Inquiry Regarding Power Supply Design for nRF52840 DK

[lancaster]

Dear Nordic Technical Support Team,

Hello!

I am currently using the nRF52840 DK and have implemented SRAM PUF. I would like to connect the development board via USB and use the J-Link command mem32 0x20000000, 0x400 to read the SRAM in order to test the charge distribution of the SRAM within the voltage range of 2V to 4V.

To achieve this, I prepared a DC power source and connected its positive and negative terminals to the button battery. However, I found that even when the button battery is removed, the USB-connected J-Link can still read the SRAM data. This leads me to suspect whether the USB is directly powering the SRAM. If the power source for the SRAM is the USB, the voltage will remain consistent, preventing me from achieving my testing goals.

Therefore, I would like to inquire about the power source for the SRAM or MCU when both USB and the button battery are connected. Is there a way to ensure that the SRAM is powered solely by the button battery while connected to USB?

Thank you for your support and assistance!

Best regards,

Yuxuan

[Susheel Nuguru]

Hi Liu,

Your observations seems correct. when the nRF52840 DK is connected via USB, the USB power is typically routed to the main power circuitry, which powers the entire system, including the SRAM. When the board is connected through USB, it provides power to the entire device, including the MCU and SRAM.When the button battery, if connected, can also power the board but if USB power is present, it usually takes precedence in powering the board, as it is routed to the regulator circuitry. 

[lancaster]

Hi  Mr. Nuguru,

Thank you for your prompt response and clarification regarding the power supply situation.

I understand that when the nRF52840 DK is connected via USB, it powers the entire system, including the SRAM. However, I was hoping to isolate the power source to the button battery for my testing. Is there a specific method to achieve this, or any recommended configurations that would allow me to power the SRAM solely from the button battery while connected to USB?

Additionally, I would like to ask whether button batteries also stabilize to a fixed voltage value when connected. This information would be very helpful for my testing setup.

Thank you for your assistance!

Best regards,
Yuxuan

[Susheel Nuguru]

lancaster said:

I understand that when the nRF52840 DK is connected via USB, it powers the entire system, including the SRAM. However, I was hoping to isolate the power source to the button battery for my testing. Is there a specific method to achieve this, or any recommended configurations that would allow me to power the SRAM solely from the button battery while connected to USB?

You have the SW9 switch that you can toggle to select to select Li-Po to power the nRF only through the battery

Button batteries typically have a nominal voltage (e.g., 3V for CR2032) and can supply relatively stable voltage during low-current operation but The voltage of a button battery drops under load due to internal resistance. For light loads like SRAM, this drop is minimal.To ensure a stable supply voltage, you might need an LDO regulator or similar voltage stabilization circuit, especially if your SRAM has strict voltage requirements or if you anticipate significant variations in current draw.