Archive for the ‘Power Supply’ Category
Uninterruptible Power Supply (UPS)
Posted by Parkzone Corsair in Power Supply on December 23rd, 2009
The human interface device (HID) class includes keyboards, pointing devices, and game controllers. With these devices, the host reads and acts on human input such as keypresses and mouse movements. Hosts must respond quickly enough so users don’t notice a delay between an action and the expected response. Barcode readers can function as HID keyboards with the barcode data emulating keypresses. Other devices with HID interfaces include uninterruptible power supply (UPS) units and display monitors that use HID for user configuration.
Some devices that perform vendor-specific functions can also use the HID class. All HID data travels in reports, which are structures with defined formats.Usage tags in a report tell the host or device how to use received data. For example,a Usage Page value of 09h indicates a button, and a Usage ID value tells which button, if any, was pressed. Windows and other operating systems have included HID drivers since the earliest editions with USB support. For this reason, the HID class has been popular for devices with a variety of vendor-specific functions. A HID can exchange data for any purpose but can use only control and interrupt transfers. Later chapters have more about using HIDs for vendor-specific functions.
A device With no Internal Power Supply
Posted by Parkzone Corsair in Power Supply on December 23rd, 2009
A convenient feature of USB is the ability to draw power from the bus. But using bus power carries the responsibility to operate within allowed limits, including reducing power in the Suspend state. This chapter will help you decide if a device can use bus power. Plus, whether your design is bus-powered or self-powered, you’ll find out how to ensure that your device follows the USB specification’s requirements for managing power.
Also covered are new power-saving options for USB 2.0 and USB 3.0. Inside a typical PC is a power supply with amperes to spare. Many hubs also have their own power supplies. Some USB devices can take advantage of these existing supplies rather than providing their own power sources. Bus power has several advantages. Users don’t need an electrical outlet near the device. A device with no internal power supply can be physically smaller, lighter in weight, and less expensive to manufacture. The device can save energy because power supplies in PCs use efficient switching regulators rather than the cheap linear regulators in the power adapters that many peripherals use. (Self-powered hubs may use inefficient supplies, however.)
Manage Power
Posted by Parkzone Corsair in Power Supply on December 15th, 2009
A device may have its own power supply, obtain power from the bus, or use power from both sources. A host can request a device to enter the low-power Suspend state, which requires the device to draw no more than 2.5 mA of bus current. Some devices support remote wakeup, which can request to exit the Suspend state. USB 3.0 hosts can place individual functions within a USB 3.0 device in the Suspend state. With host support, devices can use additional, less restrictive low-power states to conserve power and extend battery life.
All of the above tasks support the main job of a device’s USB port, which is to exchange data with the host. For most transfers where the host sends data to the device, the device responds to each transfer attempt by sending a code that indicates whether the device accepted the data or was too busy to accept it. For most transfers where the device sends data to the host, the device must respond to each attempt by returning data or a code indicating the device has no data to send. Typically, the hardware responds according to firmware settings and the error-checking result. Some transfers don’t use acknowledgements, and the sender receives no feedback about whether the receiver accepted transmitted data.
Devices send data only when the host requests data. SuperSpeed devices can send a packet that causes the host to request data from the device.The controller chip’s hardware handles the details of formatting the data for the bus. The formatting includes adding error-checking bits to data to transmit, checking for errors in received data, and sending and receiving the individual
bits on the bus.
Of course, the device must also do whatever other tasks it’s responsible for. For example, a mouse must be ready to detect movement and button clicks, a data-acquisition unit has to read the data from its sensors, and a printer must translate received data into images on paper.
