Power Computing provided the information in this article and it was deemed accurate as of 14 November 1997. Apple Computer, Inc. is not responsible for its content. This article is being provided as is and will not be updated in the future.

What is the CUDA Reset button, and why would I want to use it?

The CUDA Reset button is a small red button located on the Main Logic Board. For more information on the button's location on the various Power Computing models, please see the following Tech Info Library article:

Article 36253: CUDA Button Location and How to Get to It

Definition:
===========
Depressing this button resets the CUDA Microcontroller Chip. This chip is responsible for the following functions:

* turn system power on and off
* manage system resets from various commands
* maintain parameter RAM
* manage the Apple Desktop Bus (ADB)
* manage the real-time clock
* let an external signal from either Apple GeoPort serial port control
system power

CUDA is powered by the Motorola power manager chip which draws power from
1) the 5V rail if power supply is turned on,
2) the 5V standby supply if the power supply is plugged in but not turned on,
3) the battery otherwise.
When you push the red button, it makes no difference whether you are plugged in or not. If you are turned on when you push the button, the CUDA is reset which will turn the machine off.

Though it seems incomprehensible from an engineering standpoint, there are reputed to be problems with CUDA that require the battery be removed to completely kill it. In this case, you must unplug the line cord or CUDA will continue to be powered by the 5V standby supply. It is a good idea to leave the battery out for 10 minutes or so, since it may take that long for all of the stray capacitance to discharge, etc., and put it in a truly uninitialized state.

There is a difference when the line cord is plugged in (5V standby active) in that CUDA runs in so called "high power mode" which involves faster clock rate, more of chip powered up, etc. Though the spec. doesn't say so, this may account for some peculiarities.

When to Reset:
=============
It is rarely necessary to reset the CUDA, but many problems that appear to be hardware related can be fixed by doing so. Some of the most common reasons to reset the CUDA:

*After adding RAM, the machine will not boot.
*The machine will not power up.
*The machine powers up, but does not chime or boot.
*A serial port remains inaccessible even after booting without extensions,
resetting PRAM, etc.
*The machine will power down for no apparent reason.
*Before adding any hardware to the inside of the machine.

Of course, normal troubleshooting steps should be taken to resolve a problem before resetting the CUDA. Although resetting the CUDA will not harm the machine in any way, it is important to remember that most problems can be resolved without opening a machine.

How to Reset:
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*Power down the machine, but leave the power cable plugged in.
*Remove the cover.
*Locate the CUDA Reset button.
*Touch the metal chassis of the machine.  This will dissipate static electricity
into ground.
*Press the CUDA button for a few seconds.
*Reboot, and verify resolution of the problem you wanted to solve

Although it is rarely necessary to do so, the CUDA microcontroller can also be reset by removing the lithium battery and unplugging the machine. If you choose to do this, the machine should be left unplugged with the battery removed for 15 minutes to allow all stray charge to dissipate.

Information on the functions of the CUDA Microcontroller was taken from the following Tech Info Library article:

Article 14841: Power Macintosh: Description of Integrated Circuits

Machines Affected: All