H750VBT6 BKPSRAM can't keep DATA

[ZYubi]

 /* Enable write access to Backup domain */

 PWR->CR1 |= PWR_CR1_DBP;

 while((PWR->CR1 & PWR_CR1_DBP) == RESET)

 {

 }

 /*Enable BKPRAM clock*/

 __HAL_RCC_BKPRAM_CLK_ENABLE();

  

 *(__IO uint32_t*)(0x38800000+36) = 12345678;

run to here, DATA 12345678 is written to adress 0x38800000+36 , but I reset the chip，the DATA in adress 0x38800000+36 has been changed,I don't konw what's the reason?

[oleksandr.karbivsky]

Check power supply on VBAT pin.

[ZYubi]

VBAT is OK. I used ​the ADC3 to measure the voltage of VBAT, the value is about 3V.