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'1 wire half duplex communication between two PIC 1'
1996\06\30@032451 by NEIL GANDLER

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I have an application where I want to send half duplex serial data between
two PIC 16c74 microcontrollers through a one wire link (+ gnd of course))
The thoroughput required is 32 bits of data (the value of 4 registers)
needed to be sent 30 times per second, for LCD and misc. updating.
I want to leave the PICs out of the communication
system to avoid complexity with interrupts dependent on state changes.
I have looked at Motorola's MC145026-29 series of remote control serial
encoder/decoder IC's. They basically form a data packet with address and
data bits, have packet comparison/verification circuitry and are perfect for
one wire link systems.
They are very simple and are easy to interface. The only problem is that they
only offer a maximum of 5 bits of data to send per packet, where it would be
much easier if I could send 8 bits per packet, since I am sending values of
8-bit registers through an 8-bit serial shift register preceding the encoder.
I was wondering if anyone knows of a similar series of ICs that would be a
better alternative. Hotlek offers similar IC's with 8 bits of data but
are too slow with all of the encoding overhead and frequency limitations.
I want to keep this simple. I don't want to start playing
with UARTS and USARTS, even though a USART is built into the PIC (its seems
quite complex for a beginner like me) and a one line link (asynchronous)
is a must. Perhaps someone could suggest a simpler method. Thanks.

               Neil Gandler

1996\06\30@040928 by Lee Jones

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Hi Neil,

> I have an application where I want to send half duplex serial data between
> two PIC 16c74 microcontrollers through a one wire link (+ gnd of course))
> The thoroughput required is 32 bits of data (the value of 4 registers)
> needed to be sent 30 times per second

Since you want to send 4 octets (32 bits) 30 times per second, anything
at or above 1200 baud would suffice.  If you're going to put any sort of
checksum on each "packet", you might want to use 2400 baud.  Generally,
the slower the link, the less noise susceptibility.

Depending on the distance between the PICs, you might be able to get
away with TTL signal levels between the devices.  I'd use that for a
link up to several feet.  Beyond that distance, a differential signal
such as RS-422, RS-423, or RS-485 would work well.  Driver chips are
easier than RS-232 since they use lower, single ended voltage swings.
And you have control both the physical interface on both ends.

> I want to keep this simple. I don't want to start playing with
> UARTS and USARTS, even though a USART is built into the PIC (its
> seems quite complex for a beginner like me) and a one line link
> (asynchronous) is a must. Perhaps someone could suggest a simpler
> method.

Bite the bullet and learn about UARTs.  There's a reason that UART
stands for Universal Asynchronous Receiver Transmitter.  It does the
octet parallel to time dependant bit serial data conversion that you
say you want for this project.  If you find a chip by another name
that does the same function, you're deluding yourself if you think
you're not working with a UART/USART.

You can still get external UARTs that are programmed totally with
jumpers (i.e. no software setup).  The interface is 8 bit parallel
with a couple more lines for handshaking.  If you have enough pins
free on the PICs you're using, one end can write octets and the
other end can read octets.  Nothing simpler.

                                               Lee

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1996\06\30@141604 by james

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NEIL GANDLER wrote:
>  I have an application where I want to send half duplex serial data between
> two PIC 16c74 microcontrollers through a one wire link (+ gnd of course))
> I want to keep this simple. I don't want to start playing
> with UARTS and USARTS, even though a USART is built into the PIC (its seems
> quite complex for a beginner like me)
>                 Neil Gandler

Neil, if the USART is available (you're not already using it for
something else), it is really easy to use.  It is at least as easy as
adding more ICs and is free (you've already bought the PIC).
--
James Musselman
President
Radix/Cobalt Instruments, Inc.
PO Box 897
Clovis, CA 93612 USA
tel 209-297-9000     fax 209-297-9400

1996\06\30@172221 by Neil Gandler

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On Sun, 30 Jun 1996, James Musselman wrote:

> NEIL GANDLER wrote:
> >  I have an application where I want to send half duplex serial data between
> > two PIC 16c74 microcontrollers through a one wire link (+ gnd of course))
> > I want to keep this simple. I don't want to start playing
> > with UARTS and USARTS, even though a USART is built into the PIC (its seems
> > quite complex for a beginner like me)
> >                 Neil Gandler
>
> Neil, if the USART is available (you're not already using it for
> something else), it is really easy to use.  It is at least as easy as
> adding more ICs and is free (you've already bought the PIC).
> --

Yes, I just realized that last night. The diffult sections were the SSP,
the SCI looks pretty straightforward, thanks. I wish I looked at it
sooner.

               Neil

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