USB Power and Battery Charging

admin — Tue, 13 Apr 2010 09:16:28 +0000

The +5V output from a USB hub is permitted to range from 4.75V to 5.25V (5V +-5%). For USB2.0 this range is changed to be 4.4V to 5.25V and for USB3.0 the range starts from 4.0V.

A USB device is permitted to draw up to 100mA (1 unit load) for USB2.0, and up to 150mA for USB3.0. It may do this prior to any USB communication, so this is the current limit a USB device may draw if using a USB connector to provide the charger input without a USB interface to request a higher current from the USB hub (which it may or may not be granted depending on the hub).

USB devices that need more power than this are called high power devices (instead of low power). The maximum load that may be requested for USB2.0 is 500mA and for USB3.0 is 900mA.

The “USB Battery Charging Specification” adds optional new power modes. A host or hub port that supports charging can supply up to 1.5 A when communicating at low speed or full speed, or a maximum of 900 mA when communicating at high speed, and as much current as the connector will safely handle when no communication is taking place. USB 2.0 standard A connectors are typically rated at 1500mA. The Dedicated Charging Port shorts the D+ and D- pins with a resistance of up to 200Ω. This short disables data transfer but allows devices to detect the Dedicated Charging Port and allows simple high current chargers to be made.

TCP / UDP – Which Is Best?

admin — Tue, 02 Jun 2009 07:23:12 +0000

You’ll probably come across programmers who say this isn’t even a question – use TCP because all the hard work is done for you. In many applications this is true and using TCP can make your life as a programmer very easy. However TCP has one very big drawback in embedded applications – when a packet gets lost or a remote device doesn’t respond you have to wait for the TCP stack to retry the communication several times, each after successively longer delays before it finally gives up. This is TCP’s big advantage and drawback at the same time. With UDP there is no re-try – if a packet gets lost you need to notice that you’ve not had a reply and send it again. To the hardened PC programmer this is task often not worthy of their genius, but to an embedded programmer this is something that can make UDP incredibly powerful and fast in time critical applications. Was it ever really a problem having to notice that a remote device hadn’t responded when talking to it using RS232? Well its no different when talking using UDP and you can use the same simple approaches to defining your communications link. UDP also has one other great advantage – it allows you to broadcast packets so that every device on the local network or in the devices subnet receives them – you can’t do that with TCP.

So the answer is, it depends on what you are doing. In terms of how the packets of data travel over the Ethernet links and internet there is no difference – both get routed in exactly the same way. TCP is fantastic at offloading work from you as a programmer for many applications, but UDP is the obvious choice in a great many other applications where you need that raw control of what you are sending and receiving.

Electronic Product Design » Communications

USB Power and Battery Charging

TCP / UDP – Which Is Best?