External 32.768KHz Oscillator for RTC in STM32H743

[VShet.2]

Hi,

I want to use External 32.768KHz Oscillator as clock source for RTC. This is a reference schematic how I intend to connect Oscillator with RCC_OSC32_IN pin on my STM32H743 microcontroller. 

My Query is since oscillator requires Vcc 3.3V to operate, there are three possibilities:

1. Give 3.3V supply from my PCB, what happens when power is Off in this case ?

2. If I use 3.6V RTC battery to supply this oscillator, in that case it will use power from my battery even when the board is power On.

3. If I short my Battey with board 3.3V using a diode so that current does not flow inside battery and also, supply from PCB is used when board is Power ON and supply from battery is used when is board is power Off.

Does option 3 look feasible or else what is the ideal way to use external oscillator for RTC application ?

[AScha.3]

Hi,

1. clock away.

2. Right.

3. possible, yes.

And : if osc is low power (it is - right ? ) then could run just on Vbat and switch on charging from cpu .

(to compensate the used power from Vbat.)

[VShet.2]

I use non-rechargeable battery so, recharging is not possible.

If I only use battery for my oscillator in low power mode, in this case do you have idea how to calculate power consumption in cube Mx for RTC power domain ?

[AScha.3]

>I use non-rechargeable battery so, recharging is not possible.

Me too.  :)

But anyway...try it. (With 5k charge setting you cannot explode anything...)

With almost new LI-cell, about 3.1 V , and 3v3 supply , the 5k "charging" will be 40uA to the battery + osc.

If new cell, about 3.2V , current down to 20 uA .

btw. --- from our new friend, the AI :

Lithium Manganese Dioxide Batteries: Rechargeable

Lithium manganese dioxide batteries are also rechargeable. They are used in various applications such as cameras, watches and as a backup battery for motherboards in personal computers.