HC705 serial communication

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 communicate?

  1. 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?

  2. 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?

  1. 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.

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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.

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.

An oscilloscope would be the easiest way.

Best regards, Spehro Pefhany

"it's the network..."                          "The Journey is the reward"
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Spehro Pefhany

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?

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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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.

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Chris Pflieger

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: SERIAL COMMUNICATIONS INTERFACE ? UART asynchronous communications system ? Flexible baud rate generator ? Double buffered transmit and receive ? Optional hardware parity checking and generation ? Asynchronous communication between the MCU and a terminal, computer or a network of microcontrollers.

SERIAL PERIPHERAL INTERFACE ? Full-duplex 3-wire synchronous transfers ? 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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