the microcontroller goes into halt but does not stay and immediately wakes up.
 This seems very strange . I even cleared all external interrupt flags to make sure no interrupt is causing this although the voltage at pin 3.3V should not cause any interrupt.

Any idea why this happens with a power source other than a power supply( i use a typical hameg power supply) . i can see the mc waking up immedialtly after entering halt mode when a supercapcitor or a battery is used. Should i use some capacitors from Vcc to gnd. I am not using these as they are implemented in the board itself.
i use jp1 pin 1 and 2 to measure the supply current and it is always in mA since the mc is waking up as soon as it enters halt. 
for the battery i use a thinergy thin film battery (of course i regulate the volktage to 2 V using an ldo) and as capacitor, a .33 F supercapacitor .
Any idea why this this is happening?