HOWTO Undervolt a Pentium M CPU

From Gentoo Linux Wiki

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Pentium M

[edit] Disclaimer

One part of the kernel patch used in this howto is inspired by a similar patch made by Rickard Holmberg (the voltage limitation part). You can see his patch here: centrino-voltages.diff

Thanks to jlinkels for the pointer to this patch.

[edit] Overview

[edit] The advantages of undervolting

• reduce the power consumption of a laptop
• extend its battery lifetime
• lower temperature of the cpu
• the cpu cooler runs much quieter
• you can emerge applications at full frequency without heat shock

[edit] The disadvantages of undervolting

• it could damage your hardware
• it could run instable (even though your system doesn't crash)

[edit] Methods to undervolt a Pentium-M CPU

There are two ways to undervolt a Pentium-M CPU.

• Hardcoded Method: patch the kernel + change default voltage values of the kernel driver
• Init Script Method: patch the kernel + let a init-script change the voltage values (preferred method since Kernel-2.6.20)

Both methods consist in patching the Linux kernel source code to be able to adjust CPU voltage through a file in the sysfs filesystem.

until linux-phc-0.2.x (until Kernel-2.6.19):

/sys/devices/system/cpu/cpu0/cpufreq/op_points_table

since linux-phc-0.3.x (since Kernel-2.6.20):

/sys/devices/system/cpu/cpu0/cpufreq/phc_controls

For both methods you need to know the undervolted but stable voltage for each frequency of your CPU.

[edit] Kernel versions compatibility

The kernel patch may not work with all kernel versions.

A list of compatible kernel versions is included in the patch release tar ball. Read the file kernels.compatibility.list.txt

[edit] Prepare the Kernel

[edit] Patch the Kernel

To patch your kernel:

• ATTENTION: The latest Version of the Linux PHC patch is now only available from this source New Linux PHC Homepage
• Download the latest version of the Linux PHC patch
• Extract the files of the tarball
• Look in linux-phc-x.y.z/kernel-patch for your kernel version
• Copy the linux-phc-x.y.z-kernel-vanilla-2.6.w.patch to /usr/src/linux
• Patch the kernel

patch -p1 < linux-phc-0.2.9-kernel-vanilla-2.6.19.patch

[edit] Configure the Kernel

-> Power management options (ACPI, APM)
  -> CPU Frequency scaling
    -> CPU Frequency scaling (CPU_FREQ [y]) 

[*] CPU Frequency scaling
[*]   Enable CPUfreq debugging
<*>   CPU frequency translation statistics
[*]     CPU frequency translation statistics details
      Default CPUFreq governor (performance)  --->
---   'performance' governor
<*>   'powersave' governor
<*>   'userspace' governor for userspace frequency scaling
<*>   'ondemand' cpufreq policy governor
<*>   'conservative' cpufreq governor
---   CPUFreq processor drivers
<*>   ACPI Processor P-States driver
< >   AMD Mobile K6-2/K6-3 PowerNow!
< >   AMD Mobile Athlon/Duron PowerNow!
< >   AMD Opteron/Athlon64 PowerNow!
< >   Cyrix MediaGX/NatSemi Geode Suspend Modulation
<*>   Intel Enhanced SpeedStep
[*]     Userspace control of CPU frequency/voltage table
[ ]     Use ACPI tables to decode valid frequency/voltage pairs
[*]     Built-in tables
[*]       Built-in tables for Banias CPUs
[*]       Built-in tables for Dothan CPUs
[*]       Built-in tables for Sonoma CPUs
< >   Intel Speedstep on ICH-M chipsets (ioport interface)
< >   Intel SpeedStep on 440BX/ZX/MX chipsets (SMI interface)
< >   Intel Pentium 4 clock modulation
< >   nVidia nForce2 FSB changing
< >   Transmeta LongRun
< >   VIA Cyrix III Longhaul
---   shared options
[ ]   /proc/acpi/processor/../performance interface (deprecated)

The option

<*>   ACPI Processor P-States driver

is important for cpufreq. Without this you can't load the speedstep-centrino driver and have nothing in

/sys/devices/system/cpu/cpu0/cpufreq/

Not select the option

[ ]     Use ACPI tables to decode valid frequency/voltage pairs

is important for setting the cpu voltage. This is necessary because the driver would load the voltage values from the acpi table. Now you can save your settings.

• Save the changes
• Build the kernel
• Reboot the System

[edit] Hardcoded Method

[edit] Setting the voltages (until Kernel-2.6.19)

After configuring the kernel you have to change the voltages for your cpu in speedstep-centrino.c

vim /usr/src/linux/arch/i386/kernel/cpu/cpufreq/speedstep-centrino.c 

There are three different processor types. Banias, Dothan and Sonoma.

Banias CPUs are the first generation of Pentium-M, with a 1 MB L2 cache and 400 MHz FSB manufactured on 0.13 micron process.

/* Intel Pentium M processor 1.70GHz (Banias) */
static struct cpufreq_frequency_table banias_1700[] =
{
	OP( 600,  956),
	OP( 800, 1004),
	OP(1000, 1116),
	OP(1200, 1228),
	OP(1400, 1308),
	OP(1700, 1484),
	{ .frequency = CPUFREQ_TABLE_END }
};

The first column are the frequencies and the second are the voltages

Dothan CPUs are the second generation of Pentium-M, with a 2 MB L2 cache and 400 MHz FSB manufactured on 90 nm process.

/* Intel Pentium M processor 755 / 2.00GHz (Dothan) */
static struct cpufreq_frequency_table dothan_2000[] =
{
	OP( 600,  988,  988,  988,  988),
	OP( 800, 1052, 1036, 1036, 1036),
	OP(1000, 1100, 1084, 1084, 1084),
	OP(1200, 1148, 1132, 1132, 1116),
	OP(1400, 1196, 1180, 1180, 1164),
	OP(1600, 1244, 1228, 1228, 1196),
	OP(1800, 1292, 1276, 1276, 1244),
	OP(2000, 1340, 1324, 1308, 1276),
	{ .frequency = CPUFREQ_TABLE_END }
};

The first column are the frequencies and the second are the voltages. The other columns are for reference only (not important for us).

Sonoma CPUs are the third generation of Pentium-M, with a 2 MB L2 cache and 533 MHz FSB manufactured on 90 nm process.

/* Intel Pentium M processor 750 / 1.86 GHz (Sonoma) */
static struct cpufreq_frequency_table sonoma_1862[] =
{
	OPEX( 798, 133,  988,  988,  988,  988),
	OPEX(1064, 133, 1084, 1080, 1068, 1056),
	OPEX(1330, 133, 1180, 1172, 1132, 1124),
	OPEX(1596, 133, 1276, 1264, 1196, 1192),
	OPEX(1862, 133, 1356, 1356, 1260, 1260),
	{ .frequency = CPUFREQ_TABLE_END }
};

The first column are the frequencies, the second are the base (not important for us) and the third are the voltages. The other columns are for reference only (not important for us).

• Find your CPU in speedstep-centrino.c
• Look at the Minimal Safe Voltages table at the bottom of this site * Change the voltages of your cpu

/* Intel Pentium M processor 750 / 1.86 GHz (Sonoma) */
static struct cpufreq_frequency_table sonoma_1862[] =
{
	OPEX( 798, 133,  700,  988,  988,  988),
	OPEX(1064, 133,  780, 1080, 1068, 1056),
	OPEX(1330, 133,  876, 1172, 1132, 1124),
 	OPEX(1596, 133,  972, 1264, 1196, 1192),
	OPEX(1862, 133, 1068, 1356, 1260, 1260),
	{ .frequency = CPUFREQ_TABLE_END }
};

• Save the changes
• Build the kernel
• Reboot the System

You can see if the changes take effect with

linux # cat /sys/devices/system/cpu/cpu0/cpufreq/op_points_table
798000:700,1064000:780,1330000:876,1596000:972,1862000:1068

If you get the correct values, you are ready!

[edit] Init Script Method

[edit] Install linux-phc init script (until Kernel-2.6.19)

cp -r linux-phc-0.2.9/init-scripts/gentoo/etc /etc/

[edit] Getting the current voltage settings

The content of the file /sys/devices/system/cpu/cpu0/cpufreq/op_points_table is the list of all the frequency/voltage pairs currently stored in the CPU frequency table

linux # cat /sys/devices/system/cpu/cpu0/cpufreq/op_points_table
798000:988,1064000:1084,1330000:1180,1596000:1276,1862000:1356

[edit] Changing the voltage settings

Be careful when change this values. There are some protections in the code to block you from setting voltage values that are greater than the default settings. But if you set values that are too low, your CPU will freeze and you will have to reboot your computer.

• Go to the table Minimal safe voltages:
• Choose your CPU and take the voltages

linux# vim /etc/conf.d/undervolt

CUSTOM_VTABLE="798000:700,1064000:780,1330000:876,1596000:972,1862000:1068"

SWITCH_BACK="no"
IS_CONFIGURED=yes

Save the /etc/conf.d/undervolt

[edit] Start the init script

linux# /etc/init.d/undervolt start

linux# cat /sys/devices/system/cpu/cpu0/cpufreq/op_points_table
798000:700,1064000:780,1330000:876,1596000:972,1862000:1068

rc-update add undervolt boot

If you get the correct values, you are ready!

[edit] Install linux-phc init script (since Kernel-2.6.20)

cp -r linux-phc-0.3.0/init-scripts/gentoo/etc /etc/

[edit] Getting the current voltage settings

The content of the file /sys/devices/system/cpu/cpu0/cpufreq/phc_controls is the list of all the frequency-ID/voltage-ID pairs currently stored in the CPU frequency table.

linux # cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies
1867000 1600000 1333000 1067000 800000

linux # cat /sys/devices/system/cpu/cpu0/cpufreq/phc_controls
14:41 12:36 10:30 8:24 6:18

So the first frequency (1867000) belongs to the first frequencyID/voltageID pair (14:41) and so on. To calculate the voltageID (VID) from voltage (Vcc) you have to use this formula:

(Pentium M (centrino)) : VID = (Vcc - 700) / 16

[edit] Changing the voltage settings

Be careful when change this values. There are some protections in the code to block you from setting voltage values that are greater than the default settings. But if you set values that are too low, your CPU will freeze and you will have to reboot your computer.

• Go to the table Minimal safe voltages:
• Choose your CPU and take the voltages
• Calculate the VIDs of the voltages for the specific frequencies

So if you take VCC=1068 mV you get VID=23 
23 = (1068 - 700) / 16

linux# vim /etc/conf.d/undervolt

CUSTOM_VTABLE_0_3_X="14:23 12:17 10:11 8:5 6:0"

SWITCH_BACK="no"
IS_CONFIGURED=yes

Save the /etc/conf.d/undervolt

[edit] Start the init script

linux# /etc/init.d/undervolt start

linux# cat /sys/devices/system/cpu/cpu0/cpufreq/phc_controls
14:23 12:17 10:11 8:5 6:0

rc-update add undervolt boot

If you get the correct values, you are ready!

[edit] Trouble Shooting

If you don't get the same thing as what your have writen to the file, have a look at dmesg. If you have enabled cpufreq debug messages in your kernel, you will see errors like the following, that will give you a hint on what you did wrong:

speedstep-centrino: voltage value 0 is out of bounds: 1724 mV
speedstep-centrino: not setting control value 1 to 0c36 because requested voltage is not lower than the default value
speedstep-centrino: failed to parse voltage value 2

It may also be that the voltages have been rounded to a 16 mV boundary.

[edit] Finding missing frequencies

If you miss some frequencies you want to use check the maxium amount of frequencies your cpu support with

linux # cat /proc/cpuinfo |grep stepping
stepping        : 8

Count the amount of your frequencies after patching your kernel.

/* Intel Pentium M processor 750 / 1.86 GHz (Sonoma) */
static struct cpufreq_frequency_table sonoma_1862[] =
{
	OPEX( 798, 133,  700,  988,  988,  988),
	OPEX(1064, 133,  780, 1080, 1068, 1056),
	OPEX(1330, 133,  876, 1172, 1132, 1124),
 	OPEX(1596, 133,  972, 1264, 1196, 1192),
	OPEX(1862, 133, 1068, 1356, 1260, 1260),
	{ .frequency = CPUFREQ_TABLE_END }
};

You can see we count only five frequencies but the cpu supports eight. So copy three lines and change only the important frequency and voltage values (described under Setting the voltages)

To find out the missing frequencies looking at the Mininal Safe Voltage table

/* Intel Pentium M processor 750 / 1.86 GHz (Sonoma) */
static struct cpufreq_frequency_table sonoma_1862[] =
{
	OPEX( 798, 133,  700,  988,  988,  988),
	OPEX(1064, 133,  780, 1080, 1068, 1056),
	OPEX(1197, 133,  828, 1172, 1132, 1124),
	OPEX(1330, 133,  876, 1172, 1132, 1124),
	OPEX(1463, 133,  924, 1264, 1196, 1192),
 	OPEX(1596, 133,  972, 1264, 1196, 1192),
	OPEX(1729, 133, 1020, 1356, 1260, 1260),
	OPEX(1862, 133, 1068, 1356, 1260, 1260),
{ .frequency = CPUFREQ_TABLE_END }
};

[edit] Finding the best voltage settings

[edit] Smallest voltages before the CPU freeze

If you are not afraid to crash your system a few times, you can quickly find the lowest voltages that your CPU can acheive using the following procedure:

1. Set your CPU to the first frequency you want to test using the userspace or conservative governor (don't try this with ondemand governor - it's not working)
2. Run the voltage-ramp-down script (the script is at the end of this page)
3. Wait until your system freeze
4. Write down the voltage value of the current frequency that was displayed just before the last "OK"
5. Reboot
6. If you want to crash your system again set your CPU frequency to the next frequency and go back to step 2

If you are lucky, your CPU will be able to run at it's lowest frequency using 700 mV.

It is strongly recommended to perform this procedure in console mode with the minimum software running (shut down all the services you can).

Also it not recommended to do this if you do not have a journaled file system. And even with a journeled file system there are still chances that your file system can get corrupted if you have applications writing to the disk.

[edit] Safe voltages

I´m not sure if the method illustrated above gives you safe voltages. I myself used another way to determine the voltages and I think they are safer, than using the lowest ones before crashing your system. This method is also not 100% crash prone, while testing, but I think the voltages you get afterwards are safer.

For using this method under Linux, you will need the patch and mprime. Under Gentoo, just do emerge gimps:

emerge gimps

When running gimps, note that it requires a working directory to be set explicitly. You can perform the stress test directly by using

/opt/gimps/mprime -t -w/tmp/

Besides Windows you need RMClock [1] and Prime95 [2]. After installing both of the programs run RMClock and choose AC Power Profile "Power Saving" (this tutorial assumes you´re running on AC Power, so plug your laptop in!) then switch to the profile Power Saving. Check "Use P-State Transisitons" right click the box and Add a state. Choose the multiplier you want to test (eventually you will want to test all of them so choose the lowest and work your way up) and set the voltage to the standard voltage of your CPU. Click apply and see under the General tab if your multiplier is used. If it is start prime95 and start a torture test (maximum power consumption). Now switch back to RMClock and see if your multiplier is still unchanged. If your CPU clocked to a higher frequency because of the load something went wrong - you only define one P-State your CPU has to reside in.

Now on to the funny part. From your standard voltage undervolt your cpu one step and click apply - check in the General tab if everything went right. Look into prime if it says it got a rounding error. If it does immediately raise the voltage again (don´t forget apply) as you´re risking a bluescreen here. If nothing went wrong and Prime didn´t complain even after running for a minute with the lower voltage, lower the voltage another step (APPLY!) - repeat this until prime complains. If it does, raise the voltage another step and let prime at least run for another hour or longer after restarting the torture test again (the longer the better).

My laptop threw a rounding error on one multiplier even after a whole hour of testing, that´s why I think the voltages determined with the above script are not safe. Rounding errors are a very bad thing, even worse than crashing your machine, because you won´t know how they affect your machine. Also think about power spikes in your power supply. A stable voltage now doesn´t mean a stable voltage all the time in the future.

After you´re done testing, write down the multiplier/voltage combo, remove the P-State and start again with a new multiplier. This method is really time consuming, I know, but for me it was entirely worth it.

Btw.: Removing the heatpad from your cooler and replacing it with some cooling paste works wonders for temperature also. My dell had a really thick heatpad. Be careful while removing your cooler though, as Pentium-M cpus don´t have a heatspreader and are easily crushed by the cooler - not enough pressure on the cpu by the cooler is bad for your temps though, so be sure you know what you´re doing.

[edit] Minimal safe voltages

The tables below shows the voltage values that are considered safe by some people that have undervolted their CPU The samples are presented in 4 different tables:

• Pentium M with with FSB at 400 MHz
• Pentium M Low Voltage with with FSB at 400 MHz
• Pentium M Ultra Low Voltage with with FSB at 400 MHz
• Pentium M with with FSB at 533 MHz

Pentium M with with FSB at 400 MHz:

All voltages are in mV

Pentium M Low Voltage with with FSB at 400 MHz:

All voltages are in mV

Pentium M Ultra Low Voltage with with FSB at 400 MHz:

All voltages are in mV

Pentium M with with FSB at 533 MHz:

All voltages are in mV

* this is a Pentium M 735 that has been overclocked by increasing the FSB from 100Mhz to 133Mhz; it runs mprime stable at 38°C on a MSI 915GM Speedster mainboard. Felix: Same applies but in a dell 630m laptop. Temperature around 42°C. on idle (up to 65°C. on load).

[edit] Some handy scripts

As lowering the voltage is likely to crash the system it is safer to perform the tests in console mode to minimize the risks of file system corruption. (Using a journalized file system is also a good idea)

To perform tests in console mode you may want to use some scripts to help you change the voltages and monitor the CPU temperature and the current drawned from the battery. This chater contains some very basic sample scripts you may want to use

[edit] Measuring temperature and current

This script can be used to measure the CPU temperature and the current drawned from the battery in the command line interface. It needs to run quite a long time to have an accurate average of the temperature if there are big variations. And if the temperature does not vary at all the computed average will be no more accurate than the precision of the output of cat /proc/acpi/thermal_zone/THRM/temperature, wich usually is 1 °C.

It produces one line like the followings every 10 seconds:

2005/10/21 01:44:52  T = 39.0/40.0 °C  Avg(T) = 38.17681 °C    I = 946/952 mA  Avg(I) = 931.947 mA    F = 798000 MHz
2005/10/21 01:45:02  T = 39.0/39.0 °C  Avg(T) = 38.18414 °C    I = 946/949 mA  Avg(I) = 937.763 mA    F = 798000 MHz
2005/10/21 01:45:12  T = 39.0/39.0 °C  Avg(T) = 38.19142 °C    I = 946/949 mA  Avg(I) = 941.289 mA    F = 798000 MHz
2005/10/21 01:45:22  T = 38.0/39.0 °C  Avg(T) = 38.19562 °C    I = 946/948 mA  Avg(I) = 943.413 mA    F = 798000 MHz

#!/bin/bash
# Display CPU frequency, temperature and current drawn from the battery
#in (almost) real time

TAlpha=0.99900 # Coeficient used to compute the temperature average
               # Keep it in the ]0..1[ range.
               # Increase it to have a more smooth temperature average
               # And keep the trailing 0 it is important

IAlpha=0.950   # Same as above but for the current

SamplingPeriod=1
DisplayPeriod=10
TempUnit="°C"

echo "Sampling period: $SamplingPeriod s"
echo "Temperature average coefficient: $TAlpha"
echo "Current average coefficient:     $IAlpha"

# Variables initialization
TAvg=`acpi -t -B | cut --delimiter=" " --fields=9`
TMin=$TAvg
TMax=$TAvg
IAvg=`grep 'present rate:' /proc/acpi/battery/BAT1/state | cut  --delimiter=" " --fields=14`
IMin=$IAvg
IMax=$IAvg
TBeta=`echo "1-$TAlpha" | bc`
IBeta=`echo "1-$IAlpha" | bc`
LastDisplay=1

# Main loop
while [ 0 ]
do
  # Get temperature
  T=`acpi -t -B | cut --delimiter=" " --fields=9`
  # Compute temperature moving aveage
  TAvg=`echo "$TAlpha * $TAvg + $TBeta * $T" | bc`
  # Get current
  I=`grep 'present rate:' /proc/acpi/battery/BAT1/state | cut  --delimiter=" " --fields=14`
  # Compute current moving average
  IAvg=`echo "$IAlpha * $IAvg + $IBeta * $I" | bc`
  # Comute min/max values of current display period
  TMax=`echo "a=$T; b=$TMax; if (a > b) a else b" | bc`
  TMin=`echo "a=$T; b=$TMin; if (a < b) a else b" | bc`
  IMax=`echo "a=$I; b=$IMax; if (a > b) a else b" | bc`
  IMin=`echo "a=$I; b=$IMin; if (a < b) a else b" | bc`
  # Get CPU frequency
  Freq=`cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq`
  # Display data
  if [ $LastDisplay -eq 1 ]; then
    Timestamp=`date "+%Y/%m/%d %H:%M:%S"`
    echo "$Timestamp  T = $TMin/$TMax $TempUnit  Avg(T) = $TAvg $TempUnit    I = $IMin/$IMax mA  Avg(I) = $IAvg mA    F = $Freq MHz"
    TMax=$T
    TMin=$T
    IMin=$I
    IMax=$I
  fi
  # Wait until next sampling period
  sleep $SamplingPeriod
  LastDisplay=$(($LastDisplay+$SamplingPeriod))
  if [ $DisplayPeriod -le $LastDisplay ]; then
    LastDisplay=1
  fi
done

#!/bin/bash

# Min voltage value
# Voltages will not be set below than this value
Vmin=700

# Frequence settings
Fall=`cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies`

# run the CPU this many seconds at the given MHz to check if OK
Period=3

echo "Avail. Frequencies: $Fall"

while [ 0 ]; do

	# Get current votages
	CurVoltages=`cat /sys/devices/system/cpu/cpu0/cpufreq/voltage_table | cut --delimiter="," --output-delimiter=" " --fields=1-`
	NewVoltages=""

	# Compute new voltages as current - 16 mV
	for V in $CurVoltages; do
		V=$(($V - 16))
                
                # Ensure that voltage is not below min value
                if [ $V -lt $Vmin ]; then
			V=$Vmin
		fi
    
		if [ $NewVoltages ]; then
			NewVoltages="$NewVoltages,$V"
		else
			NewVoltages="$V"
		fi
	done

	# Display current settings from the sysfs file
        echo " "
        echo "Current settings:   "`cat /sys/devices/system/cpu/cpu0/cpufreq/voltage_table`
        
        # Display the new settings that we are going to write to the sysfs file
        echo "Requested settings: $NewVoltages"
        
        # Force the kernel to write its buffers to the hard disk
        # to reduce the risks of file system corruption in case of CPU freeze
        sync
        
        # Apply new settings
        echo "$NewVoltages" > /sys/devices/system/cpu/cpu0/cpufreq/voltage_table
        
        # now check if all voltages are OK and loop over all frequencies
	for vv in $Fall; do
		# set CPU speed
		#echo "Setting $Fcur"
		echo $vv > /sys/devices/system/cpu/cpu0/cpufreq/scaling_setspeed
		
		# sleep to see if cpu groks
		sleep $Period

		# load
		i=0
		while true; do
			i=$((i+1))
			if [ $i -gt 5000 ]; then
				break
			fi
		done
                
		echo "OK " `cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq`
        done
done

# Path to the voltage table sysfs interface file
VTABLE_PATH="/sys/devices/system/cpu/cpu0/cpufreq/op_points_table"

# Switch back to DEFAULT_VTABLE if undervoltage is stopped? [yes/no]
SWITCH_BACK="no"

# Default voltages that will be restored at shutdown if SWITCH_BACK=yes
DEFAULT_VTABLE="798000:988,1064000:1116,1330000:1244,1596000:1356"

# Custom voltages that will be applied at boot time
CUSTOM_VTABLE="798000:700,1064000:764,1333000:860,1596000:956"

# Set the following to "yes" when the all the settings are configured
# This is a safety to disable setting the voltages with the default
# values of this file that are probably wrong for your CPU
IS_CONFIGURED="no"

#Choose the governor you want to use when laptop is running on ac power
#Set the frequency:millivoltage settings to undervolt your Pentium-M in the last line
#  of the off-line case, default is what I use for my P-M 760
#(performance or ondemand recommended)
#This file created by Ryan Neufeld August 2005
#Undervolt portion added by Nick Andrade May 2006
ONAC_GOVERNOR="performance"
BATT_GOVERNOR="conservative"
range=`cpufreq-info | grep 'Hz -'`
minmhz=`echo $range | gawk '{print $3 $4}'`
maxmhz=`echo $range | gawk '{print $6 $7}'`
acstate=`grep state /proc/acpi/ac_adapter/AC/state | gawk '{print $2}'`;
case "$acstate" in
        on-line)
                #Do stuff when the ac adapter is plugged in
                cpufreq-set -g ${ONAC_GOVERNOR} -u ${maxmhz}
                logger "AC Adapter is plugged in. Setting CPU governor to ${ONAC_GOVERNOR}";
                #get the current speed in KHz
                speed=`cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq`;
                #throw it to bc and get MHz
                mhz=$(echo "scale=0; ${speed}/1000" | bc -l);
                logger "Current cpu speed is $mhz MHz";

                #set default voltage
                echo "2000000:1356,1600000:1244,1333000:1164,1067000:1084,800000:988" > /sys/devices/system/cpu/cpu0/cpufreq/op_points_table
                ;;
        off-line)
                #Do studd when the ac adapter is un-plugged
                cpufreq-set -g ${BATT_GOVERNOR} -d ${minmhz} -u ${maxmhz}
                logger "AC Adapter is un-plugged. Setting CPU governor to ${BATT_GOVERNOR}";
                #get the current speed in KHz
                speed=`cat /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_cur_freq`;
                #throw it to bc and get MHz
                mhz=$(echo "scale=0; ${speed}/1000" | bc -l);
                logger "Current cpu speed is $mhz MHz";

                #undervolt the cpu
                echo "2000000:1164,1600000:956,1330000:876,1067000:796,800000:700" > /sys/devices/system/cpu/cpu0/cpufreq/op_points_table
                ;;
esac