BQ40Z50-R2: why i can't measure a high level when leave pres pin floating or use a weak pull up gpio from MSP430？

now i want use a gpio from msp430 to control the pres pin of bq40z50R2.   dataflash NR bit = 0.  now i can control the bq CHG DSG MOS on or off use the pres pin

but when i output a high level use a weak pull up to pres pin,  the bq DSG CHG off ,    why i measure  the vol at two ends of resistor（100R） ,the vol is very low?

void battery_remove(void)
{
P3REN |= PRES_N_MCU;
P3DIR &= ~PRES_N_MCU;
P3OUT |= PRES_N_MCU; //input and out = 1, high , pull up, remove
}

and i see the pres pin have a internal pullup  resistor, so i remove the 100R  , and measure the pres pin leave it floating,,  the vol also very low ,  i found a pluse per sec,  0.02V and 0.0V toggle.

i can't get measure a high level , so i feel this not stable ,  tell me why?  thank you very much!!