I’m doing a research how to design a custom carrier board for the Verdin iMX8M Plus. I want to include support for RTC battery backup. The battery I’m planning to use is a 1/2AA, 3.6V, 1200mAh, Lithium-thionyl Chloride (Li-SOCl₂).
According to the Verdin datasheet (Table 72: Recommended Operating Conditions), the VCC_BACKUP voltage range is 1.1V to 5.5V. However, the Carrier Board Design Guide (Appendix A) states that VCC_BACKUP power input is 3.6V max.
Additionally, the reference designs and schematics show VCC and VCC_BACKUP connected via diodes.
Could you please clarify the actual maximum voltage that VCC_BACKUP can safely accept? Is using a 3.6V Li-SOCl₂ battery directly appropriate, or should I consider additional regulation or protection?
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Hi Ivan,
VCC_BACKUP Voltage Requirements
Based on the Verdin iMX8M Plus V1.1 datasheet, which is the most recent version available, I can provide clarification on the VCC_BACKUP specifications (1).
Real-Time Clock (RTC) Configuration
The Verdin iMX8M Plus module features an RTC IC on the module that is equipped with an accurate 32.768 kHz quartz crystal and can be used for timekeeping (1). The RTC power management works as follows:
Primary Power: When the main power supply is provided to the module, the RTC is sourced from this rail (1)
Backup Power: If the RTC needs to be retained even without the module’s main voltage, a coin cell must be applied to the VCC_BACKUP (pin 249) supply pin (1)
Charging Capabilities and Current Limiting
The onboard RTC has specific charging characteristics that are important for your battery selection (1):
Charging Current: The usable charging current is very low (< 500µA at 3V) (1)
Current Limiting Requirement: When using the recharging capabilities of the onboard RTC, a current-limiting resistor with a value >47kΩ must be added between the battery and VCC_BACKUP (1)
Critical Warning: If a resistor with a lower resistance is used, the module will fail to boot (1)
Battery Compatibility
Your proposed 3.6V Li-SOCl₂ battery appears to be within the acceptable voltage range based on the available documentation. However, you must ensure proper current limiting as specified above.
Additional Considerations
For higher charging current requirements, it is possible to implement a charging circuit on the carrier board (1). The Verdin family specification also indicates that VCC_BACKUP is not required for the module to start normal operation, but it is mandatory if the application requires features that rely on this supply being always on, such as the Real-Time Clock (2).
I recommend consulting the complete carrier board design guide for detailed implementation requirements, as the power supply requirements include specific details about power rating, isolation and decoupling, bulk capacitance, and characteristics such as ramp-up rate (2).
The Carrier Board Design Guide specifies a backup battery voltage range suitable for all existing and future versions of Verdin modules. The Verdin iMX8MP datasheet specifies the voltage range for that particular version.
A 3.6V battery meets both criteria, so you can use it in your design without additional regulation. However, if you’re using a non-rechargeable battery, it is still recommended to include a diode for protection
