USB (Universal Serial Bus) - is type of serial communication interface since January 15, 1996 when first USB-1.0 specification came up to day light. There were several organizations behind USB creation like Compaq, DEC, IBM, Intel, Microsoft, NEC, and Northern Telecom. There are several modifications of USB, but most common for microcontrollers are Low Speed (LS, 1.5Mb/s) and Full Speed (FS, 12Mb/s). Probably everyone knows USB because of widely used Flash memory cards, various devices that connect to personal computer via USB port like printers, cameras, scanners, media players. Today USB 1.0 is almost left behind as there is common USB 2.0 standard used which theoretical speed reaches up to 480Mb/s. Historical timeline of USB evolution:

• 1996 - USB-1.0 - up to 12Mbit/s (LS and FS) - Initial version;
• 1998 - USB-1.1 - up to 12Mbit/s (LS and FS) - Updated version;
• 2000 - USB-2.0 - up to 480Mbit/s (LS, FS and HS) - Increased speed and added miniUSB;
• 2004 - Wireless USB- up to 480Mbit/s - Wireless connection;
• 2008? - USB 3 - up to 4.8Gbit/s - on the horizon.

It is amazing how technology evolves - just imagine few years ago we were exchanging information by using 3.5″ 1.44Mbytes Floppy diskettes, now 4GB Flash isn’t enough. Same is with data transfer speed. Amount of digital information grows incredibly fast and everything has to go through some wires or wireless - who wants to wait for a long time while 4GB video will be transferred from camera to hard drive. I don’t :).

Ok let’s get back to USB. The idea of USB was to connect near devices like mouse, printer, scanners, and modems to computer with up to 3m long cables (LS) and up to 5m FS. Distance may be increased by using hubs.

USB is two point connections where one point (usually computer) is a host while another point works in slave mode. Host may have 127 slaves connected simultaneously. Of course computer has like 2 - 4 USB connectors, but using hubs number may be increased up to 127 theoretically. I don’t know if this would even work correctly.

Talking about Microcontrollers with USB connectivity- they may be connected directly to PC USB port or via hub. USB port may provide 100mA (max 500mA) of current what may be enough to power low power embedded devices. But if device needs more power device must be powered with external power source. Otherwise it may lead to damaging USB port.

Probably every microcontroller is equipped with USART module which is convenient to connect to RS232 COM port. Why almost all modern computers do not use COM or LPT ports any more? There are several reasons:

• High speed (USB is up to 100 times faster);
• Noise protection (USB uses differential signals);
• Error correction in hardware level;
• USB may ser as power source;
• Universality - simple connection cable;
• Expandability - up to 127 devices;

There are some shortcomings of USB like:

• Short operation distance;
• Often special drivers are needed;
• Complicated programming.

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This entry was posted on Wednesday, February 13th, 2008 at 3:22 pm and is filed under Misc Readings, USB and MCU. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.