As it's an embedded platform, the Raspberry Pi doesn't have a BIOS like you'd find on a conventional PC. The various system configuration parameters, which would traditionally be edited and stored using a BIOS, are stored in an optional text file named config.txt. This is read by the GPU before the ARM CPU (and Linux) is initialised; therefore it must be located on the first (boot) partition of your SD card, alongside bootcode.bin and start.elf. This file is normally accessible as /boot/config.txt from Linux and must be edited as root; but from Windows or OS X it is seen as a file in the only accessible part of the card. If you need to apply some of the config settings below, but you don't have a config.txt on your boot partition yet, then simply create it as a new text file.
Any changes will only take effect after you've rebooted your Raspberry Pi. After Linux has booted you can get the current active settings with the following commands:
vcgencmd get_config <config> - displays 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).
Note that there's a small number of config settings that can't be retrieved using vcgencmd.
As config.txt is read by the early-stage boot firmware it has a very simple file format. The format is a single property=value statement on each line, where value is either an integer or a string. Comments may be added, or existing config values may be commented out and disabled by starting a line with the # character.
Here is an example file:
# 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
GPU memory in megabytes. Sets the memory split between the CPU and GPU; the CPU gets the remaining memory. Minimum value is 16; maximum value is either 192 or 448 depending on whether you're using a 256MB or 512MB Pi. The default value is 64.
Setting gpu_mem to low values may automatically disable certain firmware features (as there are some things the GPU simply can't do with too little memory). So if a certain feature you're trying to use isn't working, try setting a larger GPU memory split.
Using gpu_mem_256 and gpu_mem_512 allows you to swap the same SD card between both 256MB and 512MB Pis without having to edit config.txt each time:
GPU memory in megabytes for the 256MB Raspberry Pi (ignored by the 512MB Pi). This overrides gpu_mem. The maximum value is 192 and the default is not set.
GPU memory in megabytes for the 512MB Raspberry Pi (ignored by the 256MB Pi). This overrides gpu_mem. The maximum value is 448 and the default is not set.
Setting this to 1 disables the CPU's access to the GPU's L2 cache; requires a corresponding L2 disabled kernel. Default value is 0.
Setting this to 1 disables adjusting the refresh rate of RAM every 500ms; this action measures the RAM's temperature. Default value is 0.
The firmware and kernel as of 19th November 2012 supports CMA (Contiguous Memory Allocator), which means the memory split between CPU and GPU is managed dynamically at runtime. However this is not officially supported.
You can find an example config.txt here.
When the GPU has less than cma_lwm (low-water mark) megabytes of memory available, it will request some from the CPU.
When the GPU has more than cma_hwm (high-water mark) megabytes of memory available, it will release some to the CPU.
The following options need to be in cmdline.txt for CMA to work:
coherent_pool=6M smsc95xx.turbo_mode=N
Setting this to 1 prevents the red camera LED from turning on when recording video or taking a still picture. Useful for preventing reflections when the camera is facing a window.
Defines the TV standard used for composite video output over the yellow RCA jack; the default value is 0.
| sdtv_mode | result |
|---|---|
| 0 | Normal NTSC |
| 1 | Japanese version of NTSC – no pedestal |
| 2 | Normal PAL |
| 3 | Brazilian version of PAL – 525/60 rather than 625/50, different subcarrier |
This defines the aspect ratio for composite video output. The default value is 1.
| sdtv_aspect | result |
|---|---|
| 1 | 4:3 |
| 2 | 14:9 |
| 3 | 16:9 |
Setting this to 1 disables colour burst on composite video output. The picture will be displayed in monochrome, but it may possibly be sharper.
Setting this to 1 uses "safe mode" settings to try to boot with maximum HDMI compatibility. This is the same as setting the following parameters:
hdmi_force_hotplug=1
hdmi_ignore_edid=0xa5000080
config_hdmi_boost=4
hdmi_group=2
hdmi_mode=4
disable_overscan=0
overscan_left=24
overscan_right=24
overscan_top=24
overscan_bottom=24
Setting this to 0xa5000080 enables the ignoring of EDID/display data if your display doesn't have an accurate EDID. It requires this unusual value to ensure that it doesn't get triggered accidentally.
Setting this to 1, will cause the GPU to read EDID data from the edid.dat file, located in the boot partition, instead of reading it from the monitor. More information is available here.
Setting this to 1 pretends that all audio formats are supported by the display, allowing passthrough of DTS/AC3 even when not reported as supported.
Setting this to 1 pretends that all audio formats are unsupported by the display. This means ALSA will default to the analogue audio (headphone) jack.
Setting this to 1 pretends that all CEA modes support 3D, even when the EDID doesn't indicate support for them.
Setting this to 1 avoids "fuzzy matching" of modes described in the EDID. Instead it will pick the standard mode with the matching resolution and closest framerate, even if blanking is wrong.
Setting this to 1 will prevent the initial active source message being sent during bootup. This avoids bringing a CEC-enabled TV out of standby and channel switching when rebooting your Raspberry Pi.
Setting this to 1 pretends that CEC is not supported at all by the TV. No CEC functions will be supported.
Force the pixel encoding mode. By default it will use the mode requested from the EDID, so it shouldn't need changing.
| hdmi_pixel_encoding | result |
|---|---|
| 0 | RGB limited (16-235) |
| 1 | RGB full (0-255) |
| 2 | YCbCr limited (16-235) |
| 3 | YCbCr full (0-255) |
This allows you to choose between HDMI and DVI output modes.
| hdmi_drive | result |
|---|---|
| 1 | Normal DVI mode (No sound) |
| 2 | Normal HDMI mode (Sound will be sent if supported and enabled) |
Configures the signal strength of the HDMI interface; the default value is 0 and the maximum is 7. Try 4 if you have interference issues with HDMI.
This defines the HDMI output group to be either CEA (Consumer Electronics Association; the standard typically used by TVs) or DMT (Display Monitor Timings; the standard typically used by monitors). This setting should be used in conjunction with hdmi_mode.
| hdmi_group | result |
|---|---|
| 0 | Auto-detect from EDID |
| 1 | CEA |
| 2 | DMT |
This, together with hdmi_group, defines the HDMI output format.
For setting a custom display mode not listed here, see this thread.
These values are valid if hdmi_group=1 (CEA):
| hdmi_mode | resolution | frequency | notes |
|---|---|---|---|
| 1 | VGA (640x480) | ||
| 2 | 480p | 60Hz | |
| 3 | 480p | 60Hz | 16:9 aspect ratio |
| 4 | 720p | 60Hz | |
| 5 | 1080i | 60Hz | |
| 6 | 480i | 60Hz | |
| 7 | 480i | 60Hz | 16:9 aspect ratio |
| 8 | 240p | 60Hz | |
| 9 | 240p | 60Hz | 16:9 aspect ratio |
| 10 | 480i | 60Hz | pixel quadrupling |
| 11 | 480i | 60Hz | pixel quadrupling, 16:9 aspect ratio |
| 12 | 240p | 60Hz | pixel quadrupling |
| 13 | 240p | 60Hz | pixel quadrupling, 16:9 aspect ratio |
| 14 | 480p | 60Hz | pixel doubling |
| 15 | 480p | 60Hz | pixel doubling, 16:9 aspect ratio |
| 16 | 1080p | 60Hz | |
| 17 | 576p | 50Hz | |
| 18 | 576p | 50Hz | 16:9 aspect ratio |
| 19 | 720p | 50Hz | |
| 20 | 1080i | 50Hz | |
| 21 | 576i | 50Hz | |
| 22 | 576i | 50Hz | 16:9 aspect ratio |
| 23 | 288p | 50Hz | |
| 24 | 288p | 50Hz | 16:9 aspect ratio |
| 25 | 576i | 50Hz | pixel quadrupling |
| 26 | 576i | 50Hz | pixel quadrupling, 16:9 aspect ratio |
| 27 | 288p | 50Hz | pixel quadrupling |
| 28 | 288p | 50Hz | pixel quadrupling, 16:9 aspect ratio |
| 29 | 576p | 50Hz | pixel doubling |
| 30 | 576p | 50Hz | pixel doubling, 16:9 aspect ratio |
| 31 | 1080p | 50Hz | |
| 32 | 1080p | 24Hz | |
| 33 | 1080p | 25Hz | |
| 34 | 1080p | 30Hz | |
| 35 | 480p | 60Hz | pixel quadrupling |
| 36 | 480p | 60Hz | pixel quadrupling, 16:9 aspect ratio |
| 37 | 576p | 50Hz | pixel quadrupling |
| 38 | 576p | 50Hz | pixel quadrupling, 16:9 aspect ratio |
| 39 | 1080i | 50Hz | reduced blanking |
| 40 | 1080i | 100Hz | |
| 41 | 720p | 100Hz | |
| 42 | 576p | 100Hz | |
| 43 | 576p | 100Hz | 16:9 aspect ratio |
| 44 | 576i | 100Hz | |
| 45 | 576i | 100Hz | 16:9 aspect ratio |
| 46 | 1080i | 120Hz | |
| 47 | 720p | 120Hz | |
| 48 | 480p | 120Hz | |
| 49 | 480p | 120Hz | 16:9 aspect ratio |
| 50 | 480i | 120Hz | |
| 51 | 480i | 120Hz | 16:9 aspect ratio |
| 52 | 576p | 200Hz | |
| 53 | 576p | 200Hz | 16:9 aspect ratio |
| 54 | 576i | 200Hz | |
| 55 | 576i | 200Hz | 16:9 aspect ratio |
| 56 | 480p | 240Hz | |
| 57 | 480p | 240Hz | 16:9 aspect ratio |
| 58 | 480i | 240Hz | |
| 59 | 480i | 240Hz | 16:9 aspect ratio |
In the table above, the modes with a 16:9 aspect ratio are a widescreen variant of a mode which usually has 4:3 aspect ratio. Pixel doubling and quadrupling indicates a higher clock rate, with each pixel repeated two or four times respectively.
These values are valid if hdmi_group=2 (DMT):
| hdmi_mode | resolution | frequency | notes |
|---|---|---|---|
| 1 | 640x350 | 85Hz | |
| 2 | 640x400 | 85Hz | |
| 3 | 720x400 | 85Hz | |
| 4 | 640x480 | 60Hz | |
| 5 | 640x480 | 72Hz | |
| 6 | 640x480 | 75Hz | |
| 7 | 640x480 | 85Hz | |
| 8 | 800x600 | 56Hz | |
| 9 | 800x600 | 60Hz | |
| 10 | 800x600 | 72Hz | |
| 11 | 800x600 | 75Hz | |
| 12 | 800x600 | 85Hz | |
| 13 | 800x600 | 120Hz | |
| 14 | 848x480 | 60Hz | |
| 15 | 1024x768 | 43Hz | incompatible with the Raspberry Pi |
| 16 | 1024x768 | 60Hz | |
| 17 | 1024x768 | 70Hz | |
| 18 | 1024x768 | 75Hz | |
| 19 | 1024x768 | 85Hz | |
| 20 | 1024x768 | 120Hz | |
| 21 | 1152x864 | 75Hz | |
| 22 | 1280x768 | reduced blanking | |
| 23 | 1280x768 | 60Hz | |
| 24 | 1280x768 | 75Hz | |
| 25 | 1280x768 | 85Hz | |
| 26 | 1280x768 | 120Hz | reduced blanking |
| 27 | 1280x800 | reduced blanking | |
| 28 | 1280x800 | 60Hz | |
| 29 | 1280x800 | 75Hz | |
| 30 | 1280x800 | 85Hz | |
| 31 | 1280x800 | 120Hz | reduced blanking |
| 32 | 1280x960 | 60Hz | |
| 33 | 1280x960 | 85Hz | |
| 34 | 1280x960 | 120Hz | reduced blanking |
| 35 | 1280x1024 | 60Hz | |
| 36 | 1280x1024 | 75Hz | |
| 37 | 1280x1024 | 85Hz | |
| 38 | 1280x1024 | 120Hz | reduced blanking |
| 39 | 1360x768 | 60Hz | |
| 40 | 1360x768 | 120Hz | reduced blanking |
| 41 | 1400x1050 | reduced blanking | |
| 42 | 1400x1050 | 60Hz | |
| 43 | 1400x1050 | 75Hz | |
| 44 | 1400x1050 | 85Hz | |
| 45 | 1400x1050 | 120Hz | reduced blanking |
| 46 | 1440x900 | reduced blanking | |
| 47 | 1440x900 | 60Hz | |
| 48 | 1440x900 | 75Hz | |
| 49 | 1440x900 | 85Hz | |
| 50 | 1440x900 | 120Hz | reduced blanking |
| 51 | 1600x1200 | 60Hz | |
| 52 | 1600x1200 | 65Hz | |
| 53 | 1600x1200 | 70Hz | |
| 54 | 1600x1200 | 75Hz | |
| 55 | 1600x1200 | 85Hz | |
| 56 | 1600x1200 | 120Hz | reduced blanking |
| 57 | 1680x1050 | reduced blanking | |
| 58 | 1680x1050 | 60Hz | |
| 59 | 1680x1050 | 75Hz | |
| 60 | 1680x1050 | 85Hz | |
| 61 | 1680x1050 | 120Hz | reduced blanking |
| 62 | 1792x1344 | 60Hz | |
| 63 | 1792x1344 | 75Hz | |
| 64 | 1792x1344 | 120Hz | reduced blanking |
| 65 | 1856x1392 | 60Hz | |
| 66 | 1856x1392 | 75Hz | |
| 67 | 1856x1392 | 120Hz | reduced blanking |
| 68 | 1920x1200 | reduced blanking | |
| 69 | 1920x1200 | 60Hz | |
| 70 | 1920x1200 | 75Hz | |
| 71 | 1920x1200 | 85Hz | |
| 72 | 1920x1200 | 120Hz | reduced blanking |
| 73 | 1920x1440 | 60Hz | |
| 74 | 1920x1440 | 75Hz | |
| 75 | 1920x1440 | 120Hz | reduced blanking |
| 76 | 2560x1600 | reduced blanking | |
| 77 | 2560x1600 | 60Hz | |
| 78 | 2560x1600 | 75Hz | |
| 79 | 2560x1600 | 85Hz | |
| 80 | 2560x1600 | 120Hz | reduced blanking |
| 81 | 1366x768 | 60Hz | |
| 82 | 1920x1080 | 60Hz | 1080p |
| 83 | 1600x900 | reduced blanking | |
| 84 | 2048x1152 | reduced blanking | |
| 85 | 1280x720 | 60Hz | 720p |
| 86 | 1366x768 | reduced blanking |
Note that there is a pixel clock limit, which means the highest supported mode is 1920x1200 at 60Hz with reduced blanking.
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=1andhdmi_mode=1) and boot up your 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:
/opt/vc/bin/tvservice -s - 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.
Setting this to 1 pretends that the HDMI hotplug signal is asserted, so it appears that a HDMI display is attached. In other words, HDMI output mode will be used even if no HDMI monitor is detected.
Setting this to 1 pretends that the HDMI hotplug signal is not asserted, so it appears that a HDMI display is not attached. In other words, composite output mode will be used even if an HDMI monitor is detected.
Set to 1 to disable overscan.
Specifies the number of pixels to skip on the left edge of the screen. Increase this value if the text flows off the left edge of the screen; decrease it if there's a black border between the left edge of the screen and the text.
Specifies the number of pixels to skip on the right edge of the screen.
Specifies the number of pixels to skip on the top edge of the screen.
Specifies the number of pixels to skip on the bottom edge of the screen.
Specifies the console framebuffer width in pixels. The default is the display width minus the total horizontal overscan.
Specifies the console framebuffer height in pixels. The default is the display height minus the total vertical overscan.
Specifies the console framebuffer depth in bits per pixel. The default value is 16.
| framebuffer_depth | result | notes |
|---|---|---|
| 8 | 8bit framebuffer | Default RGB palette makes screen unreadable. |
| 16 | 16bit framebuffer | |
| 24 | 24bit framebuffer | May result in a corrupted display. |
| 32 | 32bit framebuffer | May need to be used in confunction with framebuffer_ignore_alpha=1. |
Set to 1 to disable the alpha channel. Can help with the display of a 32bit framebuffer_depth.
Displays a test image and sound during boot (but only over the composite video and analogue audio outputs) for the given number of seconds, before continuing to boot the OS as normal. This is used as a manufacturing test; the default value is 0.
Can be used to rotate or flip the screen orientation; the default value is 0.
| display_rotate | result |
|---|---|
| 0 | no rotation |
| 1 | rotate 90 degrees clockwise |
| 2 | rotate 180 degrees clockwise |
| 3 | rotate 270 degrees clockwise |
| 0x10000 | horizontal flip |
| 0x20000 | vertical flip |
Note that the 90 and 270 degree rotation options require additional memory on the GPU, so these won't work with the 16MB GPU split.
Hardware decoding of additional codecs can be enabled by purchasing a licence that is locked to the CPU serial number of your Raspberry Pi.
Licence key to allow hardware MPEG-2 decoding, e.g. decode_MPG2=0x12345678.
Licence key to allow hardware VC-1 decoding, e.g. decode_WVC1=0x12345678.
If you've got multiple Raspberry Pis, and you've bought a codec licence for each of them, you can list up to 8 licence keys in a single config.txt; for example decode_MPG2=0x12345678,0xabcdabcd,0x87654321. This enables you to swap the same SD card between the different Pis without having to edit config.txt each time.
Set to 1 to stop start.elf from filling in ATAGS (memory from 0x100) before launching the kernel.
The alternative filename on the boot partition to read the kernel command line string from; the default value is cmdline.txt.
The alternative filename on the boot partition to use when loading the kernel; the default value is kernel.img.
The memory address into which the kernel image should be loaded.
Set to 1 to load the kernel at the memory address 0x0.
Optional filename on the boot partition of a ramfs to load. More information is available here.
The memory address into which the ramfsfile should be loaded.
This specifies both the ramfs filename and the memory address to load it at; it performs the actions of both ramfsfile and ramfsaddr in one parameter. Example values are: initramfs initramf.gz 0x00800000. NOTE: This option uses different syntax to all the other options; you should not use a = character here.
Specifies a device tree filename on the boot partition. This is not officially supported.
The memory address into which the device_tree should be loaded.
The initial UART baud rate; the default value is 115200.
The initial UART clock frequency; the default value is 3000000 (3MHz).
The initial emmc clock frequency; the default value is 100000000 (100MHz).
Wait for a given number of seconds in start.elf before loading the kernel; the default value is 1. The total delay in milliseconds is calculated as (1000 x boot_delay) + boot_delay_ms. This can be useful if your SD card needs a while to 'get ready' before Linux is able to boot from it.
Wait for a given number of milliseconds in start.elf, together with boot_delay, before loading the kernel. The default value is 0.
If set to 1, safe_mode boot won't be enabled. The default value is 0.
If set to 1, don't show the rainbow splash screen on boot. The default value is 0.
NOTE: Setting any overclocking parameters to values other than those used by raspi-config will set a permanent bit within the SoC, making it possible to detect that your Pi has been overclocked. This was originally set to detect a void warranty if the device had been overclocked. Since September 19th 2012 you have been able to overclock your Pi without affecting your warranty; for more information see the blog post on Turbo Mode.
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; if you overclock, the CPU frequency will vary with processor load. Non-default values are only used when needed according to the governor. You can adjust the minimum values with the *_min config options, or disable dynamic clocking with force_turbo=1; for more information see here.
Overclocking 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; for more information see here.
| Option | Description |
|---|---|
| arm_freq | Frequency of the ARM CPU in MHz. The default value is 700. |
| gpu_freq | Sets core_freq, h264_freq, isp_freq, and v3d_freq together. The default value is 250. |
| core_freq | Frequency of the GPU processor core in MHz. It has an impact on CPU performance since it drives the L2 cache. The default value is 250. |
| h264_freq | Frequency of the hardware video block in MHz. The default value is 250. |
| isp_freq | Frequency of the image sensor pipeline block in MHz. The default value is 250. |
| v3d_freq | Frequency of 3D block in MHz. The default value is 250. |
| avoid_pwm_pll | Don't dedicate a pll to PWM audio. This will reduce analogue audio quality slightly. The spare PLL allows the core_freq to be set independently from the rest of the GPU, allowing for more control over overclocking. The default value is 0. |
| sdram_freq | Frequency of the SDRAM in MHz. The default value is 400. |
| over_voltage | CPU/GPU core voltage adjustment. [-16,8] equates to [0.8V,1.4V] with 0.025V steps; in other words, specifying -16 will give 0.8V as the GPU/core voltage, and specifying 8 will give 1.4V. The default value is 0 (1.2V). Values above 6 are only allowed when force_turbo or current_limit_override are specified; this sets the warranty bit. |
| over_voltage_sdram | Sets over_voltage_sdram_c, over_voltage_sdram_i, and over_voltage_sdram_p together. |
| over_voltage_sdram_c | SDRAM controller voltage adjustment. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. The default value is 0 (1.2V). |
| over_voltage_sdram_i | SDRAM I/O voltage adjustment. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. The default value is 0 (1.2V). |
| over_voltage_sdram_p | SDRAM phy voltage adjustment. [-16,8] equates to [0.8V,1.4V] with 0.025V steps. The default value is 0 (1.2V). |
| force_turbo | Disables the dynamic cpufreq driver and minimum settings described below. Enables h264/v3d/isp overclocking options. The default value is 0. Enabling this may set the warranty bit. |
| initial_turbo | Enables turbo mode from boot for the given value in seconds up to 60, or until cpufreq sets a frequency; for more information see here. This option can help with SD card corruption if the Pi is overclocked. The default value is 0. |
| arm_freq_min | Minimum value of arm_freq used for dynamic frequency clocking. The default value is 700. |
| core_freq_min | Minimum value of core_freq used for dynamic frequency clocking. The default value is 250. |
| sdram_freq_min | Minimum value of sdram_freq used for dynamic frequency clocking. The default value is 400. |
| over_voltage_min | Minimum value of over_voltage used for dynamic frequency clocking. The default value is 0. |
| temp_limit | Overheat protection. This sets the clocks and voltages to default when the SoC reaches this value in Celsius. Setting this higher than the default voids your warranty; the default value is 85. |
| current_limit_override | Disables SMPS current limit protection when set to 0x5A000020; it requires this unusual value to ensure that it doesn't get triggered accidentally. This can help if you are currently hitting a reboot failure when specifying a value for overclocking that is too high. For more information see here. Changing this option may set the warranty bit. |
force_turbo=0
This enables dynamic clocks and voltage for the CPU, GPU core and SDRAM. When busy, the CPU frequency goes up to arm_freq and down to arm_freq_min on idle.
core_freq/core_freq_min, sdram_freq/sdram_freq_min and over_voltage/over_voltage_min behave in a similar manner. over_voltage is limited to 6 (1.35V). Non-default values for the h264/v3d/isp frequencies are ignored.
force_turbo=1
Disables dynamic frequency clocking, so that all frequencies and voltages stay high. Overclocking of h264/v3d/isp GPU parts is allowed, as well as setting over_voltage up to 8 (1.4V). For more information see here.
The GPU core, h264, v3d, and ISP blocks all share a PLL and therefore need to have related frequencies. The CPU, SDRAM and GPU each have their own PLLs and can have unrelated frequencies; for more information see here.
The frequencies are calculated as follows:
pll_freq = floor(2400 / (2 x core_freq)) x (2 x core_freq)
gpu_freq = pll_freq / [even number]
The effective gpu_freq is automatically rounded to the nearest even integer; asking for core_freq=500 and gpu_freq=300 will result in the divisor of 2000/300 = 6.666 => 6 and so result in a gpu_freq of 333.33MHz.
Setting this to 1 will decouple a PLL from the PWM hardware. This will result in more hiss on the analogue audio output, but will allow you to set the gpu_freq independently of the core_freq.
To view the Pi's temperature, type: cat /sys/class/thermal/thermal_zone0/temp. Divide the result by 1000 to get the value in Celsius.
To view the Pi's current frequency, type: cat /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq. Divide the result by 1000 to get the value in MHz.
To monitor the Pi's PSU voltage, you'll need a multimeter to measure between the TP1 and TP2 power supply test points; more information is available in power.
It's generally a good idea to keep the core temperature below 70 degrees and the voltage above 4.8V. Note that some 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 5V to a computer. If your overclocked Raspberry Pi is getting hot 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, available from RS Components.
Most overclocking issues show up immediately with a failure to boot. If this occurs, hold down the shift key during the next boot which will temporarily disable all overclocking; this will allow you to boot successfully and then edit your settings.
This article uses content from the eLinux wiki page RPiconfig, which is shared under the Creative Commons Attribution-ShareAlike 3.0 Unported license