As the raspberry PI doesn't have a conventional BIOS, the various system configuration parameters that would normally be kept and set using the BIOS are now stored on a text file named "config.txt". The Raspberry Pi config.txt file is read by the GPU before the ARM core is initialised.
This file is an optional file on the boot partition. It would normally be accessible as /boot/config.txt from Linux, but from Windows (or OS X) it would be seen as a file in the accessible part of the card.
To edit the configuration file, see the instructions at R-Pi_ConfigurationFile.
You can get your current active settings with the following commands:
vcgencmd get_config <config> - lists a specific config value. E.g. vcgencmd get_config arm_freq vcgencmd get_config int - lists all the integer config options that are set (non-zero) vcgencmd get_config str - lists all the string config options that are set (non-null)
- 1 File format
- 2 Memory
- 3 Video
- 4 Licensed Codecs
- 5 Boot
- 6 Overclocking
- 7 References
The format is "property=value" where value is an integer. You may specify only one option per line. Comments may be added by starting a line with the '#' character.
Note: In the newer Raspberry Pi models there is # before every line, if you want changes to have an affect then 'uncomment' meaning remove the #.
Here is an example file
# Set stdv mode to PAL (as used in Europe) sdtv_mode=2 # Force the monitor to HDMI mode so that sound will be sent over HDMI cable hdmi_drive=2 # Set monitor mode to DMT hdmi_group=2 # Set monitor resolution to 1024x768 XGA 60Hz (HDMI_DMT_XGA_60) hdmi_mode=16 # Make display smaller to stop text spilling off the screen overscan_left=20 overscan_right=12 overscan_top=10 overscan_bottom=10
And here is another example file, containing extended documentation of features.
disable_l2cache disable ARM access to GPU's L2 cache. Needs corresponding L2 disabled kernel. Default 0
gpu_mem GPU memory in megabyte. Sets the memory split between the ARM and GPU. ARM gets the remaining memory. Min 16. Default 64
gpu_mem_256 GPU memory in megabyte for the 256MB Raspberry Pi. Ignored by the 512MB RP. Overrides gpu_mem. Max 192. Default not set
gpu_mem_512 GPU memory in megabyte for the 512MB Raspberry Pi. Ignored by the 256MB RP. Overrides gpu_mem. Max 448. Default not set
disable_pvt Disable adjusting the refresh rate of RAM every 500ms (measuring RAM temparature).
CMA - Dynamic Memory Split
The firmware and kernel as of 19. November 2012 supports CMA, which means the memory split between ARM and GPU is managed dynamically at runtime. You can find an example config.txt here.
cma_lwm When GPU has less than cma_lwm (low water mark) memory available it will request some from ARM.
cma_hwm When GPU has more than cma_hwm (high water mark) memory available it will release some to ARM.
The following options need to be in cmdline.txt for CMA to work:
Video mode options
sdtv_mode defines the TV standard for composite output (default=0)
sdtv_mode=0 Normal NTSC sdtv_mode=1 Japanese version of NTSC – no pedestal sdtv_mode=2 Normal PAL sdtv_mode=3 Brazilian version of PAL – 525/60 rather than 625/50, different subcarrier
sdtv_aspect defines the aspect ratio for composite output (default=1)
sdtv_aspect=1 4:3 sdtv_aspect=2 14:9 sdtv_aspect=3 16:9
sdtv_disable_colourburst disables colour burst on composite output. The picture will be monochrome, but possibly sharper
sdtv_disable_colourburst=1 colour burst is disabled
hdmi_safe Use "safe mode" settings to try to boot with maximum hdmi compatibility. This is the same as the combination of: hdmi_force_hotplug=1, config_hdmi_boost=4, hdmi_group=2, hdmi_mode=4, disable_overscan=0
hdmi_ignore_edid Enables the ignoring of EDID/display data if your display is a crappy Chinese one
hdmi_edid_file when set to 1, will read the edid data from the edid.dat file instead of from the monitor.
hdmi_force_edid_audio Pretends all audio formats are supported by display, allowing passthrough of DTS/AC3 even when not reported as supported.
avoid_edid_fuzzy_match Avoid fuzzy matching of modes described in edid. Picks the standard mode with matching resolution and closest framerate even if blanking is wrong.
hdmi_ignore_cec_init Doesn't sent initial active source message. Avoids bringing (CEC enabled) TV out of standby and channel switch when rebooting.
hdmi_ignore_cec Pretends CEC is not supported at all by TV. No CEC functions will be supported.
hdmi_force_hotplug Pretends HDMI hotplug signal is asserted so it appears a HDMI display is attached
hdmi_force_hotplug=1 Use HDMI mode even if no HDMI monitor is detected
hdmi_ignore_hotplug Pretends HDMI hotplug signal is not asserted so it appears a HDMI display is not attached
hdmi_ignore_hotplug=1 Use composite mode even if HDMI monitor is detected
hdmi_pixel_encoding Force the pixel encoding mode. By default it will use the mode requested from edid so shouldn't need changing.
hdmi_pixel_encoding=0 default (limited for CEA, full for DMT) hdmi_pixel_encoding=1 RGB limited (16-235) hdmi_pixel_encoding=2 RGB full ( 0-255) hdmi_pixel_encoding=3 YCbCr limited (16-235) hdmi_pixel_encoding=4 YCbCr limited ( 0-255)
hdmi_drive chooses between HDMI and DVI modes
hdmi_drive=1 Normal DVI mode (No sound) hdmi_drive=2 Normal HDMI mode (Sound will be sent if supported and enabled)
hdmi_group defines the HDMI type
Not specifying the group, or setting to 0 will use the preferred group reported by the edid.
hdmi_group=1 CEA hdmi_group=2 DMT
hdmi_mode defines screen resolution in CEA or DMT format
These values are valid if hdmi_group=1 (CEA) hdmi_mode=1 VGA hdmi_mode=2 480p 60Hz hdmi_mode=3 480p 60Hz H hdmi_mode=4 720p 60Hz hdmi_mode=5 1080i 60Hz hdmi_mode=6 480i 60Hz hdmi_mode=7 480i 60Hz H hdmi_mode=8 240p 60Hz hdmi_mode=9 240p 60Hz H hdmi_mode=10 480i 60Hz 4x hdmi_mode=11 480i 60Hz 4x H hdmi_mode=12 240p 60Hz 4x hdmi_mode=13 240p 60Hz 4x H hdmi_mode=14 480p 60Hz 2x hdmi_mode=15 480p 60Hz 2x H hdmi_mode=16 1080p 60Hz hdmi_mode=17 576p 50Hz hdmi_mode=18 576p 50Hz H hdmi_mode=19 720p 50Hz hdmi_mode=20 1080i 50Hz hdmi_mode=21 576i 50Hz hdmi_mode=22 576i 50Hz H hdmi_mode=23 288p 50Hz hdmi_mode=24 288p 50Hz H hdmi_mode=25 576i 50Hz 4x hdmi_mode=26 576i 50Hz 4x H hdmi_mode=27 288p 50Hz 4x hdmi_mode=28 288p 50Hz 4x H hdmi_mode=29 576p 50Hz 2x hdmi_mode=30 576p 50Hz 2x H hdmi_mode=31 1080p 50Hz hdmi_mode=32 1080p 24Hz hdmi_mode=33 1080p 25Hz hdmi_mode=34 1080p 30Hz hdmi_mode=35 480p 60Hz 4x hdmi_mode=36 480p 60Hz 4xH hdmi_mode=37 576p 50Hz 4x hdmi_mode=38 576p 50Hz 4x H hdmi_mode=39 1080i 50Hz reduced blanking hdmi_mode=40 1080i 100Hz hdmi_mode=41 720p 100Hz hdmi_mode=42 576p 100Hz hdmi_mode=43 576p 100Hz H hdmi_mode=44 576i 100Hz hdmi_mode=45 576i 100Hz H hdmi_mode=46 1080i 120Hz hdmi_mode=47 720p 120Hz hdmi_mode=48 480p 120Hz hdmi_mode=49 480p 120Hz H hdmi_mode=50 480i 120Hz hdmi_mode=51 480i 120Hz H hdmi_mode=52 576p 200Hz hdmi_mode=53 576p 200Hz H hdmi_mode=54 576i 200Hz hdmi_mode=55 576i 200Hz H hdmi_mode=56 480p 240Hz hdmi_mode=57 480p 240Hz H hdmi_mode=58 480i 240Hz hdmi_mode=59 480i 240Hz H H means 16:9 variant (of a normally 4:3 mode). 2x means pixel doubled (i.e. higher clock rate, with each pixel repeated twice) 4x means pixel quadrupled (i.e. higher clock rate, with each pixel repeated four times)
These values are valid if hdmi_group=2 (DMT) Note: according to http://www.raspberrypi.org/phpBB3/viewtopic.php?f=26&t=20155&p=195417&hilit=2560x1600#p195443 there is a pixel clock limit which means the highest supported mode is 1920x1200 @60Hz with reduced blanking. hdmi_mode=1 640x350 85Hz hdmi_mode=2 640x400 85Hz hdmi_mode=3 720x400 85Hz hdmi_mode=4 640x480 60Hz hdmi_mode=5 640x480 72Hz hdmi_mode=6 640x480 75Hz hdmi_mode=7 640x480 85Hz hdmi_mode=8 800x600 56Hz hdmi_mode=9 800x600 60Hz hdmi_mode=10 800x600 72Hz hdmi_mode=11 800x600 75Hz hdmi_mode=12 800x600 85Hz hdmi_mode=13 800x600 120Hz hdmi_mode=14 848x480 60Hz hdmi_mode=15 1024x768 43Hz DO NOT USE hdmi_mode=16 1024x768 60Hz hdmi_mode=17 1024x768 70Hz hdmi_mode=18 1024x768 75Hz hdmi_mode=19 1024x768 85Hz hdmi_mode=20 1024x768 120Hz hdmi_mode=21 1152x864 75Hz hdmi_mode=22 1280x768 reduced blanking hdmi_mode=23 1280x768 60Hz hdmi_mode=24 1280x768 75Hz hdmi_mode=25 1280x768 85Hz hdmi_mode=26 1280x768 120Hz reduced blanking hdmi_mode=27 1280x800 reduced blanking hdmi_mode=28 1280x800 60Hz hdmi_mode=29 1280x800 75Hz hdmi_mode=30 1280x800 85Hz hdmi_mode=31 1280x800 120Hz reduced blanking hdmi_mode=32 1280x960 60Hz hdmi_mode=33 1280x960 85Hz hdmi_mode=34 1280x960 120Hz reduced blanking hdmi_mode=35 1280x1024 60Hz hdmi_mode=36 1280x1024 75Hz hdmi_mode=37 1280x1024 85Hz hdmi_mode=38 1280x1024 120Hz reduced blanking hdmi_mode=39 1360x768 60Hz hdmi_mode=40 1360x768 120Hz reduced blanking hdmi_mode=41 1400x1050 reduced blanking hdmi_mode=42 1400x1050 60Hz hdmi_mode=43 1400x1050 75Hz hdmi_mode=44 1400x1050 85Hz hdmi_mode=45 1400x1050 120Hz reduced blanking hdmi_mode=46 1440x900 reduced blanking hdmi_mode=47 1440x900 60Hz hdmi_mode=48 1440x900 75Hz hdmi_mode=49 1440x900 85Hz hdmi_mode=50 1440x900 120Hz reduced blanking hdmi_mode=51 1600x1200 60Hz hdmi_mode=52 1600x1200 65Hz hdmi_mode=53 1600x1200 70Hz hdmi_mode=54 1600x1200 75Hz hdmi_mode=55 1600x1200 85Hz hdmi_mode=56 1600x1200 120Hz reduced blanking hdmi_mode=57 1680x1050 reduced blanking hdmi_mode=58 1680x1050 60Hz hdmi_mode=59 1680x1050 75Hz hdmi_mode=60 1680x1050 85Hz hdmi_mode=61 1680x1050 120Hz reduced blanking hdmi_mode=62 1792x1344 60Hz hdmi_mode=63 1792x1344 75Hz hdmi_mode=64 1792x1344 120Hz reduced blanking hdmi_mode=65 1856x1392 60Hz hdmi_mode=66 1856x1392 75Hz hdmi_mode=67 1856x1392 120Hz reduced blanking hdmi_mode=68 1920x1200 reduced blanking hdmi_mode=69 1920x1200 60Hz hdmi_mode=70 1920x1200 75Hz hdmi_mode=71 1920x1200 85Hz hdmi_mode=72 1920x1200 120Hz reduced blanking hdmi_mode=73 1920x1440 60Hz hdmi_mode=74 1920x1440 75Hz hdmi_mode=75 1920x1440 120Hz reduced blanking hdmi_mode=76 2560x1600 reduced blanking hdmi_mode=77 2560x1600 60Hz hdmi_mode=78 2560x1600 75Hz hdmi_mode=79 2560x1600 85Hz hdmi_mode=80 2560x1600 120Hz reduced blanking hdmi_mode=81 1366x768 60Hz hdmi_mode=82 1080p 60Hz hdmi_mode=83 1600x900 reduced blanking hdmi_mode=84 2048x1152 reduced blanking hdmi_mode=85 720p 60Hz hdmi_mode=86 1366x768 reduced blanking
overscan_left number of pixels to skip on left
overscan_right number of pixels to skip on right
overscan_top number of pixels to skip on top
overscan_bottom number of pixels to skip on bottom
framebuffer_width console framebuffer width in pixels. Default is display width minus overscan.
framebuffer_height console framebuffer height in pixels. Default is display height minus overscan.
framebuffer_depth console framebuffer depth in bits per pixel. Default is 16. 8bit is valid, but default RGB palette makes an unreadable screen. 24bit looks better but has corruption issues as of 20120615. 32bit has no corruption issues but needs framebuffer_ignore_alpha=1 and shows the wrong colors as of 20120615.
framebuffer_ignore_alpha set to 1 to disable alpha channel. Helps with 32bit.
test_mode enable test sound/image during boot for manufacturing test.
disable_overscan set to 1 to disable overscan.
config_hdmi_boost configure the signal strength of the HDMI interface. Default is 0. Try 4 if you have interference issues with hdmi. 7 is the maximum.
display_rotate rotates the display clockwise on the screen (default=0) or flips the display.
display_rotate=0 Normal display_rotate=1 90 degrees display_rotate=2 180 degrees display_rotate=3 270 degrees display_rotate=0x10000 horizontal flip display_rotate=0x20000 vertical flip
Note: the 90 and 270 degrees rotation options require additional memory on GPU, so won't work with the 16M GPU split. Probably the reason for:
- Crashes my RPI before Linux boots if set to "1" -- REW 20120913.
Which values are valid for my monitor?
Your HDMI monitor may support only a limited set of formats. To find out which formats are supported, use the following method.
- Set the output format to VGA 60Hz (hdmi_group=1 hdmi_mode=1) and boot up the Raspberry Pi
- Enter the following command to give a list of CEA supported modes
/opt/vc/bin/tvservice -m CEA
- Enter the following command to give a list of DMT supported modes
/opt/vc/bin/tvservice -m DMT
- Enter the following command to show your current state
- Enter the following commands to dump more detailed information from your monitor
/opt/vc/bin/tvservice -d edid.dat /opt/vc/bin/edidparser edid.dat
The edid.dat should also be provided when troubleshooting problems with the default HDMI mode
Hardware decoding of additional codecs can be enabled by purchasing a license that is locked to the CPU serial number of your Raspberry Pi.
decode_MPG2 License key to allow hardware MPEG-2 decoding.
decode_WVC1 License key to allow hardware VC-1 decoding.
License setup for SD-card sharing between multiple Pis. Maximum of 8 licenses at once.
disable_commandline_tags stop start.elf from filling in ATAGS (memory from 0x100) before launching kernel
cmdline (string) command line parameters. Can be used instead of cmdline.txt file
kernel (string) alternative name to use when loading kernel. Default "kernel.img"
kernel_address address to load kernel.img file at
kernel_old (bool) if 1, load kernel at 0x0
ramfsfile (string) ramfs file to load
ramfsaddr address to load ramfs file at
initramfs (string address) ramfs file and adress to load it at (it's like ramfsfile+ramfsaddr in one option). NOTE: this option uses different syntax than all other options - you should not use "=" character here. Example:
initramfs initramf.gz 0x00800000
device_tree_address address to load device_tree at
init_uart_baud initial uart baud rate. Default 115200
init_uart_clock initial uart clock. Default 3000000 (3Mhz)
init_emmc_clock initial emmc clock. Default 100000000 (100MHz)
boot_delay wait for given number of seconds in start.elf before loading kernel. delay = 1000 * boot_delay + boot_delay_ms. Default 1
boot_delay_ms wait for given number of milliseconds in start.elf before loading kernel. Default 0
avoid_safe_mode if set to 1, safe_mode boot won't be enabled. Default 0
NOTE: Setting any of the parameters which overvolt your Raspberry Pi will set a permanent bit within the SOC, making it posibly to detect that you Pi has been overclocked. This was meant to void warranty if the device has been overclocked. Since 19th of September 2012 you can overclock your Pi without affecting your warranty
The latest kernel has a cpufreq kernel driver with the "ondemand" governor enabled by default. It has no effect if you have no overclock settings. But when you do, the arm frequency will vary with processor load. Non default values are only used when needed according to the used governor. You can adjust the minimum values with the *_min config options or disable dynamic clocking with force_turbo=1. 
Overclock and overvoltage will be disabled at runtime when the SoC reaches 85°C to cool it down . You should not hit the limit, even with maximum settings at 25°C ambient temperature. 
|arm_freq||Frequency of ARM in MHz. Default 700|
|gpu_freq||Sets core_freq, h264_freq, isp_freq, v3d_freq together. Default 250|
|core_freq||Frequency of GPU processor core in MHz. It has an impact on ARM performance since it drives L2 cache. Default 250|
|h264_freq||Frequency of hardware video block in MHz. Default 250|
|isp_freq||Frequency of image sensor pipeline block in MHz. Default 250|
|v3d_freq||Frequency of 3D block in MHz. Default 250|
|avoid_pwm_pll||Unlink core_freq from the rest of the gpu. Can cause low quality analog audio, which should be fixed with latest firmware. Default 0|
|sdram_freq||Frequency of SDRAM in MHz. Default 400|
|over_voltage||ARM/GPU core voltage adjust. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. force_turbo will allow values higher than 6. Default 0 (1.2V) |
|over_voltage_sdram||Sets over_voltage_sdram_c, over_voltage_sdram_i, over_voltage_sdram_p together|
|over_voltage_sdram_c||SDRAM controller voltage adjust. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. Default 0 (1.2V) |
|over_voltage_sdram_i||SDRAM I/O voltage adjust. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. Default 0 (1.2V)|
|over_voltage_sdram_p||SDRAM phy voltage adjust. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. Default 0 (1.2V)|
|force_turbo||Disables dynamic cpufreq driver and minimum settings below. Enables h264/v3d/isp overclock options. Default 0|
|initial_turbo||Enables turbo mode from boot for the given value in seconds (up to 60) or until cpufreq sets a frequency. Can help with sdcard corruption if overclocked. Default 0 |
|arm_freq_min||Minimum value of arm_freq used for dynamic clocking. Default 700|
|core_freq_min||Minimum value of core_freq used for dynamic clocking. Default 250|
|sdram_freq_min||Minimum value of sdram_freq used for dynamic clocking. Default 400|
|over_voltage_min||Minimum value of over_voltage used for dynamic clocking. Default 0|
|temp_limit||Overheat protection. Sets clocks and voltages to default when the SoC reaches this Celsius value. Setting this higher than default voids warranty. Default 85|
|current_limit_override||Disables SMPS current limit protection when set to "0x5A000020". Can help if you are currently hitting a reboot failure when overclocking too high. |
enables dynamic clocks and voltage for the ARM core, GPU core and SDRAM. When busy, ARM frequency go up to "arm_freq" and down to "arm_freq_min" on idle. "core_freq", "sdram_freq" and "over_voltage" behave the same. "over_voltage" is limited to 6 (1.35V). Non default values for the h264/v3d/isp parts are ignored.
disables dynamic clocking, so all frequencies and voltages stay high. Overclocking of h264/v3d/isp GPU parts is allowed as well as setting "over_voltage" to 8 (1.4V). 
The GPU core, h264, v3d and isp share a PLL, therefore need to have related frequencies. ARM, SDRAM and GPU each have their own PLLs and can have unrelated frequencies.
The following is not necessary with "avoid_pwm_pll=1".
pll_freq = floor(2400 / (2 * core_freq)) * (2 * core_freq) gpu_freq = pll_freq / [even number]
The effective gpu_freq is automatically rounded to nearest even integer, so asking for core_freq=500 and gpu_freq=300 will result in divisor of 2000/300 = 6.666 => 6 and so 333.33MHz.
The following table shows some successfull attempts of overclocking, which can be used for orientation. These settings may not work on every device and can shorten the lifetime of the Broadcom SoC.
There are reports that Hynix RAM is not as good as Samsung RAM for overclocking.
SD Card Usage with Overclocking
SD Card Setup redirect : http://elinux.org/RPi_Easy_SD_Card_Setup
Usage of Class 6 & 10 SD Cards ( SHDC / SHDX ) with Overclocking may make the SD Card Filesystems on RPI Instable after few days or weeks. It´s Equal what ext4 , NTFS or other . It´s Equal what SD Card vendor. It´s Equal what PI Models. This don´t mater any SD Card size - verified truely shure at 16Gb and up. ! It Matters when you UNDER-Power your PI ( i.e less then the PI base setup SPECs ! https://github.com/popcornmix Comment at https://github.com/raspberrypi/linux/issues/280: "Overclocking can cause sdcard errors. It tends to be board specific (i.e. some Pi's are more tolerant to overclock and sdcard corruption). I believe it is generally core_freq that causes the sdcard corruption (rather than arm_freq or sdram_freq)" At type of Writing ( April 2013 ) this Note there are 137 Threads at http://www.raspberrypi.org/phpBB3/index.php with SD cards Issues, allmost from Overclocking .
If you use Class 6 & 10 SD Cards and want an stable running PI : DON´t try Overclocking or you Losing probably DATA
Monitoring Temperature and Voltage
To monitor the Pi's temperature, look at:
To monitor the Pi's current frequency, look at:
To monitor the Pi's PSU voltage, you'll need a multimeter, across the power supply test points, or the expansion header.
It's generally a good idea to keep the core temp below 70 degrees, and the voltage above 4.8V. (Note that some, not necessarily cheap, USB power supplies fall as low as 4.2V, this is because they are usually designed to charge a 3.7V LiPo battery, rather than to supply a solid 5V to a computer). Also, a heatsink can be helpful, especially if the Pi is to be run inside a case. A suitable heatsink is the self-adhesive BGA (ball-grid-array) 14x14x10 mm heatsink, part 674-4756 from RS Components.
Overclock stability test
Most overclocking issues show up right away with a failure to boot, but it is possible to get filesystem corruption that arises over time. Here is a script to stress-test the stability of the system, specifically the SD-card. If this script runs to completion, without any errors showing in
dmesg, then the Pi is probably stable with these settings.
If the system does crash, then hold down the shift key during boot, which will temporarily disable all overclocking. Also, note that SD-card issues are usually affected by the core_freq, rather than the arm_freq, and that there is a surprisingly big jump in this (from 250 MHz to 500 MHz) between the High speed (950 MHz) and Turbo (1 GHz) presets in raspi-config.
#!/bin/bash #Simple stress test for system. If it survives this, it's probably stable. #Free software, GPL2+ echo "Testing overclock stability..." #Max out the CPU in the background (one core). Heats it up, loads the power-supply. nice yes >/dev/null & #Read the entire SD card 10x. Tests RAM and I/O for i in `seq 1 10`; do echo reading: $i; sudo dd if=/dev/mmcblk0 of=/dev/null bs=4M; done #Writes 512 MB test file, 10x. for i in `seq 1 10`; do echo writing: $i; dd if=/dev/zero of=deleteme.dat bs=1M count=512; sync; done #Clean up killall yes rm deleteme.dat #Print summary. Anything nasty will appear in dmesg. echo -n "CPU freq: " ; cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq echo -n "CPU temp: " ; cat /sys/class/thermal/thermal_zone0/temp dmesg | tail echo "Not crashed yet, probably stable."
- Introducing turbo mode: up to 50% more performance for free
- What this means is that you can specify -16 and expect about 0.8V as the GPU/core voltage. This is 0.4V below the normal value of 1.2. If you specify 16, you'd get 0.4V ABOVE the normal value of 1.2V, or 1.6V. The fact that someone carefully specified "8" and "1.4V" as the upper limit in the examples leads me to think that it is likely to shorten the life of your raspberry pi significantly if you would specify values above "8". So: don't specify values above zero, but if you do, don't go above 8.