iPhone keeps disconnecting and reconnecting to MacBook via USB

Question

This post/question is referencing the iphone macbook connections issue reported by @Avelj (thanks for your details analysis, that helped a lot) in detail in this thread:

iPhone-Mac connection issue and what does the usbd process do?

But since you have to have "reputation" points to submit an answer not least a comment I am forced to open this new thread instead!!! (Well, not such a good system at last)

I might ramble a bit, tl;dr at the bottom...

The issue:

As presented in the referenced post connecting an iPhone to my MacBook Pro Mid 2015 (MacBookPro11,5) would result repeatably in disconnecting and reconnecting the iPhone. This would make the connection impossible to use for Ethernet or Sync and affects also the performance of the MacBook: Every time the iPhone would disconnect the MacBook would freeze for half a second, being very visible when watching a movie/show on Netflix. This bugged me for years as I could not understand what the issue might be.

It was reported that this could be solved with either a new lightning cable or disabling the usbd process on the MacBook, but in case of the cable would probably resurface only month later. Both solutions are dissatisfying, as I for one wouldn't want to purchase a lightning cable every month or so. And second disabling the usbd process via sudo killall -STOP -c usbd would solve the freezing but reduce the charging power to abysmal 300 mA.

The investigation:

I tested various iPhones (XS, XS, 8, 5, 4) and all showed the same issue. I used different lightning cables, no change. I used a different Macbook Pro (2008) and a a Mac Mini - same with the old MacBook, but the Mac Mini works like a charm.

So I took a closer look at the software stage but soon realized that the issue could not be resolved this way, as it probably hat to do with charging power and connection issues from the iPhone itself, as the connection would only fails as the charging power increases.

Therefor I checked all parameters regarding this and found iStatMenu to be a very good tool to check the charging power. The 5V USB rail showed a consumption of 0 mA for a suspended state and ~20 mA for active state with no USB accessories connected. This consumption ist most likely the standard current draw from the Trackpad and Keyboard connected to the USB controller internally on the MacBook Pro 2015.

Then I connected the iPhone to either USB ports and could monitor the current consumption: With a disabled usbd process it would rise to 500 mA consistently and be very stable. But charging the iPhone this way would take ages and in some cases, when the iPhone was used aside the MacBook result in no charge at all.

When resuming the usbd process with sudo killall -CONT -c usbd the current draw would increase to 600 mA or more but be very unstable and drop back down to 400 mA resulting in a buzz from the iPhone reporting a dis- and reconnect.

You can actually see this in the following picture: A depicts the active state without USB connections. B the connected iPhone with usbd disabled. C with usbd enabled. The current draw reaches 600 mA and brakes down before reconnecting again and again.

The final solution and my opinion, on what causes this issue please read my own answer below... :)

TL;DR: The issue of reconnecting iPhones on Macbook/Pro seams to be repeatably caused by the USB sockets in the MacBook and/or the USB connectors on the used lightning cable. Contact resistance of the connector prongs makes the iPhone disconnect and reconnect in high power charging mode.

Answer 1

So, in regard to my own question, here is what I found works:

The solution:

I looked at the USB socket and cable and first cleaned them with isopropyl alcohol and then shifted and pressed on the connection in different directions and with different force. It can clearly be seen that the current consumption increased up to 1000 mA (the nominal current draw I would expect from an iPhone with <50% SOC), but was not stable, when releasing the pressore or grip on the USB connector.

So I tried different positions of the connector in the socket and various positions of the cable tied to the MacBook case and in some instances the current draw stayed constantly high (~800-900 mA). Interestingly the freezing issues subsided when the current reached this high and stayed off even, when the current draw would subside back to 400 mA if the pressure on the connector was released. In this case the current would falls all the way down to 260 mA not charging the iPhone. Only physically removing the cable from the socket and retrying the fit would resume charging with showing the freezing issue until the charging was consistently above 800 mA.

So at last I found a position on the right USB socket that utilized the full charging power with 950 mA. The freezing stopped and the iPhone charged normal (and fast). The tinkering on this port might have lead to solving the problem for now as I can repeatably use this port now to charge the iPhone and not have to stop the usbd process. The other socket still shows the same old problem.

The most likely reason for this issue:

So from this investigation and experiments I can confidently say that the issue with reconnecting iPhones on my MacBook Pro from 2015 is due to high contact resistance on the connection prongs inside the USB socket and USB connector from the cable used. This also explains why a new lightning cable or even third party cables might resolve this issue for a certain amount of time.

A permanent solution is probably rebending the socket prongs and cable prongs inside the connector/socket. I believe that they are manufactured with a certain preload which subsides with constant use very quickly. Also maybe the connectors are gold plated to reduce contact resistance and repeated use scrapes off the plating from the connector (which can be seen on cheap USB sticks). In any circumstance the connection surface area of these prongs is very very small and might lead to high resistances (as USB was never intended to supply more than 500 mA). The iPhone charging chip is obviously sensitive enough to react to the reduced voltage resulting from the contact resistance and disconnects the iPhone as a precaution.

As I will replace my battery in the next weeks I will open the MacBook and work on the USB sockets. Don't do this while the battery is connected inside the MacBook as you might short something.

Maybe someone else can try to replicate my results and report here, that would be awesome. This problem bugged me for years and nearly prompted me to by a new MacBook Pro (until I saw the current prices - I nearly fainted). Good to have a real solution.

And many thanks to the author of my referenced thread here, disabling the usbd process kept me going for years on!!!

TL; DR

A solution might be to clean or bend the connectors contacts within the USB socket. For me cleaning solved the issue on one port repeatably! A deeper cleaning or reworking the sockets might fix it permanently.

Answer 2

I resolved this issue on my M2 MacBook Pro 13" 2022.

I replicated this on three other MBPs from 2018 to 2023. Currently, I am running Sonoma 14.5 (23F79). The issue also occurred when using 128 GB USB-A flash drives and transferring data of >100 MB onto it. Constantly getting the drive disconnected message, then the drive would auto mount so I would try to transfer again and it would disconnect in under 3 seconds.

Like you, I thought it was a power draw or stability issue. I cleaned the ports, I was using the Apple USB-C multi-port adapter (HDMI + USB-A + USB-C) when experiencing this error. Thinking it was perhaps the adapter, I bought another adapter but was unable to get rid of the problem. Pretty much giving up and resorting to online file transfer service, I realized that transferring 37 GB would take a very long time.

Finally, I found a USB-C to USB-A adapter from SYNTECH (XR001RWB52N) that is an inline solid adapter with no cable. Switching to this adapter finally did the trick, the drive works every time it's connected and does not disconnect while transferring 37 GB to it. I find it incredibly strange that Apple adapters fail (with and without external power plugged in), the terminal command to stop and start USB fails, and multiple reboots and SMC resets fail to resolve this but this Chinese solid USB-A to USB-C adapter works like a charm.