AVR-Chat

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

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 ?

At 02:25 PM 21/01/04 -0800, you wrote:
>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!

Is the RS232 (Max 232??) interface chip designed for 3.3V or 5V operation?
It may have stopped working!!

Regards

John Samperi

******************************************************
                        Ampertronics Pty. Ltd.
  11 Brokenwood Place Baulkham Hills, NSW 2153 AUSTRALIA
         Tel. (02) 9674-6495       Fax (02) 9674-8745
               Email: samperi@ampertronics.com.au
                 Website  http://ampertronics.com.au
* Electronic Design   * Technical Services   * Contract Assembly
******************************************************

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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Mike Harrison wrote:
> 
> 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 ?


samperi@ampertronics.com.au wrote:
> 
> Is the RS232 (Max 232??) interface chip designed for 3.3V or 5V operation?
> It may have stopped working!!
> 
> Regards
> 
> John Samperi

Al Welch wrote:
> 
> 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
.
.
.
> Al Welch


The drivers don't seem to be the problem . The STK501 uses a MAX3232 rated at 3.0-5.5v 
to drive the 9-pin J301 connector. My board uses a pair of MAX3244's also rated for
3.0-5.5v. The problem looks like the Dell PCs' +6v threshold.

I guess my real question, for the embedded types on the list, is has anyone else run into
this issue talking to other brands of PC's or is this just a Dell box thing?

As an aside, I noticed that the AVR-JTAG pod didn't seem to have any trouble talking to
my PC even when powered by the system 3.3v, so I opened it up. Low and behold - a boost 
supply shoving 7.3v into a 3-pin 5v linear regulator and a good ol' MAX202 5v only part.
So simple...

Bruce

Not so much with RS232 but a lot in the old days with the printer ports and
dongles. We used to program security dongles from Vault and they would only
program with some printer port cards and not others. Once again the levels
differ by brands. If you Dell has a USB port you may be able to use one of
those USB to RS232 adapters.

Al

-----Original Message-----
From: Bruce Parham [mailto:obparham@jpl.nasa.gov]
Sent: Thursday, January 22, 2004 10:42 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] RS-232 Levels???


Mike Harrison wrote:
>
> 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 ?


samperi@ampertronics.com.au wrote:
>
> Is the RS232 (Max 232??) interface chip designed for 3.3V or 5V operation?
> It may have stopped working!!
>
> Regards
>
> John Samperi

Al Welch wrote:
>
> 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
.
.
.
> Al Welch


The drivers don't seem to be the problem . The STK501 uses a MAX3232 rated
at 3.0-5.5v
to drive the 9-pin J301 connector. My board uses a pair of MAX3244's also
rated for
3.0-5.5v. The problem looks like the Dell PCs' +6v threshold.

I guess my real question, for the embedded types on the list, is has anyone
else run into
this issue talking to other brands of PC's or is this just a Dell box thing?

As an aside, I noticed that the AVR-JTAG pod didn't seem to have any trouble
talking to
my PC even when powered by the system 3.3v, so I opened it up. Low and
behold - a boost
supply shoving 7.3v into a 3-pin 5v linear regulator and a good ol' MAX202
5v only part.
So simple...

Bruce



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