RTC backup battery on i.MX6 and MMPF0100

[danielelicini]

Dear community,

I have developed a board based on i.MX6 Dual Lite and PMIC MMPF0100.

I need RTC function.

My system hasn't a main battery but is supplied by a wall mount power supply. I need that RTC will keep the time when the main power supply will be disconnected for a little.

So, I have provided a supercap but it seems that it isn't enough.

Now I'm revising my board and I would provide a small lithium rechargable battery.

But, I have some doubts.

1) How is the voltage required to mantain RTC time when the main power is OFF? In the i.MX6 datasheet I read  that "Backup battery supply range (VDD_SNVS_IN) is minimum 2.9V and maximum 3.3V". Is it correct?

2) I read that the Maximum supply current of VDD_SNVS_IN is 275uA

With the supply current specified (275uA), the small coin lithium batteries (like ML614 or ML621) has a voltage drop and doesn't assure that the voltage will be above 2.9V, even if it is full charged.

Should I provide a bigger battery or there is something that I miss?

Onto the SABRE SDP is present a ML414 that for sure can't assure a voltage above 2.9V with a current sink of 275uA.

Thanks very much,

Daniele.

[gusarambula]

Regarding the RTC battery, this is a good observation. VDD_VSNVS_IN is the input of the on-chip LDO that supplies the power to the Secure Non-Volatile Storage domain and RTC. Since the target output voltage of this LDO (that can be measured on the VDD_SNVS_CAP pin) is about 1.1V, it remains operational even if the VDD_VSNVS_IN voltage lowers to about 2.5V. However, a strict lower limit has not been qualified and the optimal range is [2.8 3.3]v.

Maybe some users have more information on behavior on the field when using certain batteries.

The RTC chip it’s another elegant solution.

[danielelicini]

Dear gusarambula,

many thanks for your details and thanks also to whom that has contributed to reply to my topic.

Could be a possible solution to provide an external LDO that takes battery voltage and generate 1.1V that I could connect directly to VDD_SNVS_CAP pins? Maybe providing a diode to ORing the two power supplies.

Otherwise I can provide a uPower step up regulator that regulates the battery power to 3V, but is more expensive in term of BOM.

Thanks very much,

Daniele.

[pankajrana]

Hi Daniele,

In addition to the other valuable comments/answers, I would suggest that, you can go for an external RTC chip, connected to the processor through I2C or SPI or so....

I am not pushing you to go for this , but as per my information, the Coin Cell will get drained up soon going with the Processor RTC, while in external RTC case it will last longer. Also some of the RTC chips do provide the input to the XTAL pins of the processor , so you can further avoid the 32.768 KHz crystal as well.

Part number : PCF2127  , just in case .:smileyhappy:  (Uses the board power when board is on, & Coin Battery when the main supply is removed)

--Pankaj Rana

[jaymind]

Hello danielelicini,

As per suitable way also another word that one of the way for RTC interface with i.MX6 is that use good RTC chip (e.g. IC M41T83SQA6F STMicroelectronics) which is good and also suitable and working way.

Battery which connect to RTC chip is about to 3.5V Lithium coin cell.

This chip is work on serial I2C protocol and connect to uP via I2C interfacing.

some circuit connections and image show below.

RTC chip to uP connections

May above information helpful to you.

Regards

Jaymin D