I have had problems with USB serial interfaces. The utility works
flawlessly at 115.2K on my desktop but can be a dog on some types of
serial ports. I also had a PCMCIA serial card on my notebook that worked
fine with TeraTerm yet would give me problems when I tried to use the
Flash utility. You didn't say what i/f or o/s you were using. Check your
settings, fiddle with the buffer thresholds etc. I suspect the utility
could handle the timeouts better. Don't forget to decouple the MAX.
RS-232!
IMO, the lower the drive voltage the better, far less for the drivers to
slew through at high data rates. I much prefer an interface using 0 to
+3-5V drive through a biased 422 chip. They don't have all the symmetry
problems etc that come with the standard drivers. Nobody would think of
putting +/- voltage drivers on an I2C or SPI bus just to drive it a few
meters, would they?. Makes even less sense when you run a mere few
meters to your PC at a snail's pace of 115.2K or less into what is a
unipolar receiver with a switching threshold of 1.5V (check the specs!).
Ahhhh, tradition! Professional gear will always need the +/- voltage for
standards "compliance", but not necessarily for compatibility.
If you want to find out more about running "RS-232" at lower voltages,
just read a Maxim app note.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/1834
Excepts:
"The old quad-receiver MC1489 has typical high and low receiver-input
thresholds of 1.3V and 1.0V. The corresponding MAX3225E thresholds are
specified at 1.5V and 1.2V, as are those of other interface parts.
Therefore, the assumption that most positive thresholds are below 2.0V
and negative thresholds above 0V allows an additional voltage drop on
the cable."
"Most of today's RS-232 communications, however, require cables of less
than three meters, e.g., connecting a cell phone or PDA cradle to a
notebook computer, or a small PBX to a desktop PC. Those applications
reside in the office or in the home, so noise immunity is not a big issue"
*Peter*
acetoel wrote:
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>Yesterday I finished my LPC board. I used a MAX3232 CPE instead of the
>MAX232 that I used on the protoboard. The 3.3V and 1.8V regulators are
>working okey. And I have check the voltage on pin 6 and 2 of the
>MAX3232. In Pin 6 it was -5.5V and in pin 2 +5.5V.
>
>When I open the Flash Utility and try to stablish a communication,
>everything works okey. I can set the baudrate at 9600bps and read the
>signature and the bootloader version. But when I tried to program the
>micro, it get's halted says "cannot communicate with board". So I slow
>down the baudrate to 4800, and the same happen. Until I get to 2400bps
>and it works okey, but I have to wait a long time...
>
>Is -5.5V and 5.5V a good voltage for the max? Or I will have to go
>back to the MAX232 (with 5V), whihc gave near -8V and +8V?
>