RTC Not Working When Using VDD for VBAT on STM32L4

[Sujith]

Hello STM32 Community,

I’m encountering an issue with the RTC on my STM32L4 microcontroller. When I connect VDD to the VBAT pin (without using a battery), the RTC is not functioning. However, if I use a battery for VBAT, the RTC works fine.

Here’s the situation:

• I’m not using a battery for VBAT, and I’m connecting VDD directly to VBAT.
• The RTC does not work in this configuration.
• When I use a battery for VBAT, the RTC works as expected.

My Questions:

1. Is there an issue with supplying both VDD and VBAT at the same time, without using a battery for VBAT?
2. Why does the RTC stop working when VDD is connected to VBAT directly, but works fine with a battery?
3. What is the correct setup for powering the RTC when VBAT is connected to VDD instead of using a battery?

Any insights or recommendations for solving this would be greatly appreciated!

Thank you for your help!

[Richard Li]

For test purpose, you can add one resister (220Ohm? depends your power consume) between Vbat and Vdd ,add one capacitor 100 uF , provide power to Vbat. This will emulate Vbat and Vdd delay.

It may give you idea.

[Sujith]

1. Internal without Battery:

• BDCR: 0x0000820B
• Differences from default:
• LSEON (bit 0) = 1 (LSE enabled).
• LSERDY (bit 1) = 1 (LSE is ready).
• LSEDRV (bits 3-4) = 01 (Medium-low drive strength).
• RTCSEL (bits 8-9) = 01 (RTC clock source is LSE).
• RTCEN (bit 15) = 1 (RTC is enabled).
• RTC_TR: 0x00000033: Time shows 00:00:33.

Comparison with Default:

• LSE is enabled, and the RTC is running properly with the LSE clock source.
• RTC has incremented from the default time of 00:00:00 to 00:00:33.

2. Internal with Battery:

• BDCR: 0x0000820B (Same as previous).
• RTC_TR: 0x00000115: Time shows 00:01:15.

Comparison with Default:

• Similar to the previous case, but the time has incremented to 00:01:15.
• The battery ensures that the RTC keeps counting even if the main power is lost, unlike the default state where the RTC is disabled.

3. External with Coin Battery:

• BDCR: 0x0000810B
• Differences from default:
• LSEON (bit 0) = 1 (LSE enabled).
• LSERDY (bit 1) = 1 (LSE is ready).
• LSEDRV (bits 3-4) = 00 (Low drive strength).
• RTCSEL (bits 8-9) = 01 (RTC clock source is LSE).
• RTCEN (bit 15) = 1 (RTC is enabled).
• RTC_TR: 0x00000006: Time shows 00:00:06.

Comparison with Default:

• LSE is enabled, and the RTC is counting up from 00:00:06.
• The coin battery allows the RTC to keep running without VDD.
• By default, the RTC would be disabled and not operational.

4. External without Battery (Drive Medium-Low):

• BDCR: 0x00008100
• Differences from default:
• LSEON (bit 0) = 0 (LSE not enabled).
• LSERDY (bit 1) = 0 (LSE is not ready).
• LSEDRV (bits 3-4) = 01 (Medium-low drive strength set).
• RTCSEL (bits 8-9) = 00 (No clock source for the RTC).
• RTCEN (bit 15) = 0 (RTC clock is disabled).
• RTC_TR: 0x00000000: Time shows 00:00:00 (default).

Comparison with Default:

• The RTC is effectively disabled, similar to the default state.
• Without a valid clock source, the RTC does not function, and time remains at 00:00:00.

5. External without Battery (Drive High):

• BDCR: 0x00008100 (Same as the previous case).
• RTC_TR: 0x00000000 (Same as default).

[Richard Li]

You find the BDCR bit 8 need change to "1" , could you write to this bit?

This time the register is locked, you need follow reference manual page 1038 , enable this register first.

[Richard Li]

@SofLit, @STTwo-32

For this case, reset cause different, any suggestion from ST?

[Sujith · Accepted solution]

Issue Solved - RTC LSE Oscillator Configuration

Hi everyone,

The issue has been resolved! I made a few configuration changes:

1. First, I disabled the LSI.
2. Then, I set the LSE drive to high.
3. Finally, I enabled the LSE.

After these adjustments, the oscillator is now working as expected.

Thank you for your support!

Best regards,
A. Sujith