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wrting to BKPSRAM with 3.3 volts applied to v-bat

obdgenie obdgenie

using the stm32f2xx_hal library I seem to be able to enable access to the BKPSRAM with

__HAL_RCC_PWR_CLK_ENABLE();
HAL_PWR_EnableBkUpAccess();
__BKPSRAM_CLK_ENABLE();

Once this is done I can see the 0x40024000-0x40024fff populate with data of some kind. I'm not sure if this is just garbage ram or back up data. I figured at this point one can write with.

*(__IO uint8_t *)BKPSRAM_BASE = 2;

If the power is still connected and I reset the chip (with debugger) I would expect to see this memory change to a 2. This never takes place. I'm supplying the vbat with a constant 3.3 (maybe I should put a small cap on this?) I would also expect the data to change in debug stepping true code. The memory view and data at *(__IO uint8_t *)BKPSRAM_BASE remain as it was.