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'usart to usart'
2000\05\20@164631 by Greg Hartung

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  I am trying to get a 16f877 to talk to another one over the built-in
serial usart.  I don't think I need a Maxim or Dallas rs-232 chip.  Can
I simply hook tx to rx?  Receiving 877 never gets anything.  I tried a
series resistor and a pullup.  No luck.

2000\05\20@173457 by Thomas McGahee

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If you want to talk over only a short distance, then you can directly
connect the two usarts by using a simple crossover connection:

tx1 to rx2    tx2  to rx1

The rs232 inverter stuff is to allow transmission of signals
over longer distances.

If you are having trouble receiving at all (or transmitting at all)
see my tutorial/program available at:

http://redrival.com/mcgahee

Please read the whole thing and make sure that you have taken care of
all the gotcha's mentioned.

Fr. Tom McGahee

----- Original Message -----
From: Greg Hartung <spam_OUTghartungTakeThisOuTspamUSWEST.NET>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Saturday, May 20, 2000 4:44 PM
Subject: usart to usart


>    I am trying to get a 16f877 to talk to another one over the built-in
> serial usart.  I don't think I need a Maxim or Dallas rs-232 chip.  Can
> I simply hook tx to rx?  Receiving 877 never gets anything.  I tried a
> series resistor and a pullup.  No luck.

2000\05\20@175148 by M. Adam Davis

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If you are hooking the two chips together, it would be a good idea to
look into synchronos operation of the USART.  It is much faster, and it
was designed for this purpose.

As far as the other questions, have you gotten either chip to talk any
other serial device (computer, etc) to verify that your code works?  The
TX can be connected to the RX without any other components.  Send your
sample code to the list and we can see what's wrong.  I assume otherwise
the chips are working correctly (got a blinking debug LED going?)

-Adam

Greg Hartung wrote:
>
>    I am trying to get a 16f877 to talk to another one over the built-in
> serial usart.  I don't think I need a Maxim or Dallas rs-232 chip.  Can
> I simply hook tx to rx?  Receiving 877 never gets anything.  I tried a
> series resistor and a pullup.  No luck.

2000\05\20@211454 by Greg Hartung

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  Excellent comments in your code.  I wish I'd read it 6 months ago!!!  I
missed a few things like three reads and a fake write, so I gave them a try
but didn't really expect them to work, since both processors will run my
serial echo code just fine.  I am getting signalling at the receiving RX pin,
but the receiver never sees RCIF.  It's just stuck this loop

WaitForChar
       btfss PIR1, RCIF
       goto WaitForChar

Thomas McGahee wrote:

{Quote hidden}

2000\05\24@111142 by Dan Mulally

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----- Original Message -----
From: "Greg Hartung" <ghartungspamKILLspamUSWEST.NET>
To: <.....PICLISTKILLspamspam.....MITVMA.MIT.EDU>
Sent: Saturday, May 20, 2000 2:44 PM
Subject: usart to usart


>    I am trying to get a 16f877 to talk to another one over the built-in
> serial usart.  I don't think I need a Maxim or Dallas rs-232 chip.  Can
> I simply hook tx to rx?  Receiving 877 never gets anything.  I tried a
> series resistor and a pullup.  No luck.

I'm using a PIC 16F873A to talk with a PIC 16F874A by doing essentially what
you described and it works. Although it's not a direct connection (I have an
FSK modulator/demodulator so I can multiplex several signals on a single
coax), it should do the same thing. I'm using the CCS C compiler to develop
my code.

Dan

2000\05\24@175223 by Anthony Clay

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----- Original Message -----
From: Dan Mulally <EraseMEdtconspam_OUTspamTakeThisOuTRAPIDNET.COM>
To: <PICLISTspamspam_OUTMITVMA.MIT.EDU>
Sent: Wednesday, May 24, 2000 10:05 AM
Subject: Re: usart to usart


{Quote hidden}

No real logic goes with this, but it's worth a try, how about a TTL buffer?

Anthony

2000\05\24@182345 by Thomas McGahee

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I hesitate to even ask this question, but you DO have a common
ground line between both PICs, don't you?

Fr. Tom McGahee

>
> > {Original Message removed}

2000\05\26@002430 by Greg Hartung

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> I hesitate to even ask this question, but you DO have a common
> ground line between both PICs, don't you?

  Good question.  I wish it had been that easy.  Yes, I do.  This is the same
circuit I've been building for the last few months.  Pressure sensor, LCD, etc.
This second chip will be a servo controller.  They share a 5V regulator.
  An interesting symptom has cropped up in the last few days of trying to figure
this USART problem out...  I am using a combination of 16F877 and 16C774.
Sometimes I use 2 877's, sometimes a combination, depends on whether I need to
test 12bit ADs or not.  Anyway, the point is, the 877's freak out when I touch
the case or RB3 with my finger.  It skips lines of code, things ground out.
LED's go off.  buzzers go on...   Swap a 774 into the circuit's ZIF, and all the
flaky stuff goes away.
  The reason I found this was trying to debug an LED I had connected for
debugging the wait_for_serial_in routine on the slave 877.  I noticed the
datasheets indicate it is the PGM on the 877 and LVDIN on the 774.  Maybe someone
could 'splain why the PGM pin is so sensitive?  But more interesting, why the
ceramic package itself does this also?
  But anyway, back to the USART, the receiving 877 has nothing except a crystal,
an LED, and RX/TX attached to it.
  I swapped a couple brand new 877's in today, just in case I'd fried
something.  No luck, tho.

2000\05\26@043228 by Alan B Pearce

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>Anyway, the point is, the 877's freak out when I touch
>the case or RB3 with my finger.  It skips lines of code, things ground out.
>LED's go off.  buzzers go on...   Swap a 774 into the circuit's ZIF, and all
the
>flaky stuff goes away.

This sounds like someone elses problem on this list in the last week. The
problem they found was removed by disabling the LV programming pin (RB3?) IFRC.

2000\05\26@073902 by fernteix

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Hi


<Anyway, the point is, the 877's freak out when I touch
>the case or RB3 with my finger.  It skips lines of code, things ground out.
>LED's go off.  buzzers go on...   Swap a 774 into the circuit's ZIF, and
all the
>flaky stuff goes away.

But more interesting, why the
>ceramic package itself does this also?

I all this happened in analog world, without reading other details, I would
think in electric field produced by a finger not grounded ! , injecting
current in high impedance inputs;  and the effect of  humidity lowering the
high impedance of ceramic surface
Is a possibility only, of course.

Fernando

2000\05\26@223244 by uC Novice

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Your right, you don't need an RS-232 transceiver chip
if you are transmiting from one uC to another on the
same board; however if you try to do that that at any
significant distance and speed you better grab
yourself an RS-232/RS-485 transceiver. You must have
configured the USARTs wrong. If you think you will
have an easier time with the built in SPI peripheral
you can always use the MAX3100 SPI front end UART from
Maxim. Then again if you are just sending serial data
from one uC to another on the same board why not just
use exclusively SPI.

Chao for now,

uC
--- Greg Hartung <RemoveMEghartungTakeThisOuTspamUSWEST.NET> wrote:
>    I am trying to get a 16f877 to talk to another
> one over the built-in
> serial usart.  I don't think I need a Maxim or
> Dallas rs-232 chip.  Can
> I simply hook tx to rx?  Receiving 877 never gets
> anything.  I tried a
> series resistor and a pullup.  No luck.


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