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'Unwanted chars on terminal screen ??'
1999\05\05@091309 by Goovaerts

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Hello, got a question about a RS232-connection between a PC( Hyper-terminal
program of Windows 98) and a PIC16C63.

When I turn off the power to the PIC, I always get some unwanted characters
appearing on the screen. How can I fix this ??? Ideas anyone ?? I used to
have the same problem when turning
on the power supply, doing an external POR (power-on reset) ! I fixed that
by using a RC filter, designed to cause a delay of a few ms, so that the
power source could stabilize ! But the
problem when turning it off still remained. Could someone please help me,
I'm kind of in a
hurry to finish my design.

Glenn Goovaerts

1999\05\05@093139 by Benjamin Petersen

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> When I turn off the power to the PIC, I always get some
> unwanted characters
> appearing on the screen. How can I fix this ??? Ideas anyone
> ?? I used to

Without knowing much about rs232, it seems that one or more pin on your pic
project is floating (when powered off) tie one or more of the signal lines
to ground via a resister of 10k ?. Anyone ?
I am sure that there will be an easy solution for you... Just wait about 24
hours ;-)

Regards
Benjamin Petersen

1999\05\05@102352 by Wagner Lipnharski

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Goovaerts wrote:
>
> Hello, got a question about a RS232-connection between a PC( Hyper-terminal
> program of Windows 98) and a PIC16C63.
>
> When I turn off the power to the PIC, I always get some unwanted characters
> appearing on the screen. How can I fix this ??? Ideas anyone ?? I used to
> have the same problem when turning
> on the power supply, doing an external POR (power-on reset) ! I fixed that
> by using a RC filter, designed to cause a delay of a few ms, so that the
> power source could stabilize ! But the
> problem when turning it off still remained. Could someone please help me,
> I'm kind of in a
> hurry to finish my design.

RS232 uses the start bit to signal the other side that will follows
timed bits.
When you turn on or off your PIC, the tx circuit is entering momentarily
in this "start bit" condition, what makes the PC to "time slicing" the
RX pin and displaying the not existent character.

Suppose that the "start bit" voltage level at the PIC exit is "high", it
would means a "low" level at the RS232 (that is inverted from TTL
logic).  When you off the unit, the power supply voltage drops toward
zero, and in the way it crosses some internal "voltage sensing" (if it
exists) that could trigger an internal reset (it could raise all port
pins to up state, causing the TX pin to signal an invalid "start bit"),
or the electronics craziness with invalid voltage can do anything else,
causing the same problem.

You don't say if you are using some kind of MAX232 chip to do the level
conversion, if you do, just attach a 1N4004 diode to a 1-10uF capacitor
to the PIC, so when removing power from the board, the MAX232 would lose
power before the PIC, and it would be not able to propagate the PIC
craziness to the PC. It means that the PIC would "power off"
milliseconds after the max232.

--------------------------------------------------------
Wagner Lipnharski - UST Research Inc. - Orlando, Florida
Forum and microcontroller web site:  http://www.ustr.net
Microcontrollers Survey:  http://www.ustr.net/tellme.htm

1999\05\05@202308 by Mike Keitz

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On Wed, 5 May 1999 15:10:35 +0200 Goovaerts <spam_OUTgoofy1TakeThisOuTspamGLO.BE> writes:
>Hello, got a question about a RS232-connection between a PC(
>Hyper-terminal
>program of Windows 98) and a PIC16C63.
>
>When I turn off the power to the PIC, I always get some unwanted
>characters
>appearing on the screen. How can I fix this ???

Most devices using RS-232 aren't intended to do anything predictable
while one end is switched on and off.  The standard doesn't define what a
reciever should receive if the transmitter is not connected or powered
on.  If you absolutely must switch the PIC on and off without getting
spurious characters over the line, use a brown-out circuit to hold the
PIC in reset while it is powering up and down.  Also connect a pull-up
resistor on the TTL line from the PIC to the RS-232 driver.  When the PIC
is in reset (all pins switch to input mode), the resistor should hold the
line inactive.  Of course this won't help you if the RS-232 driver does
strange things while its power is going up or down.

Again, many devices such as external modems send a few characters of
garbage when their power is cut off.  I don't consider that faulty
operation.


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