AVR-Chat

On Wed, 21 Jan 2004 14:25:06 -0800, you wrote:

>Okay, I give. How do I talk to a PC using 3.3V? (A bit OT but...)
>
>I have a short program running on a M128 that puts out a time of day string on one of the comm ports
>every second. The AVR side is an STK500/501 set with the comm using the #2 comm translator/connector on
>the 501. The other end of the cable is a desktop Dell pc running HyperTerminal. This setup has been working
>just fine for some months. 
>
>Today I adjusted the board voltage down to 3.3V and the PC quit receiving. The AVR is still running just fine
>but the comm Tx level drops enough that the PC no longer receives. A scope shows the Tx levels are -6.0 and +7.2
>when running at 5.0V, and -5.8 and +6.0 when set to 3.3V. Not a big change - and still over the +/-5v that I
>thought was the "RS-232" spec - but enough to kill comm!
>
>This has my shorts knotted because I just received my first prototype board from the fab house and, sure
>enough, it has an M128 with two Maxim translator chips for comm all running at 3.3V.
>
>BTW, I've tried this with two other desktops and a laptop (all Dells) with the same result; they all stop
>at around 3.8V. Any ideas or words of wisdom?

You can get away with 5V for RS232, so if your levels ate -5.8 and +6 I would be highly surprised if
this was the problem, unless your driver IC has delay or risetime issues at lower supply voltages. 
Are the RS232 chips you are using actually designed for 3.3v operation ?