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Zegar & Kalendarz
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Konwerter USB-RS232 na procesorze AVR
AVR-CDC is an USB-RS232C interface using CDC (Communication Device Class) protocol on USB 1.1. Although CDC is a part of USB 2.0 standard, it works on low speed USB with slower speed. ATtiny45 can handle the 4800bps 8N1 well enough.
1. No dedicated driver necessary. CDC loads Windows built-in usbser.sys.
2. Very low cost. With ATtiny45, this is the cheapest solution for the USB-RS232C interface.
You need an information file (avrcdc.inf) when you connect it first to Windows PC. The Virtual COM Port appears after the connection established. Macintosh OS X requires none for the CDC connection.
Figure 1: AVR-CDC circuit for ATtiny45
The circuit is quite simple. The Vcc should be lower than 3.6V to avoid SYNC errors. I recommend using a quartz crystal instead of a ceramic resonator for stable connection. Program size is 2.8KB. Since ATtiny45 has no USART, 8bit timers make 4800bps timing. The USART version is also developed for ATmega8/48/88 and achieves the higher transfer rate.
I implemented AVR-CDC to add USB interface to an ATmega88 system. The system is for educational purpose and has tiny PostScript-like compiler inside.
The AVR-CDC is based on Object Development's AVR-USB. I publish the portion of code that I added as free software (GPL2). See the License.txt together with the Object Development's license agreement.