why nRF51822 cannot connect to J-LINK?

[sunnyjet16]

I soldered 5 pieces of PCBA with nRF51822, but I got only one PCBA which can connect to J-LINK. I checked the bad four and found that SWDIO of nRF51822 on the four board is lower than 1V. The right one is 3.3V. My question is:

1. Is SWDIO has to be equal to VDD? Otherwise, the interface does not work.
2. What is the root cause that SWDIO is not equal to VDD? Soldering issue, bad chip,or external circuit? To program the chip, I put only a few circuits on the board including nRF51822 chip, 3.3V LDO which supplys 3.3V to VDD and AVDD.

Regards Alex Chen

[MartinBL]

Hi

1. Yes. And this is important. The SWDIO pin is also an active low reset pin and should be pulled high (to Vdd) whenever you are not programming the chip. However, the pin has an internal ~13 kOhm pull up resistor so it should stay high unless you have connected anything to the pin externally. You could try to add an external pull up resistor of ~1kOhm, but even if this solves the problem it doesn't really explain why it happens.
2. Hard to say. It could very well be bad soldering or a bad circuit. I find it unlikely that 4 out of 5 chips are bad.

What chip revision do you have? Refer to the Compatibility matrix.

Could you upload your schematic and/or PCB layout? If you are not comfortable uploading it here you can create a MyPage account and submit a support ticket there. Then you will get private support and you can get a completely confidential review of your schematic/board.