STM32F103 / STM32F407 boards, enhancement request 32khz RTC crystal and VBAT

STM32F103 and STM32F407 has built-in RTC (Real time clock), what is needed is a 32Khz watch crystal. Some design needed there. Check ST documents.

In addition, VBAT pin need to power from 3v coin cell (e.g. cr2032) to keep the RTC running. I think VBAT can leave open, not connected and goto a header pin if you don't want to put battery on board. Those who want to use VBAT pin can connect their own cr2032 battery, or you can sell as separate add on module.

Why is this important? Because with VBAT, it is possible to use backup registers or backup SRAM. These are there inside the STM32F103 and STM32F407 chip. e.g. they can be used to store some important printer settings and still keep memory after switch off.

The RTC is also important, it keeps time. So for example, can do time based automation, e.g. set alarm clock and goto sleep. wake up in the morning and start heated bed and start printing. But those things need the RTC. RTC also useful for monitoring print time and a lot of other things.

makerbase-mks / MKS-Robin-E3-E3D

STM32F103 / STM32F407 boards, enhancement request 32khz RTC crystal and VBAT #141