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[on:]

wan't quite sure where to post this, but...

UNO: what does the ATmega8 that's used as USB interface actually do

I can see there is level adjustment (I think)
what else does it have to do?

subsidiary question
if it is essential, can you get ATmega already programmed to do the job?

cheers
Mike

[Reply #1 on:]

USB is not a simple serial line that just needs a level shift to work with TTL Serial.  It uses a complex set of communication protocols for handling things like serial communications on the same wire as disk access.

If you have an ICSP/ISP programmer you can buy a blank ATmega chip and program it yourself.

[Reply #2 on:]

thanks John

no programmer, but that's fixable

what code to upload is the bigger question!

or should I just bite the bullet and get a chip (FT232RL) direct from FTDI?

cheers
Mike

[Reply #3 on:]

Many simple Arduino clones skip the on-board USB to Serial conversion and just include a 6-pin header for a $20 FTDI cable.  If you want to save money you can find some $3 cellphone USB cables that do the USB to TTL Serial conversion.  Many of those use the 3.3v LVTTL logic levels so be sure to get one that matches your operating voltage.

[Reply #4 on:]

most helpful

what I'm working on is my own ATmega328 UNO clone

design is fine stand-alone, but need to be able to talk to it

cheers
Mike

[Reply #5 on:]

Many simple Arduino clones skip the on-board USB to Serial conversion and just include a 6-pin header for a $20 FTDI cable.  If you want to save money you can find some $3 cellphone USB cables that do the USB to TTL Serial conversion.  Many of those use the 3.3v LVTTL logic levels so be sure to get one that matches your operating voltage.

The main disadvantage of those cheap serial cables is that they don't bring out the DTR signal wire used to perform the auto reset function that the arduino IDE uses to activate the bootloader at the correct time. You can master performing a manual reset but it takes practice to learn the correct timing for that operation. Also finding the proper USB driver for some of those serial cables can be a problem depending on the OS version you have.

Lefty