BeagleBone; where is the system time set?

I just recently got a BeagleBone. I know the AM355 has an onboard
RTC. How and where is the time initially set? I am not running NTP.


Hi Fred,

use the linux date command

for example - set the current date and time to Dec 04, 2012 04:30 you type:

date --set=“20121204 04:30”

The is no backup battery, so the time will not be kept after disconnecting power


Time resets to Jan 1 2000 if the RTC loses power. The RTC will lose
power every time you turn off the bone. Also see the am335x errata
about the RTC in the first silicon rev (tl;dr: it's broken, don't use
it :slight_smile:


Fred, you had replied to only me and I back to you. I'm now including
the beagle list as others may be interested.

>> Thanks, I verified the Jan 1 2000 behavior. What is the exact
>> problem with the RTC hardware on the first rev. silicon? I read
>> the errata and it sounded like it has a problem keeping time when
>> the voltage falls too low? If the voltage is stable the data
>> sheet implies the RTC does work. Is this not the case?
> You are correct, except the entire point of a RTC is to keep time
> when the voltage is gone for the main processor. The errata says
> that if the core voltage (the one driving the Cortex-A8) goes away,
> the RTC will not keep track of time properly any more. Due to
> this, if you can enter a mode where the RTC is still powered but
> the core isn't (aka: TPS65217 SLEEP mode where VRTC is still up but
> all other rails are down on the bone) then you can't keep track of
> time properly.
> An external RTC is needed until TI fix this in the next silicon rev.
> If you have found a way around this, I'm interested to hear more.
What happens in the case where we don't enter any sleep modes; either
operate at full voltage or turn off (with battery still backing the
RTC). Does the RTC still not work in that case?

If you operate at full voltage, the CPU is on and the RTC is on. If you
do software resets, the RTC should not lose the time. If you turn off,
either by going to TPS65217 OFF mode or by removing power from the
bone, the RTC will lose input voltage and not keep time.

As far as I know, if you want to be in a mode that most would consider
to be "off," the TPS65217 OFF mode is the only one that the bone can
do (partly due to the RTC errata). In order to keep voltage to the
RTC, TPS65217 SLEEP mode is the lowest power mode that can be used,
except when core voltage goes away (which does happen in TPS65217 SLEEP
mode, the RTC will lose time and wake the TPS65217 back to ACTIVE mode).

The am335x low power modes (like laptop suspend) shouldn't cause the RTC
to lose time but I have not attempted this. My interest has been in
using TPS65217 OFF or SLEEP modes while on lithium battery. Keeping
the RTC running in these modes does not seem possible. To solve this
issue I'm using an external I2C RTC chip.

Also anyone know what silicon rev. we're up to, and how to get this
rev. no. from the board (BeagleBone).

Current bones ship (as far as I know) with the first rev of am335x
silicon and the currently posted TI errata applies to all of them. The
am335x technical reference manual should have more info about what
registers would show you the silicon rev (sorry, don't know off hand).

Hopefully future revisions to the silicon from TI will include fixes
for all currently known errata, including the RTC.

The am335x low power modes (like laptop suspend) shouldn’t cause the RTC
to lose time but I have not attempted this. My interest has been in
using TPS65217 OFF or SLEEP modes while on lithium battery.

Andrew, I’m curious which battery you’re using?

  • Grant

We're using an UltraLife UBP002 [1].