nRF5280 PDK Floating VDD_nRF_SENSE

[sas]

Hi,

Suddenly I can not connect to my nRF5280 PDK (PCA10056 0.11.0 2018.3)

1. I tried using JLinkExe and also SES, the same result, I got error "Can not connect to JLink via USB". All LEDS off.
2. Double cheking all switch, it was correct as it worked before. I am using linux, so I did "lsusb" but the PDK did not show up on lsusb output.
3. Accidentally I switch on SW10 and then I didn't know how LED5 was on. I run lsusb on the terminal which gave me output "segger", which is correct.
4. JLinkExe and SES can connect to interface MCU, but can not connect to target.

I switch off SW10, measured VSENSE_SW_OUT voltage, it showed 1.5V. This is interesting as I believe it should be around 3V as it is connected to VDD to power up the board.
Measuring VSUPPLY I got 2.89V, which is close to 3V. I suspected something wrong with U17. So I shorted SB39 and viola... the system run as expected. I can connected to SES and JLinkExe, also I can go to BOOTLOADER MODE and download blinky firmware via SES.

I concluded something was wrong with U17. I dig deeper, and found VDD_nRF_SENSE was floating if SW9 was in VDD position and SW10 was in OFF posisition.

I could be wrong here, could you please verify my finding?

Thank you,
-sas

[sas]

additional note:
I think my question related to  nRF52840-PDK USB not recognized problem.

[ketiljo]

If switching SW10 to ON, the power supply of nRF52840 is routed directly to P21 (External supply) connector, as long as SW9 is in VDD position. With no power supply on P21, nRF52840 has no power and it is reasonable you could not connect to target.

 Some thoughts:

• The 1.5V you see is most likely that the nRF52840 gets reverse powered through some GPIOs.
• This will have some diode drop to VDD_nRF.
• The VDD_nRF is then fed back to VSRC_nRF through SW9
• VSRC_nRF connects to VDD_nRF_SENSE through SW10
• VDD_nRF_SENSE then activates the voltage follower U19 and activates the switch U16 which has an EN threshold voltage of 1.1V
• With VDD_nRF_SENSE floating when SW10 is switched to OFF might leave the voltage “hanging” there.
• You could try to populate a resistor on R88, 1M ohm or so to see if U16/U17 switches correctly then.

[sas]

Ketiljo,

SW10 is ON:
1. Yes you were correct, there is no power to nRF52840. But Interface MCU is still powered. This is the expected behavior I think.

VDD_nRF_SENSE is floating:
1. Unfortunately I don't have the right tool to install R88. But I confirm that when I shorted both pad of R88, my laptop can communicate with the PDK smoothly., I can detect the board using lsusb command, I can download the code to nRF5280 chip.
Shorting R88 pads means pull Q2 gate low, on which pull U17_ENABLE high and at the same time pull U16_ENABLE low. No doubt add R88 should fix VDD_nRF_SENSE floating.
2. I believe this is not a good practice to left Q82 gate, which is VDD_nRF_SENSE, floating. If it is, then why nordic didn't populate R88? I saw some thread setters asking why suddenly their computer can not detect PDK. I conclude populate R88 is the solution to their problem. Cmiiw.

Thanks,

[ketiljo]

I agree that R88 should be populated, I'll forward the feedback.