Pcs With Energy Saving Cpu Technology - THORLABS DCU223x Operation Manual And Sdk

DCx Cameras

6.1.1 PCs with Energy Saving CPU Technology

This application note is related to all DCx USB cameras connected to PC systems using current CPU models that
implement modern energy saving technologies.
Symptoms:
- Low USB bandwidth provided by the PC system
- TransferFailed errors occurring even at moderate pixel clock settings
- Camera operates at low speed only
Summary:
Current CPUs with modern energy saving features can cause bandwidth limitations on USB. The only available
approach to this issue is to disable CPU sleep states. Unfortunately this is not possible for all systems.
Detailed explanation:
Modern CPUs like Intel i5 & i7 and others make use of advanced energy saving technologies ensuring a low power
consumption and long battery life for mobile
devices. Additionally those CPU implement features for increasing the performance of single cores if there is
enough thermal headroom available when other cores have little load.
A basic idea to achieve this is to put a CPU core to sleep while there is nothing to do for it. Various different activity
states of CPU cores are available in modern CPUs. These CPU states are referred to as "C-states". C0 is the
working state of a core.
Increasing numbers refer to less activity and longer wake up times. Current CPU fall down to variations of the C3
state which are referred to as "Sleep", "Deep Sleep" and similar.
Unfortunately negative effects of the sleep states have shown up. It is observed that the available bandwidth of PC
busses drops significantly when part of the CPU enters these states.
The operation of DCx USB cameras is affected by the sleep states because they reduce the speed of the USB
system. The available bandwidth on the USB may drop down to around 30% of the maximum bandwidth when the
CPU, or one of its cores, enters sleeping states.
One would expect that a CPU core will not fall into a sleep state while it is obviously needed for the operation of the
USB. But obviously USB data transfers do not prevent the CPU from falling to sleep. If the code execution load of a
CPU core is low enough it will fall asleep and immediately reduce the USB bus speed.
For operation at high frame rates DCx cameras require an adequate USB bandwidth which might not be available
when CPU cores are in sleep states.
Advice:
If you seem to be running into this low bandwidth issue please check and try the following. These first hints are
general recommendations for issues with the USB
data transfer. You can check the USB performance with the "Optimum" pixel clock settings checkbox in uc480
Demo software. A good USB system should be able to reach a pixel clock setting near the maximum value.
Please remove other USB devices from the system (USB keyboard and mouse are fine). Run tests with only one
camera connected at once.
Make sure using a USB port directly on the mainboard. Front panel or other ports are connected to the
mainboard with poor cabling quality frequently.
Make sure to use USB2.0 certified cables to connect the camera.
If you are using USB hubs or extensions: Run a test without these devices, connect the camera directly to the
PC.
Disable other equipment that is connected via USB. For example WLAN and Bluetooth adapters might use USB
to connect.
If you are using a mobile PC: run it on mains power, not battery.
Check your energy saving options in the operating system. Disable energy saving features and set the available
features to "full performance" or similarly named options.
If you checked the above and still observe low USB performance you might be experiencing the issue with CPU
sleep states.
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