Identification of 51822 and SWDCLK resistor

[cristian.anceschi]

Hi all

We are experiencing high current draws that lead to 51822 chip failure under some circumstances. By looking at this thread 

https://devzone.nordicsemi.com/f/nordic-q-a/3954/nrf51822---swdio-swdclk-internal-pull-resistors 

it seems that an external resistor is needed, rather than not,  depending on the revision of the 51822 chip - rev. 2 and rev. 3 don't need any.

Our chips are: NRF51822 QF AA G2    and     NRF51822 CE AA E0 - therefore respectively revision 2 and 3.

Therefore, if we understand what was discussed in the thread linked above, no resistor is needed on SWDCLK pin.

However, with no resistor, and J-Link Ultra Plus dev. board, our 51822s seem to invariably start sinking large current and are damaged.

Same thing happens with a 12K Ohm resistor.

We are now testing with 470 Ohm, and for the last hours it behaved well, but we're worried as we need to enter production soon and therefore would like to know whether the 470 Ohm resistor is needed, or not. To us, it seems that such resistor on SWDCLK is mandatory, not simply optional. Can you confirm please.

Thanks

[Kenneth]

You will notice that the latest reference design use a 1kohm pull down resistor on SWDCLK, this applies to all revisions of the nRF51:

http://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf51/dita/nrf51/pdflinks/ref_layout.html?cp=3_6 

Using a strong pull down (like 1kohm) reduce risk of external noise to cause the chip to enter debug mode (for instance when PCB is exposed). It's possible to even use smaller resistor, like 470-680ohm, but make sure the drive strength of the programmer can drive the SWDCLK pin to logic low (<0.3*VDD) and logic high (>0.7*VDD) in such case.