Hi,
the wakeup is triggered by the rising edge of the internal PTB3 signal therefore its required to ensure the internal PTB3 is low. Important to notice is that the timer clock is changing in low power mode. To get exact timing the Timer should be disabled, configured ... Entering Stop mode will then enable the Timer.
can you try the following code:
// PTB3 must be set to output
B_GPIO_CTL = B_GPIO_CTL_DIR3M_MASK | B_GPIO_CTL_DIR3_MASK;
// route Timer OC3 to PTB3
B_GPIO_OUT3_WKUP = 1;
B_GPIO_OUT3_TCOMP3 = 1;
// set Timer Ch3 to OC
B_TIOS_IOS3 = 1;
B_OC3M_OC3M3 = 1;
// force internal PTB3 to low
B_TSCR1_TEN = 1;
B_TCTL1 = 0x80; // clear on OC
B_CFORC_FOC3 = 1; // force OC
B_TSCR1_TEN = 0;
// setup a rising edge in 10s
B_TCTL1 = 0xC0; // set on OC
B_TC3 = B_TC3 + 0x2710; // reload 10s
B_TFLG1_C3F = 1;
//enable wakeup on ptb3 (rising edge)
B_PCR_WUE_WUPTB3 = 1;
...
W.