Max Capacitance on NANO Reset pin?

Anyone know how much external capacitance, if any, can be connected to NANO's reset pin?

Apparently 0.1uF is too much, as I cannot download via the USB port if this cap is attached to the reset pin (NANO pin 3).

I'm assuming the problem is that the RC rise-time, given NANO's 1K pullup on the Reset line, is too long, and it is squelching the PC's initial attempts, when the USB cable is inserted, of communicating with NANO via USB.

In my application NANO will be under a shield, and so I've brought the reset line to a 2-pin header outside of the shield. The cap is from this line to ground, and it is there to attenuate any RF picked up via this header and its wiring (the application will be in a high-RF environment).

Thanks!

No capacitance to ground.

The capacitor is already there - it is connecting Reset to the USB chip. There is no reason to connect any other capacitance to ground. (If you wish to be perverse and absolutely insist upon it, connect the capacitor from the USB chip side of the reset capacitor to ground.)

If RF is a concern, put a RFC in series with your wire.

Anyone know how much external capacitance, if any, can be connected to NANO's reset pin?

Why would you do that? The only reason to to that is to actually disable the auto reset circuit when you are using an Arduino as an ISP (In Circuit Programmer )

Thanks for the replies...

Grumpy_Mike:
Why would you do that? The only reason to to that is to actually disable the auto reset circuit when you are using an Arduino as an ISP (In Circuit Programmer )

There's another reason -- my application places the Nano in an environment with potentially high RF fields in the range of 3 - 30 MHz.

(If interested, here: K6JCA: Antenna Auto-tuner Design, Part 7: Build, Phase 1)

My experience (painfully gained from EN61000 immunity-testing of other designs) is that strong RF fields can cause circuits to barf, and so it's best to keep that RF away from device pins. (And best to design in this mitigation at the start of a design, rather than at the end, when the boss is wondering why there's another board spin).

There are many ways to make circuitry immune to external RF fields, but I've found that a simple (and cheap) way is to cover the sensitive circuitry with a shield (which can actually be quite inexpensive) and then use single-pole R-C filters to low-pass filter any RF that might be sneaking in on signal traces (or cabling) that passes between the outside (RF) world and the shielded circuitry.

Usually I just use series 100 ohms and put a cap-to-gnd on the trace going from the resistor to the circuitry under shield (i.e. low-pass filtering any RF pickup that might be arriving from the outside world).

Now, because my Nano will be under a shield, its reset button is inaccessible. So I thought I'd parallel it with a header at my board edge, just in case I want to reset it without cycling power. This header isn't under the shield. Therefore it is an antenna. And I'd like to keep RF picked up by this antenna from Nano's Reset pin (and from the USB chip's DTR# pin, too, which is essentially on the same net). So I thought -- let's add a low-pass filter between this antenna and the Reset pin.

Paul__B:
No capacitance to ground.

The capacitor is already there - it is connecting Reset to the USB chip. There is no reason to connect any other capacitance to ground. (If you wish to be perverse and absolutely insist upon it, connect the capacitor from the USB chip side of the reset capacitor to ground.)

If RF is a concern, put a RFC in series with your wire.

Ah-ha! Many thanks for pointing out that cap! I hadn't seen it (probably because I hadn't expected it). Is this how the board is reset if, for example, invoking the serial port?

Regarding RF chokes -- at 3 MHz a choke that would provide significant attenuation without additional capacitance-to-ground would be, well, significant. Ferrite beads, for example, won't cut it. And you'd probably still want some C to gnd, just to get an adequate rolloff at low frequencies. As I mentioned above. a single-pole R-C filter is simple, cheap, and, if implemented with two 0805 (or 0603) passives, it has a very small footprint.

Adding capacitance-to-gnd at the DTR# pin is an interesting idea, thanks! The only potential downside I see is that there would be an unfiltered signal running for a significant distance under the shield (and thus potentially coupling onto other nets) -- something I try not to do. Never the less, a possibility. Except -- I've already soldered my Nano to my circuit board (well, proto-board), so the USB part is no longer accessible!

RF proofing a reset pin is not best done with a capacitor. That pin is more robust than many. I have done my fair shair of RF immunity work in the past and a capacitor alone is not the way to do things. A differential RF choke made with a 6 hole ferrite beed is far better. If you must use caps then 0.1uF is way way out by many orders of magnitude, there is no need to go over 100pF and you can use feed through capacitors if you want the signals inside a can.

Grumpy_Mike:
RF proofing a reset pin is not best done with a capacitor. That pin is more robust than many. I have done my fair shair of RF immunity work in the past and a capacitor alone is not the way to do things. A differential RF choke made with a 6 hole ferrite beed is far better. If you must use caps then 0.1uF is way way out by many orders of magnitude, there is no need to go over 100pF and you can use feed through capacitors if you want the signals inside a can.

Thanks for the feedback.

I agree with you if I were just designing to meet EN61000 immunity, where the minimum frequency is 80 MHz and where the field strength is, say, 3V/m or 10V/m. In such instances I would typically use a series R of 100 ohms and a 100 pF cap to ground. (Actually, I usually put a 100 pf cap to ground on both sides of the resistor – one side attenuates for immunity testing, the other side attenuates for emissions testing).

But at 3 MHz, 100 pF has an impedance of about 500 ohms – that’s significant. And working against, say, 100 ohms series R, it would provide little attenuation.

Plus, there’s the potential that the field strength could be huge. I’d like this tuner to be able to handle 800 watts, peak power. And the circuitry carrying that power would be, literally, just a few inches away from the Nano. Very near-field! There’s a grounded bulkhead between the two, but how effective will it be? I have no idea. So the potentially high field strength has me worried – I’m going where I’ve never been before.

Is 100 nF overkill? Honestly, I sure hope so! I’d rather overkill and not have to touch the board again rather than discover that there are significant problems with smaller caps. (And, for all I know, I might have significant problems even with 100 nF caps).

Anyway – many thanks for the comments and ideas. I very much appreciate the chance to bounce ideas off of you and others.

  • Jeff

P.S. As I discovered, 100 nF on the Reset line is a bad idea. But 10 nF seems to be OK (Z ~ 5 ohms at 3 MHz), which is not too surprising given that the series-C to the USB chip’s DTR# pin is a factor of 10 larger (it being 100 nF).

P.P.S. Regarding my use of R’s and C’s for RF mitigation – for many years I worked for a company designing products in plastic cases (often with long cables attached to them), and for whom bottom-line BOM costs were very important. So I got into the habit of trying to find inexpensive ways to comply with RF emissions and immunity. R’s and C’s are dirt cheap, so they were usually my first go-to. But for some of the designs (e.g. POE or, say, off-line switching supplies), differential chokes are absolutely required. I’m just worried about their efficacy in this very-near-field environment (and, of course, the room they’d take).

I think trying to design such a thing with one board spin is a bit unrealistic given these parameters. As you know this is not an exact science.
Back in the 60s our standard source of interference was a half horsepower electric motor with a hole drilled in the mains plug over the live terminal at 240V. Into this was inserted a round file, and the other motor wire was brushed against file to generate sparks. We reckoned if it could withstand that it would withstand anything.