Why does the arduino need a USB-to-serial chip?

Why does the arduino need a USB-to-serial chip? Given that USB stands for Universal serial bus, I don't understand why it is apparently converted to serial again using the "USB-to-serial chip"... I'm completely new to micro controllers.. I've read many of the pages on the arduino web site but none of them explain this particular point.

mahela007: Why does the arduino need a USB-to-serial chip? Given that USB stands for Universal serial bus, I don't understand why it is apparently converted to serial again using the "USB-to-serial chip"... I'm completely new to micro controllers.. I've read many of the pages on the arduino web site but none of them explain this particular point.

The very first arduino boards were built to interface to PC RS-232 comm ports. That is how the PC arduino IDE software uploads new programs onto the Arduino board via the bootloader program also stored on the boards processor chip. Then more and more PCs stopped having hardware RS-232 com ports installed onto them. The USB serial converter chip is just a solution to allow PC software to allow application software like the Arduino IDE to talk to a device via a software comm port interface, but using the USB bus to send the data.

The USB serial converter chip is said to be a 'virtual com port' device. Does that make sense?

Lefty

USB is a specific set of protocols for information transmission, which happens to send data in a serial fashion.

"Serial" in the context of programming the Arduino micro-controller is another, different (and much simpler), protocol. It's very similar to RS232 as used for decades by PCs and other devices, but the voltage levels are different. AFAIK there is no official name for this communication type so it ends up just being called "serial", or "TTL serial" to distinguish it from RS232.

This "TTL" serial variant is much used for communication over short distances because it is electrically simpler than RS232. Older devices convert the TTL voltages to genuine RS232 when they need to connect to the outside world, but these days RS232 is dying out. Hence many modern devices - like the Arduino - convert to USB instead.

PS. There are micro controllers now with built-in USB interfaces. The Arduino Uno board uses such a chip to translate from USB to "serial" instead of a dedicated converter chip (so there are actually two micro-controllers on the Uno board). There are also Arduino-like boards which use just a single chip with built-in USB. It is even possible for USB-less micro-controllers to emulate USB in software and therefore do without a converter.

Thanks guys. That clears up that question.