Can VBAT be left unconnected?

[shawn]

Posted on July 31, 2008 at 18:05

Can VBAT be left unconnected?

[shawn]

Posted on May 17, 2011 at 12:40

Hello,

I know that the design manual says that Vbat 'must' be connected, but is there really any reason? If the switch shown in the figures in the datasheet is true to implementation, how different is it to leave it unconnected than to just wire it up to VDD which gets turned off anyway? The Eval board works just fine if you remove the battery or remove the Vbat jumper entirely. My application desire is to allow the user to add/remove a battery as memory retention is needed without requiring a switch to do so. Thanks, Shawn

[asterix]

Posted on May 17, 2011 at 12:40

Hi Shawn,

I believe that, what you would like to do is reliable since you have anyway to replace the coin battery in case of drain. This is the way how STM32 is working. However, this not the case for applications which are not using ''at all'' battery and low power operations to keep RTC running, or RAM back-up for critical data. So in board design this VBAT should connected or left open ? . Manuals says Vbat ''must'' be connected to VDD : Making a search in RM0008 Reference Manual I found : i) Section RCC : ''Backup domain reset'' A backup domain reset is generated when one of the following events occurs: 1. Software reset, triggered by setting the BDRST bit in the Backup domain control register (RCC_BDCR). 2. VDD or VBAT power on, if both supplies have previously been powered off.ii) Section ''Backup domain control register (RCC_BDCR)'' Note: LSEON, LSEBYP, RTCSEL and RTCEN bits of the Backup domain control register (RCC_BDCR) are in the Backup domain. As a result, after Reset, these bits are write protected and the DBP bit in the Power control register (PWR_CR)has to be set before these can be modified. Refer to Section 5 on page 59 for further information. These bits are only reset after a Backup domain Reset and VBAT power on.Any internal or external Reset will not have any effect on these bits. So VBAT must powered on and so connected to a battery or VDD to reset properly some critical bits that are useful in such applications. Do you agree with my analysis ? please correct me if I'm wrong. Regards , Magigimix. :o [ This message was edited by: asterix.magigimix on 31-07-2008 21:38 ]