CPUClock

Important: CPUClock is only of use on hardware that is capable of switching the CPU between two speed settings. This means hardware such as the OMAP3 BeagleBoard and the OMAP4 PandaBoard ES.

Background

The following description uses extracts from a post by Jeffrey Lee in the ROOL forum (with permission).

RISC OS does several things to control the speed of the CPU:

• There is the CPU clock driver sitting in the HAL. This allows the Portable module to change the clock speed between a number of preset settings (referred to as "OPP"s in TI’s docs). Each OPP defines a clock speed and core voltage. Although the hardware is capable of using arbitrary clock speeds and voltages, TI’s docs suggest that sticking to the standard OPPs is the best in terms of reducing heat/power drain. On BB-xM the clock driver also makes use of SmartReflex, which allows the hardware to fine-tune the core voltage in real time, typically resulting in a lower voltage being used than that specified by the OPP.
• In terms of how this all hooks into RISC OS, Portable_Speed, which is the original speed control SWI that was used by the A4, is used to select between preselected “fast” and “slow” speeds. Portable_Speed2 is a new SWI which allows the available speeds to be enumerated, and selection of which speed is the fast one and which is the slow one. By default the fastest speed (1GHz for xM, 620/700MHz for BB) is chosen as the “fast” one and the slowest speed (300MHz for xM, 125MHz? for BB) is chosen as the “slow” one. The main user of Portable_Speed is the Wimp, which will switch the CPU to the slow speed if it thinks the machine is idle.
• The Portable module also provides wait-for-idle functionality (Portable_Idle), which allows the CPU to be put to sleep until the next interrupt. Portable_Idle is a new SWI that was added for the Stork, and due to the cancellation of the project it probably isn’t being used in all the places that it could be. However the Wimp does know how to use it, as well as a couple of other places (e.g. OS_Byte 19 & OS_ReadC), so if you’re sat idling in the desktop the machine should spend most of its time with the CPU halted.
• If you want to force the CPU to run at a particular speed, the recommended way is to use Portable_Speed2 to set the slow & fast speeds to the desired values. There is nothing in the OS that will override these settings.

Installation

For a new installation, simply drag and drop the CPUClock application into the directory of your choice.

If you have a previous copy of CPUClock you have two choices.

• Move the old version of CPUClock somewhere safe and replace it with the new version
• Simply copy the new version over the old version. Set your filer options to Force and Newer.

Basic operation

To run the application, double click on it's icon in a filer window. Clicking on the iconbar icon with Select will open the CPU status window.

The Iconbar Menu

A screen shot of the iconbar menu is shown right.

Info  –  A standard application info box, including a button to go to the web site to check for later versions.

Help...  –  Runs the help file (this one).

Status...  –  Opens the CPU clock speed status window (see below). Clicking on the iconbar icon with Select will also open the CPU status window.

Quit  –  This terminates the application immediately.

The CPU clock speed status window

The CPU clock speed can be set to a 'fast' and a 'slow' speed. The OS can switch between the two speeds depending upon how hard the CPU is working. The three display fields show information on the CPU clock speed. The top field shows the current setting for the 'fast' speed. The middle field shows the setting for the 'slow' speed. The bottom field shows the current clock speed of the cpu. This will switch between the 'fast' and 'slow' setting as the work load of the cpu changes. By default the fast and slow settings will be the maximum and minimum speeds at which the cpu can be clocked.

Note that CPUClock updates the current speed value every 1 second using wimp NULL poll events. If the wimp is not multitasking at that instant, e.g. processing an image, then the display cannot be updated. However, in general use you should see the speed being switched between fast and slow as you do things in the desktop.

The harder the CPU works, the warmer it will get. It might be useful at certain times of high ambient temperature to limit the speed of the CPU to keep its temperature down.

Clicking on the popup menu button to the right of the 'Fast' or 'Slow' fields will raise a menu listing the allowable CPU clock speeds. Selecting one of these speeds will reset the 'Fast' or 'Slow' setting as appropriate to this new value.

Reducing the 'Fast' speed may be beneficial in high ambient temperature conditions. It is unlikely there will be any benefit in raising the 'Slow' speed, since this is used when the processor is idling.

Hardware - CPUClock runs without problem on the BeagleBoard and on the PandaBoard ES using more recent ROM versions.

For anyone who is interested I have an alternative version of CPUClock available. This is a Boot-Configuration tool version. Once installed it will appear in Boot-Configuration (double click !Boot in the root directory). It allows the cpu speed to be set, and the setting will be retained over a reboot or reset. This version does not give a continuous display of cpu speed, but is intended only to set the cpu speed - useful in hot weather to keep the cpu temperature down. Contact the author direct to obtain this version.

Contact

All communication about CPUClock should be directed to Chris Johnson (Email:chris@chris-johnson.org.uk). Suggestions for new features are always welcome.

Web site

Chris Johnson's web site is at http://www.chris-johnson.org.uk/index.html, with his RISC OS software at http://www.chris-johnson.org.uk/software/index.html.

Screenshots

All the screen shots in these pages were obtained using Snapper, a versatile screen capture application.

Snapper is available from http://www.chris-johnson.org.uk/software/snap.html

Licence

Copyright (c) 2012, Chris Johnson
All rights reserved.

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

• Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.
• Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.