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I've been resetting the SMC in many macbooks in my job as a technician. Yet, it's not entirely clear to me if the SMC is reset in this case:

Please let me explain myself properly: According to Apple's official article: How to reset the System Management Controller (SMC) on your Mac - Apple Support

in Macbooks with removable batteries (these are the older macbooks in which the user can push a clip or clips and easily detach the battery), the SMC reset is done by removing the battery and pressing the Power button for five or more seconds.

My doubt is:

  • Will the SMC reset if you disconnect the battery and press power for 5 seconds in newer Macbooks?
  • Will the SMC reset if you press the keyboard combination Shift-Ctrl-Alt and Power in older Macbooks with user-removable batteries?

My idea is that in both cases the SMC will reset. But I'm not altogether sure, which has me repeating SMC unnecessarily in my daily troubleshooting.

Any suggestion will be welcome.

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This answer is based on the datasheet from a similar power management controller unit, or PMU (what Apple calls SMC - they're the same thing) as datasheets for those chips are not available.

I am using the datasheet from the TM4C1231C3PM which is approximate to the (Apple SMC) LM4FS1AH5BB which are both made by Texas Instruments.

There are 6 conditions which will cause a reset:

  • Power on reset (POR)
  • Brown out detection
  • Software initiated reset
  • A Watchdog timer reset
  • MSOC (system clock) failure

The POR will reset the SMC

Wait....but I turn on/off my laptop all the time.

This is the power on state of the chip, not the machine. As long as power is applied, it will hold the settings which is why disconnecting the the battery resets the SMC. This is the initial power up of the chip.

The internal POR is only active on the initial power-up of the microcontroller and when the microcontroller wakes from hibernation.

So, to answer the questions...

Will the SMC reset if you disconnect the battery and press power for 5 seconds in newer Macbooks?

Yes. Removing the power and discharging any residual charge will reset the SMC.

Will the SMC reset if you press the keyboard combination Shift-Ctrl-Alt and Power in older Macbooks with user-removable batteries?

It's impossible to know without actually testing them as datasheets for older SMC chips are even more difficult to source. (IMO) it's unlikely since the desktops of that vintage didn't use the keypress and relied solely on removing power, the laptops did as well since they had removable batteries. It wouldn't hurt to test this.

  • Nice answer! +1 – user3439894 Nov 5 '17 at 16:32
  • That answer, bueno! From experience with an old MacBook with a user removable battery, the keyboard shortcut does not reset the SMC (didn't resolve high fans when tried, but removing the battery and holding power button did) – FreelancerJ Nov 7 '17 at 10:41
  • Also, anecdotal, but I'm under the impression that on MacBook computers with non-user replaceable batteries have those keys additionally wired for the combination, because I've been able to reset the SMC on MacBook Airs and Retina Pros that the keyboards are otherwise completely unresponsive in the OS and on Startup manager, which might further explain why the shortcut wouldn't work on earlier Macs – FreelancerJ Nov 7 '17 at 10:42
  • My guess is the keypress disabled one of the watchdog timers putting them out of sync which forced a reset. Unfortunately, I have no way to test this theory.... – Allan Nov 7 '17 at 11:27
  • I also noticed the SMC combo works with 2015 macbook early 13" pros when the keyboard isn't working. For these we always replace the top case, we're an apple repair centre. I didn't test with the other macbooks. – Antonio23249 Jan 4 '18 at 1:44

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