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Implementation USB into microcontroller Igor Atmel-USB device: 
USB-RS232 converter + USB-8/16bit converter + USB-EEPROM scratch pad
based on cheap AVR microcontroller

  Purpose of this article is to inform readers about implementation USB to RS232 converter into singlechip microcontroller, which this interface directly not supports. Simply: implementation USB interface on firmware level (similar as emulation of RS232 interface in microcontrollers, which not have RS232 support). Given project is the next follower of my project of basic USB module IgorPlug-USB (AVR) (computer remote control IgorPlug with USB).

For use in user applications I developed improved USB to RS232 converter (+ another interfaces) with next features:
Device is intended for embedding into your products (source codes of firmware and software is free). For better choice I have prepared two versions, which are different in used processor type. Simpler version is based on AT90S2313 processor. Improved version is based on ATmega8 (see and read bellow its features).

Receiving FIFO buffer :
 - AT90S2313: 32 byte FIFO
 - ATmega8: 800 byte FIFO 

Communication speed and link parameters: 
 - AT90S2313: from 4800 baud to 57600 baud, 8 databits, 1 stopbit
 - ATmega8: from 300 baud up to 115200 baud + possibility to change databits (5,6,7,8), stopbits (1, 2), parity (none, odd, even, mark, space)

Additional build in interfaces and possible extensions: 
 - AT90S2313: 

• USB converter to 8-bit input-output port : each pin can be as input or output, option of pull-up resistor on input pin
• USB read/write into internal data EEPROM memory : 128 byte EEPROM to store own data (e.g. calibration data of your product)

 - ATmega8:

• USB converter to three 8-bit input-output ports (16-bits together) : each pin can be as input or output, option of pull-up resistor on input pin

• USB read/write into internal data EEPROM memory : 512 byte EEPROM to store own data (e.g. calibration data of your product)
• eventually possibility to use ATmega8 peripherals (not yet implemented): 
  - 6-channel AD converted: 4x10bit + 2x8bit
  - PWM channel
• possibility to add own functions: cca 3K memory words (from whole 4K words memory) of Flash is free for next user extensions - specific functions (crypto functions for hardlocks, USB to "something" converter - e.g. USB<>I2C, USB<>PS2, ... . )
• eventually possibility of firmware upgrade through USB (now not implemented - project for another developers :-) )

Application access: 

• Device communicate with user application through driver DLL library: is possible to communicate from more application in the same time (advantage with comparison with standard serial port) 
• Receiving buffer  is exclusive for whole system - received data from RS232 are delivered to all communicating applications (every application obtains all received data)
• Receiving of data is in one thread in DLL library (this receiving thread is only one in system)
• Transmission of data is designed for use in control process - synchronous transmitting: after the transmit function call to transmit buffer is control returned back from DLL when whole buffer is transmitted (including last stopbit of last byte). This behavior is proper in handshake communication (and main advantage towards to classic Windows COM port).

Practical experiences: 

• Up to speed 14400baud 57600baud (improved own driver) is possible continuous receiving - FIFO buffer is not increasing (transfer from device to computer is faster as RS232 receiving transfer)
• Above speed  14400baud 57600baud is receiving possible up to FIFO buffer size - FIFO buffer is filling (transfer from device to computer is slower as RS232 receiving transfer) and is necessary some pause in RS232 receiving to transfer data from FIFO to computer. So in case of handshake communication at high speed is necessary using data packets with size smaller than FIFO size (this condition is in practically mostly fulfilled - 800byte FIFO inside ATmega8 may be enough for everybody ;-) )

And here is example: my testing application "IgorPlugUSBdemo.exe" (receiving of file "msdos.sys" from RS232  terminal TeraTerm.exe):

    Next information will be frequently added (last update: 20.10.2004).
Whole project will be in future accessible on webpages of Atmel company as Application Note. On the following picture is Project structure (source codes, firmware, binaries, documentation, ...) :

English Application Note is here and full project is available as Zip archive (cca 3MB) for DOWNLOAD. Now is here all source codes !!!

www.cesko.host.sk - Hardware section
http://www.cesko.host.sk/IgorPlugUSB/IgorPlug-USB (AVR)_eng.htm - baseline project of simple USB device
www.cesko.host.sk/girderplugin.htm  - infrared remote control of computer
www.appliedp.sk - company in which I am working and developing something from another brand
www.usb.org - all about USB bus
and another: ...

This webpage will be continuously updated - new information (about which you are interested).

author: Ing. Igor Češko:
            e-mail: cesko@internet.sk
            homepage: www.cesko.host.sk