VDD_LOG powers internal logic circuitry such as SRAM, clock & reset Unit, PLL, internal MCU. VDD_CENTER powers mostly vpu, encoder/decoder but it also powers DDR controller.   Recommended operating condition for those supply is 0.9V but we have found other RK3399 board, RockPro64, increase this to 0.95V to improve stability https://github.com/u-boot/u-boot/commit/f210cbc1f3d4f84baa595d83933369b4d6a529ea https://github.com/u-boot/u-boot/commit/5a6d960b22220