# How can we maintain RTC in WP76xx without VBAT

**URL:** <https://forum.sierrawireless.com/t/how-can-we-maintain-rtc-in-wp76xx-without-vbat/9814>\
**Category:** WP Series\
**Created:** [February 14, 2018, 7:55am UTC](https://forum.sierrawireless.com/t/how-can-we-maintain-rtc-in-wp76xx-without-vbat/9814 "2018-02-14T07:55:34Z")\
**Posts on this page:** 4\
**Page:** 1

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**Author:** ![annaertd](https://avatars.discourse-cdn.com/v4/letter/a/13edae/32.png) [@annaertd](https://forum.sierrawireless.com/u/annaertd)\
**Post date:** [February 14, 2018, 7:55am UTC](https://forum.sierrawireless.com/t/how-can-we-maintain-rtc-in-wp76xx-without-vbat/9814/1 "2018-02-14T07:55:34Z")

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The WP76 series doesn’t have an BAT\_RTC pin compared to WP8548.  
How can we maintain the RTC if we don’t use an external battery and charger circuit?

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**Author:** ![mlw](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.sierrawireless.com/mlw/32/3706_2.png) [@mlw](https://forum.sierrawireless.com/u/mlw)\
**Post date:** [February 14, 2018, 8:47am UTC](https://forum.sierrawireless.com/t/how-can-we-maintain-rtc-in-wp76xx-without-vbat/9814/2 "2018-02-14T08:47:49Z")

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Hi,

Not sure what you mean, pin 21 is BAT\_RTC for the WPx5 and the WP76 families, the WPx5 family can charge the cap where the WP76 can’t so you just need to put a precharged a coin cell in place and the unit will draw on this when VBATT is not available.

Regards

Matt

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**Author:** ![annaertd](https://avatars.discourse-cdn.com/v4/letter/a/13edae/32.png) [@annaertd](https://forum.sierrawireless.com/u/annaertd)\
**Post date:** [February 14, 2018, 2:07pm UTC](https://forum.sierrawireless.com/t/how-can-we-maintain-rtc-in-wp76xx-without-vbat/9814/3 "2018-02-14T14:07:15Z")

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Hi,

the Product Technical Specification of WP8548 describes the internal RTC as below:  
“The AirPrime WP75xx/AirPrime WP8548 module‘s internal RTC is powered from the VBAT\_BB supply (when available).  
The module also provides an interface (BAT\_RTC) for using a coin cell to maintain the internal RTC when VBAT\_BB is removed from the module.”

The WP76 specification doesn’t mention that there is an internal RTC and does not say that we can connect a coin cell.  
The definition of pin 21 says “Reserved - No connection”.

Did you get your information from another specification?

Greetings,  
annaertd

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**Author:** ![mlw](https://sea1.discourse-cdn.com/flex025/user_avatar/forum.sierrawireless.com/mlw/32/3706_2.png) [@mlw](https://forum.sierrawireless.com/u/mlw)\
**Post date:** [February 14, 2018, 2:21pm UTC](https://forum.sierrawireless.com/t/how-can-we-maintain-rtc-in-wp76xx-without-vbat/9814/4 "2018-02-14T14:21:34Z")

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Annaertd,

I was looking at old documentation, the latest PTS and scaleability guide do indeed not show an external supply on pin 21. So I guess its Vbatt or you lose the RTC.

Regards

Matt