AVR-Chat

Look at the MAX3228 instead of the MAX232 for lower supply.

According to Maxim...

The MAX3228/MAX3229 are +2.5V to +5.5V powered EIA/TIA-232 and V.28/V.24
communications interfaces with low power requirements, and high data-rate
capabilities, in a chip-scale package (UCSP™).

The MAX3228/MAX3229 achieve a 1µA supply current with Maxim's AutoShutdown™
feature. They save power without changes to existing BIOS or operating
systems by entering low-power shutdown mode when the RS-232 cable is
disconnected, or when the transmitters of the connected peripherals are off.

The transceivers have a proprietary low-dropout transmitter output stage,
delivering RS-232 compliant performance from a +3.1V to +5.5V supply, and
RS-232 compatible performance with a supply voltage as low as +2.5V. The
dual charge pump requires only four small 0.1µF capacitors for operation
from a +3.0V supply. Each device is guaranteed to run at data rates of
250kbps while maintaining RS-232 output levels.

Al Welch

-----Original Message-----
From: Bruce Parham [mailto:obparham@jpl.nasa.gov]
Sent: Wednesday, January 21, 2004 2:25 PM
To: AVR-Chat@yahoogroups.com
Subject: [AVR-Chat] RS-232 Levels???


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?

Bruce




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