How to solve reset problem at ESD status?

[Devin]

Hi, Dear Expert:

I am an Electronic Engineer. We used nRF51422 or nRF51822 at our products, but found the products was easily reset when did +/-1KV contact ESD testing at charging pogo pin. So, I added a 1uF capacitor between SWDIO pin and the ground for filtering the interference, and the product didn't reset even at +/-6KV . But Someone said could not add this 1uF capacitor, it would make the product unstable. I am confused, don't know how to do. Look forward to your reply,Thanks a lot! DevinZhou

[Kenneth]

Hi, the main problem with adding a 1uF is that it will prevent programming of the device, as the nRF51xxx will not be able to drive the SWDIO line during programming with such a large capacitor. The SWDIO pin is a shared pin with the nRESET pin. However using a <1nF can work, it might reduce programming speed, but it should still help to avoid pin reset due to ESD event in the final product.

[Devin]

Hi，Thanks for your quick reply! There is a short circuit jumper between 1uF capacitor and ground. During programming, let the jumper open; After programming, short circuit the jumper to add a 1uF capacitor between SWDIO pin and ground. 1uF will be better than 1nF for filting the inferrence signal, is it OK?

[RK]

yes that's fine - or just program the device first and then solder in the cap.

I'd have thought even at 1uF you might be able to program the device v.e.r.y s..l..o..w..l..y but I suspect you'd be down to 1kHz on the clock line before the capacitor didn't shunt the signal straight to ground.

[Devin]

Thanks a lot!