hoakley July 27, 2023 Macs, Technology

How and why to reset your Mac’s SMC and NVRAM

Among the characteristics of Intel Macs is their reliance on a System Management Controller (SMC), and on persistent settings saved to non-volatile memory (NVRAM, formerly PRAM). As both of those affect a wide range of different functions, resetting either or both can fix such a wide range of problems that this has become something of a panacea when troubleshooting. That has changed completely in Apple silicon Macs.

Apple silicon Macs

Most of the functions of the SMC are performed within Apple’s new SoC, which even has an SMC system to monitor internal temperatures and state, and may work in conjunction with its Always-On Processor (AOP) to run the Apple silicon version of power management software. If you want to re-initialise these, simply shut your Mac down, disconnect all non-essential third-party peripherals and the mains power lead, wait at least 30 seconds, then reconnect and start your Mac up again. The SMC and AOP should be re-initialised soon after startup, load their firmware, and be ready to work normally again.

NVRAM is more extensive than that in Intel Macs and stored in NOR Flash memory, but much of it is protected, and it’s not intended to be meddled with by the user. The only way to set it back to factory condition is to perform a full restore in DFU mode, which also wipes the entire contents of the internal SSD and returns the whole Mac back to factory condition. The Asahi Linux project has documented much of the contents of NVRAM, and you should leave it well alone. Trying to reset NVRAM using the key combination for Intel Macs does absolutely nothing to an Apple silicon Mac.

Intel SMC

Features controlled by the SMC include:

the cooling fans; if these seem stuck full on even when your Mac is just idling, then that suggests an SMC or sensor problem.
accessory lights, such as keyboard backlighting, status indicator light, battery indicator lights on non-removable batteries, those around the I/O ports on recent Mac Pros.
power button; if this is not a mechanical problem (older models), suspect the SMC.
opening and closing the lid of a notebook.
sleeping and shutdown; if your Mac sleeps or shuts down when you don’t think it should, suspect the SMC.
battery charging (notebooks), and the MagSafe power adaptor light.
napping and processor slowing; if your Mac performs poorly but the CPU isn’t showing high load, suspect the SMC.
Target display mode; if this doesn’t work properly, or it switches unexpectedly, suspect the SMC.
recognition of peripherals connected to a USB-C port; if your Mac doesn’t recognise these, suspect the SMC.
the Sudden Motion Sensor (SMS), ambient light sensing, and selection of an external video source for some iMacs.

How you reset the SMC depends on the Mac, with different routines for notebooks with and without a removable battery, and for desktops.

To reset the SMC in a notebook with a non-removable battery (without a T2 chip):

Shut the Mac down.
Connect its mains adaptor and ensure that power is being supplied to it.
On the built-in keyboard, hold down Shift-Control-Option on the left side and press the Power button with them held down, for 10 seconds.
release all the keys, wait a few seconds, then power up normally, preferably to reset the NVRAM next.

To reset the SMC in a notebook with a user-removable battery (older models):

Shut the Mac down.
Disconnect any mains adaptor.
Remove the battery.
Press and hold the power button for 5 seconds.
Insert the battery, then connect the mains power adaptor and ensure that power is being supplied to it.
Power up normally, preferably to reset the NVRAM next.

To reset the SMC in a desktop without a T2 chip:

Shut the Mac down.
Optionally disconnect all peripherals apart from any Ethernet cable, keyboard, mouse/trackpad, and any display. For the procedure to be most reliable, connect any wireless keyboard using its USB charging cable. Then disconnect the mains power cable from the back of the Mac.
Wait for 15 seconds, then reconnect the mains cable.
If you are going on to further system diagnostics or testing, leave peripherals disconnected. Otherwise you can reconnect the peripherals that you need.
Wait for 5 seconds, then power up normally, preferably to reset the NVRAM next.

Although you don’t have to reset the NVRAM straight after resetting the SMC, because of potential overlap between their functions and problems, it’s wise to do so.

Disconnecting peripherals is an additional measure not included in the standard SMC reset procedure. I recommend it because you may well want to have those peripherals clear when you are trying to sort this type of problem out. If you’re sure that you’ll just proceed to reset the SMC (and possibly the NVRAM), you do not need to do that.

Macs with a T2 chip incorporate the SMC, image signal processor, audio controller, SSD controller, and the secure enclave processor in the single SoC (system on a chip), which behaves quite differently from previous SMC chips, although signs of malfunction remain the same.

To reset the SMC functions in a notebook with a T2 chip:

Shut the Mac down.
Once shut down, press and hold its Power button for 10 seconds.
After releasing the Power button, wait a few seconds, then press the Power button to start the Mac up.

If that doesn’t work, try this:

Shut the Mac down.
Once it is shut down, press and hold the right Shift key, the left Option key, and the left Control key for 7 seconds. Then keep holding those keys while you also press and hold the Power button for another 7 seconds. During this, the Mac may turn on and off again.
Release those keys and the Power button, and wait a few seconds before pressing the Power button to start it up again.

To reset the SMC functions in a desktop with a T2 chip:

Shut the Mac down.
Disconnect its mains power lead for 15 seconds.
Reconnect its mains power lead, and wait for 5 seconds.
Press the Power button to start it up.

These procedures are detailed here.

Intel NVRAM

NVRAM stores key settings that your Mac cannot obtain from disk during startup. Apple explains it in a separate article about NVRAM.

The NVRAM contains settings for:

sound output volume,
display resolution, and sometimes other display settings,
startup volume (disk),
any recent kernel panic, to enable a panic log to be retrieved,
other model-specific features.

If you’re curious, you can use the Terminal command
nvram -xp or nvram -p
to list all the variables stored in your Mac’s NVRAM. The output will be formatted slightly differently according to model, and you may find it best to try both, to work out which is more meaningful.

To reset the NVRAM on any Intel Mac:

If your Mac has a firmware password set, you must turn that off before attempting to reset the NVRAM, or the procedure will fail. To do that, restart in Recovery mode and select the Firmware Password Utility or Startup Security Utility from the Utilities menu.
If your Mac is running, shut it down. For the procedure to be most reliable, connect any wireless keyboard using its USB charging cable.
Position your fingers on the Command, Option, P and R keys ready to press and hold them down.
Power your Mac up using its Power button as normal.
Immediately after pressing the Power button, press and hold those four keys. If you’re worried about doing this quickly enough, you can hold the four keys down as you press the Power button.
If your Mac plays a startup chime (chord), hold those four keys until your Mac sounds the startup chime a second time, when you should release them.
If your Mac doesn’t play a startup chime (chord), hold those keys until the display backlight turns on a second time, or the Apple logo appears and disappears for the second time.
Release the keys. If you don’t release them now, your Mac will restart again, and may reset the NVRAM a second time.
Allow your Mac to start up normally.

If you want to be certain that you have reset the NVRAM properly, the most reliable way is to compare its contents before and after resetting, using the command given above. Following successful reset, the NVRAM contains a lot less information in fewer fields.

Remember that when you reset the NVRAM, all its settings will revert to their defaults. If you have changed third-party kernel extension policy using spctl, for instance, then you will need to repeat that change of setting.

One simple way to reset both the SMC and NVRAM without keyboard gymnastics is to start up in Single-user Recovery mode using Command-S-R, then enter the two commands
pmset -a restoredefaults nvram -c
before restarting with reboot. This should be possible even when a hardware password is set.

Resetting the SMC and NVRAM are not panaceas for every problem. They will not fix disk problems, nor failing graphics cards, nor a great deal else. Used appropriately, they can transform a Mac that seems to be dying into one that is in fine fettle.

19Comments

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1

EcleX on July 27, 2023 at 6:50 am

Great reference article. Thanks!

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- 2
  
  hoakley on July 27, 2023 at 10:59 am
  
  Thank you.
  Howard.
  
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3

Udo Thiel on July 27, 2023 at 10:12 am

Great summary. Slightly off-topic question: PCs still boot in 16 bit even today. Do we know if Macs nowadays boot straight into 64 bit?

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- 4
  
  hoakley on July 27, 2023 at 10:58 am
  
  AFAIK all Mac models released since around 2010 (maybe earlier) have 64-bit ROMs. It was one of the failings of my old 8-core Mac Pro, that it didn’t, so had a short life of upgrades. And as far as I’m aware, the cores in Apple silicon don’t even support 32-bit code. So I’m fairly confident that no Mac has booted into anything other than 64-bit code for many years.
  Howard.
  
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5

Duncan on July 27, 2023 at 1:55 pm

“simply shut your Mac down, disconnect all non-essential third-party peripherals and the mains power lead, wait at least 30 seconds…”

I assume this is to discharge a capacitor or something similar, but where does the ’30 seconds’ figure come from? How much of a safety margin does that provide?

(I would imagine the duration is closer to something like 10 seconds, with 30 seconds providing ample time to make sure the charge is depleted, but I’ve never seen this quantified anywhere.)

Also:

“Following successful reset, the NVRAM contains a lot less information in fewer fields.”

Grammatically, that is a beautiful sentence. I’m reminded of this:

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- 6
  
  hoakley on July 27, 2023 at 3:11 pm
  
  Thank you, Duncan.
  Before going any further, I should point out that I’ve never experienced or come across any problems that might be related to the SMC in an Apple silicon Mac. While they could of course occur, I think we’re into rocking-horse manure territory here!
  From all that I know about the boot processes in Apple silicon Macs, I don’t think for a moment that a cold boot is necessary to get the SMC to re-initialise – a warm restart will be just as good.
  However, as you’ll recall from all the discussions we had about cold boots, there are those who insist that’s the only way to perform a full re-initialisation. As no one appears able to explain exactly why, I’m in the dark as to how long might be required to make a boot cold. The consensus from those who advocate this (to me, pointless) manoeuvre seems to be around 30 seconds. Ten might do fine, but as no one seems to have assessed this (and I don’t know how they could), to me it’s just a wild guess. I’d love to see some real evidence for an SoC like the M-series, but frankly I don’t think that anyone has a clue.
  Howard.
  
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  - 7
    
    Duncan on July 27, 2023 at 3:28 pm
    
    Thank you for the reply.
    I have an issue with squishy time estimates on any sort of user action, as there seems to be a growing reliance on ‘long presses’ on buttons to change the button’s behavior. The same applies to ‘unplug and wait 30 seconds’ troubleshooting instructions. If these behaviors have been intentionally programmed into a user-action, then the designer/programmer should tell us the target duration along with the allowable margin where it would still work as intended.
    
    Otherwise we are left wondering if the action didn’t produce the desired results, was it because there is a different problem involved, or did we judge the time duration incorrectly. If the latter, then that’s a terrible user interaction to design towards.
    
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8

joevt on July 27, 2023 at 5:07 pm

nvram -xp or nvram -p only list Apple related NVRAM variables on Intel Macs. Other NVRAM variables are related to UEFI. You may have some NVRAM variables related to Microsoft if you boot Windows. To see all the NVRAM variables, you can use UEFIExtract or UEFITool with a ROM dump, or use dmpstore in UEFI Shell. the nvram command can access those NVRAM variables if you provide a GUID with the variable name.

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- 9
  
  hoakley on July 27, 2023 at 5:13 pm
  
  Thank you.
  Howard
  
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10

Inwerp on July 29, 2023 at 6:21 pm

Basically, there is a bit more to the “SMC” part of the T2.
In fact, SoC Rom on board stores information about the battery, which means its corruption can lead to common “smc reset solvable” problems like battery not charging properly or unit not even powering on.

This method can be seen on many chineese videos as “T2 ROM Repair” including one from REWA, which actually misses the SN part. (Missing SN in SoC ROM will make Mac Activation impossible since apple would not be able to match T2 to the machine SN. )
So long story short, in some rare cases “full SMC reset” means performing a revive procedure in AC2.

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- 11
  
  hoakley on July 29, 2023 at 10:10 pm
  
  Thank you. I don’t think that you mean to refer to “SoC Rom”. If it’s ROM, then nothing can write to it, so it can’t be corrupted. I suspect this is NVRAM, which is why it will be restored to factory condition by performing a DFU mode revive. This is definitely the sort of problem to pass on to Apple technicians, and not to attempt yourself.
  Howard.
  
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  - 12
    
    inwerp on July 29, 2023 at 11:19 pm
    
    SOC Rom is a SPI ROM Chip name from logicboard schematics. This chip contains iboot bootloader, platform information and some T2 variables including battery information. SPI ROM chips are written dynamically on modern laptops. for example, Intel EFI (on pre-t2 devices) is stored on EFI SPI chip and basically is a filesystem container with even more dynamic things like intel ME logs. T2 SOC Rom corruption is a common reason for no power issues on T2 macbooks. As an example waterdamage on power button might cause such damage. There is always a chance that random power failure during write might cause some issues with ROM contents.
    Since T2 incorporates all functions of SMC, basic functions like battery charging, sensors or SMC power sequence is programmed and controlled by iBoot/SOC ROM firmware.
    
    As a more readable explanation:
    SMC chip is a multicontroller IC which runs on internal/external (depends on particular controller) firmware. is is not that different from arduino which has some configurable GPIO pins which are used to route and process some signals. for example it sees power button on GPIO Pin X and it translates it to the PCH via GPIO pin Y waiting for it to answer for a power state switch which then triggers it to enable more power rails.
    Since T2 macs do not have dedicated SMC anymore and basically similar to an old iPhone CPU, they also need some basic firmware which is stored on SoC Rom. To start up, it needs same GPIO configuration and power sequence, it needs to know which signals mean that power is present, how to turn on SSD power, etc. Like any controller it has some dynamic settings which should be stored during runtime. For example, things like “battery charge limit to 70%” profile would be exactly the thing to store in SOC Rom since we need it to be independent from operating system.
    
    I fix pc laptops/macbooks for a living, and also “apple certified” as if it had any meaning in such context.
    
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    - 13
      
      hoakley on July 30, 2023 at 2:48 pm
      
      I’m sorry, but while I’m sure you’re a superb notebook technician, you seem to be a little confused here.
      ROM is an abbreviation for ‘read-only memory’. While it can be programmable, and there’s PROM, EPROM and even EEPROM which are generally considered to be ROM, you’re referring here to Flash memory, often known as non-volatile memory, or by the abbreviation of NVRAM.
      EFI firmware and iBridge firmware for T2 chips isn’t stored in ROM: that’s where the boot ROM is. The following stages of pre-boot are normally stored in Flash (or, in later stages, on the internal SSD), so that they can be rewritten whenever the firmware is updated. Try that with even EEPROM, and you’d end up with it dying relatively fast, because there’s a limit to the number of times it can be reprogrammed.
      You also refer to the T2 firmware incorrectly as “iboot”. iBoot is the second-stage firmware or pre-boot in M-series chips. In T2 chips, that’s known as iBridge or BridgeOS, and is fairly frequently updated in macOS updates. It’s also that (and not ROM) that is refreshed when you do that in DFU mode from Apple Configurator 2.
      “There is always a chance that random power failure during write might cause some issues with ROM contents.” That’s well known for Flash and disk storage, and can occur when updating iBridge firmware during a macOS update. In notebooks, because of the way their power is delivered, it’s exceptionally unusual. It’s less uncommon on desktop Macs operated without a UPS, when mains power outage during firmware updates can corrupt the firmware, and require DFU mode and Configurator to revive the firmware.
      But none of this has anything to do with the subject of this article.
      Howard.
      
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    - 14
      
      inwerp on July 30, 2023 at 4:22 pm
      
      well, you are right that technically it is not “read only memory”, but as i mentioned, it is the way Apple engineers name it on every single logicboard. the SPI Flash memory on board is called SoC ROM even in line names, so i see no problem in using it as reference. On older boards they did call it “SPI ROM”, they also call other Spi chips “TBT ROM”, “BT ROM”, etc.
      
      as for iBoot, it is easy to confirm well known Davidov’s T2 description( https://i.blackhat.com/USA-19/Thursday/us-19-Davidov-Inside-The-Apple-T2.pdf ). According to Davidov, There is T2 “on die” Bootrom with the very basic functionality(DFU communication, internal LDO configuration and SPI interface to run iBoot) which starts iBoot which is stored on SPI flash chip called(im sorry but it is what it is called) SOC ROM which then later starts iBridge firmware from the SSD drive.
      It is indeed called iBoot and it is clearly visible in Hex editor if you read SoC Rom dump from the board. (here is a screenshot of iBoot firmware, flashed with AC2 Revive on the clean SoC ROM chip: https://ibb.co/hY4yHsP )Also, if you search for your battery’s serial number, it will also be found in this chip as well as some more dynamic information. Interestingly, you can even see if board seen other batteries before.
      To even start SSD Drive power rails and run some further firmware/bridgeOs you are mentioning, T2 needs some instructions for its SMC functionality, and since it acts as an SSD controller it needs configuration for that as well (for example, part of its RAM is used as cache, and it also binds multiple SIP SSDs into one volume).
      
      SOC Rom corruption is basically a very common reason for T2 Mac nopower/nocharge issue, so every time you hear that laptop stopped charging and it was fixed with AC2 revive – it the case. From the experience waterdamaged power button which probably pulls ONOFF signal low for a long period of time seem somehow to affect SoC Rom contents, i’ve seen it dozens of times. DFU Revive/Restore writes both to SoC Rom chip and to the BridgeOS partition on the SSD drive. In some rare cases it does not revive untill you completely clear SoC Rom externally, but i had it just two times in my practice. I think it is because AC2 first reads and parses original SoC contents to keep machine’s SN number. Main difference between Revive and Restore is that Restore re-creates lower SSD partitioning(also resetting User partition and encryption key, of course), creating BridgeOS/User Area partitions from scratch. For example, it is needed if SSD chip set is replaced.
      
      Back to my very first comment, long story short, in sense of “SMC Reset”, AC2 revive procedure is, in fact, much closer to being SMC reset rather than PMIC/T2 reset performed with long power button press. SMC reset on pre-T2 boards cleared variables stored internally in SMC chip, AC2 Revive/restore clears variables stored in SoC Rom on board.
      
      The similarity is quite symmetrical: just like SPI ROM contents and variables (again, Apple’s name for the SPI Flash memory with EFI on board) now stored on BridgeOS partition in EFI Image file, the “SMC settings” and variables are now stored in SoC ROM.
      
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    - 15
      
      hoakley on July 30, 2023 at 6:36 pm
      
      If you tell an advanced user that they need to reset or restore the ROM, they’ll be totally confused, and probably laugh at you. It doesn’t matter what you claim “Apple engineers” call it, it’s not ROM (and the firmware engineers I know wouldn’t refer to it as such, as they know the difference). I also note that in Davidov’s presentation, nowhere did he refer to it as ROM either, but as Flash.
      As for iBoot, thank you for showing that hex dump, because if you read a few bytes before where it says iboot, you’ll see that it refers to llb. Since Davidov’s presentation 4 years ago we’ve seen and analysed Apple’s M-series chips and their secure boot process. In case you’re not familiar with this, I have diagrammed it here. iBoot there is actually the later part of pre-boot, and it’s preceded by the LLB, just as your hex dump indicates.
      So perhaps unsurprisingly the T2 follows the same sequence as M-series chips, with Boot ROM first, followed by LLB, then iBoot, and finally launches its own BridgeOS kernel. In Apple’s terminology, the whole of this is referred to as iBridge as far as the T2 is concerned: that’s what you refresh or install in DFU mode, and it has a single exposed version number.
      The instructions that I have given in this article for resetting the SMC in an Intel T2 Mac are correct. While in some cases of sick Macs, the whole firmware may need to be restored, that’s not how a user should reset the SMC in that chip. If your Intel T2 Mac has sustained water damage, then it goes to an approved service provider to deal with, as it may well need a replacement logic board. That’s not something the user should be messing with, unlike the symptoms that I have described above.
      Howard.
      
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    - 16
      
      inwerp on July 30, 2023 at 8:10 pm
      
      Well, thats the same in repair community. If you call SoC Rom something like ‘T2 SPI Flash”, that would be technically right but you will have to add “i mean SoC ROM”, because it is not that i “claim” something, but i am simply referring to a part of the logicboard i am talking about ( https://ibb.co/TTyQHPz ). Every single SPI chip on apple Logicboard is labeled ROM.
      More of that, if you take random PC board, say, Compal, you might see that on block diagram they call it “SPI ROM”. Again, not “my” claim, just a fact ( https://ibb.co/d5HV4ht ).
      As i mentioned, iBoot is located in SoC ROM or T2 SPI Flash if you prefer, and it also contains instructions and settings for SMC chip emulation of the T2. AC2 revive typically solves very same problems “SMC Reset” did, because technically power-cycling/switching T2 to DFU does not completely reset settings which are stored in SMC emulator config on SoC ROM. As an practical example, it might solve lack of battery communication, weird fan behaviour, non working lid sensor and temp.sensor problems unless there is actual hardware damage to these components.
      
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    - 17
      
      hoakley on July 30, 2023 at 8:55 pm
      
      Actually, if you corrupt the LLB, it will have the same effect, as the boot process will halt with the Boot ROM and DFU mode. So it’s immaterial and probably impossible to tell whether the corruption has occurred in either part of pre-boot. This is exactly the same with M-series chips. But fixing that isn’t a matter of resetting the SMC in either case, it’s a matter of replacing the ‘firmware’, which in the case of the T2 includes everything from LLB to BridgeOS proper.
      Howard.
      
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18

Marshal on July 31, 2023 at 9:06 pm

Just what I needed to know! Thank you!

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- 19
  
  hoakley on July 31, 2023 at 9:07 pm
  
  You’re most welcome.
  Howard
  
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