i.MX28 RTC Backup Battery Connection

[markwilliams]

Hi all,

I have a board that uses the 5V input to the i.MX28 PMU for the power supply and just requires a small NiMH backup battery to keep the RTC alive. The NiMH is trickle charged from the VDD4P2 rail when the device is powered on to keep it topped up.

I am a little confused about the connections for the PMU battery and DCDC batt pins. When I started my design the AN4199 document from Freescale did not cover this so I laid the board out based on the descriptions of the connections. Since obtaining my prototypes Freescale have updated AN4199 to add more detail on systems with just a backup battery and now I am not so sure of what I have done!

If you look at the diagram of the PMU in AN4199 it shows that the BATTERY pin is the output from the internal charger, and the DCDC_BATT pin is the input to the DC-DC from the battery.

In the datasheet for the i.MX28 it states that the BATTERY pin provides current to charge the battery, and the DCDC_BATT pin provides power into the iMX28 PMU from a battery. This agrees with the diagram.

Now, assuming that you are not using the internal battery charger naturally you would think to leave the battery pin disconnected. However, in the data sheet it states: "Application should include a Schottky diode between BATT and VDD4P2.". Which I have done.

I am feeding my battery backup into the DCDC_BATT pin and as from the above I believed this was the input to the board for the battery. Even if just used for RTC backup.

I have since found out that the RTC backup battery should be fed into the pin labelled BATTERY (which is described as the charger output in the documentation?!) and not DCDC_BATT.

My schematic is below, thankfully I added R24 which is a big 1206 pad in case I needed to make a mod!

In this schematic I have the diode the datasheet asks for from BATTERY to VDD4P2. I am trickle charging the NiMH via D5 and R65 from VDD4P2.

D8 is to prevent the possibility of charging the battery back from DCDC_BATT (I guess nothing should be output from this pin anyway). R67 is there to essentially protect the battery should the board try and boot from it. The Vdrop accross R67 is negligible under backup mode but would kill the supply if the board tried to boot.

The problem I have is that I have now found out that BATTERY is not just the charger output as described in the documentation. It is also the input for the RTC backup. Apparently it is also the pin on which the internal battery monitor measures the battery voltage too. Can anyone confirm this for me?

I can mod my circuit above as I only have a small prototype run and have the necessary pads brought out from the device.

I have seen the power scheme suggested in the revised AN4199. This connects the battery to the BATTERY pin. It then has resistors from DCDC_BATT  and BATTERY to VDD4P2.

Can anyone from Freescale who actually knows the internal architecture offer any advice on my circuit above?

Thanks, Mark

[YixingKong]

Weidong

Please keep following up this customer's further question.

Thanks,

Yixing

[markwilliams]

Hi Weidong,

Since the last post above I have been experimenting with the RTC backup. I am currently having issues shutting the processor down on 5V removal.

I have the recommended Freescale circuit from AN4199 with the 1k resistors from the battery pin to VDD4p2 and from DCDC_Batt to VDD4p2.

When I remove the 5V power I find that the battery pin tries to take 2.7mA from the battery. The various rails (VDDIO, VDDA etc) are sitting at odd voltages (0.7V, 1.1V). If I remove the jumper link connecting the battery then these rails immediately drop to 0V.

The only load on the battery is the BATTERY pin on the i.MX283. I even tried removing the 1k resistor from BATTERY to VDD4P2 in case there was some leakage through this path. For some reason on 5V removal the processor does not shut down completely.

If I re-apply 5V power while in this state then the board does not boot up. If I remove the battery jumper and 5V (to completely power down the board) then it clears the condition and I can boot again.

I am not sure what could cause the battery pin to take this additional current? The battery voltage is around 3.8V. The 24MHz xtal is configured to shut down during power-off and the 32.768kHz xtal remain active. The current should be around 20uA from the battery in this state when 5V is removed.

Do you know what could be causing this? Is there any advice from Freescale as to how to configure the board to shut down completely when 5V is removed but a small battery is present on the battery pin?

Thanks for your help, Mark