[NPMUG] When to reset the PRAM and SMC/PMU

[Dave Sevick]

Good article from Ted Landau at MacFixit :

http://www.macfixit.com/article.php?story=20090409081833464

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When to reset the PRAM and SMC/PMU

The PRAM

The Parameter RAM is a small amount of "nonvolatile" RAM (NVRAM) that  
holds various settings that the system can use before the operating  
system loads, and maintains these settings even when the computer is  
turned off. There is a small battery on the computer's mainboard that  
helps maintain the PRAM settings when power is off.

Many times when people's computers get bogged down and they are  
experiencing problems, advice will be given to "Reset the PRAM." In  
certain situations this can fix things, but many times people just  
blindly throw that advice out there. Resetting the PRAM may sound like  
an exotic fix that somehow may magically cure something; however, many  
times doing this is just a shot in the dark. As such, it's not  
necessarily bad for the computer, but may lead to customized settings  
such as alternate boot devices and speaker volumes being put back at  
default values. If you have problems with any of the following then a  
PRAM reset might help you out:

Volume changes or won't stick.
Video resolutions not sticking or not all available
Time zone information and clock settings
Boot volume isn't set (question mark shows briefly before booting)
Keyboard repeat rates
Mouse input rates (click and tracking speeds)
Default system fonts
Prior to OS X, the PRAM held information for networking, but that has  
been removed so any network troubles should not be affected by PRAM  
settings. Sometimes there have been odd problems that have been cured  
by PRAM resets, including one reported on recently by MacFixIt  
regarding spell-checking not working. It is possible that settings in  
the PRAM may indirectly affect various system functions like the spell  
checker, but knowing this is anyone's guess.

If these settings constantly get reset even when you have not manually  
reset the PRAM, the computer's logic board battery may need replacing.

To reset the PRAM, reboot the computer and hold the options-command-P- 
R keys at the same time. The computer will chime, and then continually  
reset and chime while these keys are held. Allow it to cycle a couple  
of times and release the keys and allow the computer to boot normally.

NOTE: Resetting the PRAM may require you to set some settings such as  
mouse speed and keyboard rates again.

The SMC/PMU

The System Management Controller (Intel Macs) or Power Management Unit  
(PowerPC Macs) is a chip that manages the power for various system  
components such as the fans and backlights, as well as the circuits  
that initiates power to the whole system when you turn on your computer.

Like the PRAM, many people readily offer suggestions for resetting the  
SMC or the PMU. However, there are specific behaviors that will  
benefit from SMC/PMU resets, and others that won't. For the most part,  
the power management in the computer should only be reset if you  
experience problems with seemingly "stuck" settings, or nonfunctional  
indicators, especially any on the hardware itself (power adapter  
indicator, or battery life for laptops). The following list of  
situations are some instances where a SMC/PMU reset can be beneficial:

Battery isn't charging properly
Display brightness won't work properly
Keyboard backlight won't work
Fans blaring all the time
Power button not functioning properly
Closing/opening laptop lid doesn't sleep/awaken the computer
External ports not receiving power
External devices not recognized
Internal components such as Airport or Bluetooth not starting up
Unexpected shutdown
Resetting the SMC/PMU will depend on the machine you have, but here  
are some resources for a few Mac models:

Mac Pro
iMac and Mac Mini
MacBook and MacBook Pro

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