feipoa wrote on 2025-11-19, 13:03:

I finally got around to testing these on a Biostar MB-8433UUD. There is a small difference regarding the RAM clear input betwee […]
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I finally got around to testing these on a Biostar MB-8433UUD. There is a small difference regarding the RAM clear input between the BQ3285 and the DS12887A real-time clock chips. On the BQ3285, the datasheet states:

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1A low level on the RCL pin causes the contents of each of the 114 storage bytes to be set to FF(hex). The RCL' input is only recognized when held low for at least 125ms in the presence of V CC when the oscillator is running.

Conversely, the DS12887A datasheet states:

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1The RCLR pin is used to clear (set to logic 1) all the general-purpose RAM, but does not affect the RAM associated with the RTC. To clear the RAM, RCLR must be forced to an input logic 0 during battery-backup mode when V CC is not applied.

In short, DS12887A doesn't want the motherboard powered when clearing the NVRAM, while the BQ3285 wants the motherboard powered. I wanted to point this out for anyone else gets stuck on this like I did.

Thanks feipoa. How does this affect general use and how did you work around it, if you did?

feipoa wrote on 2025-11-19, 13:03:

I finally got around to testing these on a Biostar MB-8433UUD. There is a small difference regarding the RAM clear input betwee […]
Show full quote

I finally got around to testing these on a Biostar MB-8433UUD. There is a small difference regarding the RAM clear input between the BQ3285 and the DS12887A real-time clock chips. On the BQ3285, the datasheet states:

Copy code to clipboard

1A low level on the RCL pin causes the contents of each of the 114 storage bytes to be set to FF(hex). The RCL' input is only recognized when held low for at least 125ms in the presence of V CC when the oscillator is running.

Conversely, the DS12887A datasheet states:

Copy code to clipboard

1The RCLR pin is used to clear (set to logic 1) all the general-purpose RAM, but does not affect the RAM associated with the RTC. To clear the RAM, RCLR must be forced to an input logic 0 during battery-backup mode when V CC is not applied.

In short, DS12887A doesn't want the motherboard powered when clearing the NVRAM, while the BQ3285 wants the motherboard powered. I wanted to point this out for anyone else gets stuck on this like I did.

Hm, very interesting. With other words, if there is a jumper on the mainboard to reset the CMOS data, with DS12887A you have to use it on a system turned off and with the BQ3285 when it is turned on. I'll mention it in the documentation on the project site. If you have more to add, may be share your experience how it affected you, please open a ticket on the project site and I'll condense it into a documentation note. Such notes are the best "thank you" for the open source projects like this.

The only affect this has is when you want to clear your BIOS NVRAM without entering the BIOS. Sometimes you may want to force a BIOS clear and aren't able to get into the BIOS. The result is that you need to power down, put jumper on, power up, power down, remove jumper. As opposed to power down, put jumper on, remove jumper, power up.

I have never encountered a BIOS' RTC which can only be jumper-cleared while the system is live. It's a small difference that needs to be remembered when using these. The alternative is to remove the battery, but my battery is so tight, I could almost not get it in. It took me about 5 minutes of grunting, however I ordered cheap battery holders from AliExpress.

feipoa wrote on 2025-11-19, 22:05:

The only affect this has is when you want to clear your BIOS NVRAM without entering the BIOS. Sometimes you may want to force a BIOS clear and aren't able to get into the BIOS. The result is that you need to power down, put jumper on, power up, power down, remove jumper. As opposed to power down, put jumper on, remove jumper, power up.

I have never encountered a BIOS' RTC which can only be jumper-cleared while the system is live. It's a small difference that needs to be remembered when using these. The alternative is to remove the battery, but my battery is so tight, I could almost not get it in. It took me about 5 minutes of grunting, however I ordered cheap battery holders from AliExpress.

Ah, I see. Thanks for the heads up!

If the extra steps to clear the NVRAM/CMOS are too bothersome with the BQ3285S, I think DS12885S might be more suitable. It is also plentiful and inexpensive.