AM3352 and PMIC RTC

Hi Stephen,

1.We want to have an RTC in our application. Is there any advantage in selecting Processor or PMIC RTC? I just need to get time information at the next bootup for logging purposes.

My personal opinion is that an external RTC will give you a much longer battery life.

2.Referring to swcu093c i see the TPS65910A31 can supply processor VDDS_RTC while TPS65910A3 cant. If we have to use processor RTC with TPS65910A3, an external buck should be used to power VDDS_RTC from a coin cell or supercap right?

This can be an LDO, but yes, the AM335X RTC needs a stable 1.8V power supply.

3.If we are using PMIC RTC with TPS65910A3 ,we can use an external buck to power VDDS_RTC, while the buck is powered from Main VBAT?

Yes, this is correct (see comment about LDO above).

4.The EEPROM Boot mode as described in swcu093c selects CK32K_CTRL bit of DEVCTRL_REG as 1 i.e internal RC oscillator. I suppose the EEPROM settings cant be changed in field from (http://e2e.ti.com/support/power_management/pmu/f/43/t/324147.aspx) So does that means we can only use internal RC oscillator if i select BOOT[1,0]=10 or we can change the clock source in run time.? We would like to use external 32kHz crystal oscillator for PMIC RTC.

It's possible to overwrite EEPROM bits in runtime, but this will have to be done every time you power cycle the PMIC.

5.We want our board to boot as soon as i provide the main battery voltage VBAT. So if i follow the connections in    Figure 1 swcu093c is it possible?or this application circuit power up the processor only when the push button connected to PWRON is pressed? If it is so is there any workaround for me to apply.

You will need to connect the AM335X PMIC_PWR_EN signal to the PWRHOLD input of the PMIC.

7.If I'm using PMIC RTC with battery what will be the typical current consumption while RTC is running? i.e when main battery is OFF.

PMIC RTC: 15-20uA typ.

AM335X RTC: 20-30uA typ.

External RTC: 0.5uA typ.

Thank you very much for the quick reply,

I would like to learn a little bit about point 4. the PMIC datasheet says,

The TPS65910 device can provide a 32-kHz clock to the platform through the CLK32KOUT output,the source of
this 32-kHzclockcanbe:
• 32-kHzcrystalconnectedfrom OSC32INto OSC32KOUTpins
• Asquare-wave32-kHzclocksignalappliedto OSC32INinput (OSC32KOUT keptfloating).
• Internal 32-kHzRCoscillator,to reduce the BOM,if anaccurateclockis notneededbythe system.
Defaultselectionofa32-kHzRCoscillatorversus32-kHzcrystaloscillatororexternalsquare-wave32-kHzclock
dependsonthe Bootmodeordeviceversion(EEPROM programming):
• BOOT1=0,BOOT0=1:quartzoscillatororexternalsquarewave32-kHzclockdefault
• BOOT1=0,BOOT0=0:32-kHz RC oscillator default
Switching from the 32-kHz RC oscillator to the 32-kHz crystal oscillator or external square-wave 32-kHz clock
can also be programmed though DEVCTRL_REG register, taking benefit of the shorter turn-on time ofthe
internal RC oscillator

I assume that once the Main voltage VBAT is lost , but the VBACKUP supply is working the PMIC will switch on the RTC domain to battery and the RTC keep on working. Also assume after the first boot up  i had changed the 32kHz clock source to crystal oscillator.So now when the PMIC goes to Backup state will the crystal oscillator or internal RC oscillator will be used..?

Thank you very much,

Stephen

I don't know the TPS65910A in such detail. You can ask this on the PMIC forum: http://e2e.ti.com/support/power_management/pmu/f/43.aspx