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'transferring data : Two-Way with One Pin'
1999\09\16@111838 by Thomas McGahee

Yes, you can transfer data using a single pin.
If you only need one-way communication, then you wire one PIC
as the transmitter, and the other PIC as the receiver.
You can do this with ANY microcontroller.

The next level of complexity allows a single i/o pin on each PIC
to allow TWO-WAY communications between the PICs. This
trick can only be done by microcntrollers that have programmable i/o pins.

Here's the basic method:
A single wire connects both PICs (ya gotta have a common ground, of
course!)  BOTH PICs initially have their i/o pin connected to this line
as an INPUT.  A 10k pullup resistor connected to +5 ensures that this line
will be pulled high whenever both PICs are listening.

OK, eventually one of the PICs has something to say. It first ensures that
the other PIC is not talking. Once it is assured of this, it programs its
line as an output and sends out a low. The PIC that is still listening will
detect this high-to-low- transition (perhaps via an interrupt), and will
see it as the beginning of  a Start Bit. If you are using standard UART
techniques, then the Start Bit will be followed by the usual 8 data bits
and a minimum of one stop bit (I prefer two stop bits)..

The receiving PIC places the received data into a register and terminates
interrupt. The receiving PIC can then move, process, or otherwise handle
the received byte. It should then set the original receiving register to
Thus the receiving byte can be tested. If it is 0x00 it means there is no
received. If there is anything *else* there, then it is valid data, ready to
processed. If 0x00 is not a good value for you to use as an empty marker,
choose some other value, *or* set a software flag to indicate that data is
ready, and clear the flag once the data has been moved.

To determine if  it is OK to talk it is best to make sure that
(a)  the shared line is high, and
(b) that the shared line has *been* high for at least the time it would take
     output a complete byte.

I usually accomplish (b) by maintaining a "sticky" counter. At the END of
a reception this counter is set to 1 or some other number. Periodically in
the program I increment this counter *if* the shared line is still high.
it rolls over back to 0x00 I *leave* it there. If the PIC wants to send data
it checks the value of the counter. If it is not 0x00, then it increments
counter, does a wait for some tens of microseconds, and then loops around
and checks the counter again. Eventually the counter will reach 0x00, and
the PIC can commence transmission. (note that the interrupt receive routine
will pre-set this counter if a reception occurs, so there is no need to
check the line itself in the counter wait loop)

The inital number stuffed into the counter at the end of the reception
routine should be chosen so that  the minimum time before rollover of
the counter to 0x00 is greater than the time needed to send a complete
byte.  You can give one of the PICs a lower priority for transmissions
by giving it a longer count interval than the other PIC has.

If you are the adventurous type, then you might decide to make the max
wait delay equal to the time it takes to send only a single BIT. I prefer
safety to speed most of the time.

Fr. Tom McGahee

----- Original Message -----
From: Gennette Bruce <spam_OUTbruce.gennetteTakeThisOuTspamTAFE.NSW.EDU.AU>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Thursday, September 16, 1999 4:06 AM
Subject: Re: transferring data

{Quote hidden}

1999\09\17@064624 by Duilio Foschi


wow! This was a master course in uP data transmission !

Thanks a lot


At 10.52 16/09/99 -0400, you wrote:
{Quote hidden}

>> > {Original Message removed}

1999\09\18@060545 by Morgan Olsson

picon face
Hej Duilio Foschi. Tack fšr ditt meddelande 12:45 1999-09-17 +0200 enligt nedan:
>wow! This was a master course in uP data transmission !
>Thanks a lot

Yes it is nice.

One problem though is what happens when both processors see the line free, and start transmitting at approximately the same time.

This can happen in the time it takes them before they see the line free until they start sending.

Remedy proposal:
First insert a series resistor so nothing serious happens when both transmits.
Secondly, make a protocol where they demand some acknowledge from each other (which should be impossible to be misinterpreted from a collision or other error)  And they should have different delay before retransmitting.

Another way is that one is master and can send at any time, and also polls the slave regularly saying "Send something" and the slave immediatelu replies with status etc, then keep silent until it get a new request.

There can then easily be many slaves with each different adresses.

For long distance differential signalling is superb, i.e RS-485.


{Quote hidden}

Morgans Reglerteknik, HŠllekŒs, 277 35 KIVIK, SWEDEN
  tel +46(0)414-446620, fax -70331,

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