HC705 serial communication

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I have a device that has a MC68HC705J1ACDW microcontroller on it that
I would like to try to emulate in software.  The device is a display
unit that has a couple status leds and a seven segment display.  I
would like to route the 4 wire proprietary connector to a RS232 serial
port on my pc so that I can write my own software to control the
display.  There is one button on the display, so I would need to
emulate the communication in both directions.  I am not worried about
the software part, but I need to some help on the hardware side.  I
have a couple of questions, in case anyone has some insight for me.

1.  I am assuming that there is not a lot of communication going on,
because of the small number of LEDs on it.  Is it realistic for me to
observe the data sent from the main device and learn how they

2.  How do I go about creating the cabling?  I have determied which
wire is power and which is ground.  I am assuming that each of the
others is for communication.    How do I determine which is which?

3.  It seems to me that the only way that I can really look at the
data being transmitted is to connect the source device to my pc via
one serial port, the display via the other serial port, and pass the
data through, while logging it. Is this reasonable?  Is there freeware
available to do this, or am I going to have to write it myself?

4.  Any books or online resources that I can use?  I am trying to get
a copy of "Serial Port Complete" by Jan Axelson, which is the only
book I have heard fo so far that comes close.

I am sure that I have a lot of work ahead of me, and I am sure I
forgot something, but thanks for any help.

Re: HC705 serial communication
On 4 Aug 2003 00:18:30 -0700, the renowned dolphin_moon_ snipped-for-privacy@yahoo.com

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You'd better do that first. If it's a synchronous communication
protocol (eg. SPI - /CE, SDIO, GND, Vdd) you won't be able easily to
convert it to asynchronous (serial port). You could control it with a
parallel (printer port), however.

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Look at it on a 'scope. Follow the wires to the processor and see what
pins it goes to, compare to the datasheet. Those micros have fixed
pins for the peripheral functions so you can tell a fair bit.

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An oscilloscope would be the easiest way.

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Best regards,
Spehro Pefhany
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: HC705 serial communication

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Looking at the PCB, it is kind of hard to tell what is going where.
The microcontroller has a 6 bit I/O port and an 8 bit I/O port, but
the connections quickly get lost (to my eye).  I am not confident
enough in my soldiering skill to pull any of the electronic from the
PCD.  My understanding of all this is VERY basic, (I am trying to
learn) but since there are only two wires connecting the device and
display, I was assuming (hoping is more a more accurate description)
that it would be using some sort of serial communication.  I found an
article for the design of a software SCI for this chip, but I have
know way of knowing if this is used.

It seems like creating a parallel interface would be a lot more
complicated (for me) and I might have to shelve the project, because
that might be a bit beyond my knowledge and the time that I can devote
to this.  If I try to create a serial interface and hook it up, what
will I see if it is some sort of synchronous protocol?  Will it just
look like noise?  How can I differentiate this from using the wrong
communication settings?

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The plan that I have in my previous thread should allow me to look at
the signals on both wires, and determine which is which (assuming it
is using a protocol I can look at).  I'll have to try to find a friend
with an oscilloscope  since it isn't in the budget to buy one at this
time.  The only tool I have is a multimeter...  My main concern now is
to safely attach the devices, and to decipher what I see, if possible.

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Re: HC705 serial communication

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It's highly unlikely that it's a  software SCI implementation. It's
rather constraining to do with this chip. It's more likely that this is
a synchronous serial bus. Spehro's right about connecting to a PC, you
won't see anything. Try using your multimeter to ring out the pins from
the connector to vdd, ground and the microntroller pins. If it's just
four pins there may be power and ground and the other two could be a
clock and data line. Although the maybe the device attached is powered
from another source. This could mean there is ground, clock, data and a
select line. This would comply with the SPI protocol, for a simplex network.
Actually the best tool for this job is a logic analyser. It can allow
you to look a wide segement of time on multiple points.

Re: HC705 serial communication
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I hooked everything up tonight, and it looks like you both are right.
I am not getting anything (except 0x00 when I turn off the power, 0xFF
if I lower the bitrate from 9600).  I was hoping for the software SCI
implementation because 1.  I saw an article on the motorola site with
source and a description how it could be done.  2.  The cabling was
simple to make.  3.  It didn't seem like it would be that hard to
figure everything out if it was.  But, it doesn't look like that is
the case.

I am okay with this being a bit harder than I had first hoped, but
this is obviously going to take some more reasearch on my part to even
figure out your suggestions (I am a software guy and have only a
couple college electives I barely remember to work from).

I am not sure what you mean by "ring out" ('scuse my ignorance in all
of this).    From the pdf I found, there is two I/O ports on the chip
(one 6 bit and one 8 bit, taking 14 of the pins) and the others are
OSC1, OSC2, RESET, IRQVPP, VDD and VSS.  Other than that, there are a
lot resistors and other chips I can't identify, and I am having a hard
time just tracing the lines directly from the chip because they go
under it, etc.

Any suggestions on where to go from here?  I am going to be needing a
lot of basic info before I tackle this.  Is it even doable?  I think I
may have found some freeware (PonyProg2000) that might be able to help
me make sense of the signals, but I am not sure how to hook it up to
my computer.  Is there any place I can buy (cheaply) or make (simply)
a parallel interface like Spehro  mentioned?  I am going to have to
spend a lot more time looking around the newsgroups before I can ask
some more intelligent and original questions (there seems to be a lot
of talk on this subject, and I don't want everyone to have to repeat
themselves if they already answered a similar question).

The main device has a mc68hc908mr16cfu chip in it.  The spec sheet has
this to say:
UART asynchronous communications
Flexible baud rate generator
Double buffered transmit and receive
Optional hardware parity checking and
Asynchronous communication between
the MCU and a terminal, computer or a
network of microcontrollers.

Full-duplex 3-wire synchronous
Maximum master bit rate of 4 MHz for
8 MHz system clock
High-speed synchronous communication
between multiple MCUs or between
MCU and serial peripherals
Cost-effective serial peripheral
expansion to EEPROM, high-precision
A/D and D/A converters, real-time
clocks, etc.

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