PIC LED DVM

[KRISTOFFER]

The older PIC LED DVM modules had a resolution of 1 decimal point and could read 00.0V to 99.9V. The newer versions seem to have a range change built in, reading from 0.00V up to around 9.99V then chaging to 10.0V all the way up to 99.9V. Handy if that is what you are after but unfortunately I am not. EBay seems to have non of the older non-ranging modules left. I was wondering if I could lift the PIC program from one of the older versions PIC and use it to reprogram a newer autoranging version back to the older non autoranging version.

I seem to remember reading on the internet that there were some break out points on the pcb's where the manufacturers programmed the chips after production. Either way can I do it with some computer software and some temporary wires connected to the board. Or, is the program protected in some way to prevent it being read from the chip. I am fine with analog electronics, struggle with programming, but persistent enough to get there.

[Kleinstein]

With most products the µC is protected, so that it is not possible to simply read out the program.  It is mainly with hobby and open source projects that may allow to read the program.
Even of one could it would be very difficult to modify that program in the binary form. To modify the program one would need access to the source code.
On the other side it is often possible to overwrite the program with your own or a copy to get from somewhere.

There are quite a few DVM / panel meters available - few use a PIC at all. Some may share a similar hardware and could thus run the same program, but some can be different. Even more minor differences (e.g. use different pins, different clock, more modern chip) can be enough to not work with the same binary code.
I would consider the fixed range the more normal case with those DVM modules. If one does not want to change the code, it would not matter what µC is used.

[jpanhalt]

SparkFun used to sell a 4x20 line LCD module with SPI interface.  I bought one and found the programming was unusable.  It had a PIC16F88 which is pin compatible with a later 16F18xx.  So, I removed that chip, added the more modern chip and wrote my own program for it. 

Yes, what you suggest might be doable.  How much work is involved would depend on the details.  Are you sure the MCU is a PIC?  If so which one?  Can you attach a picture of the board so we can see whether the ICSP connections are present?  Can it be read?  If not, then you will have to write your own.  Writing your own is not that bad, if you can determine the layout.  Is the display a standard display that you can access or at least determine the pinout to the MCU? 

I agree that simply copying from your older version to the newer version is unlikely to be successful because the code is probably protected.  As a first step, see if you can read the code in your current meter.

[rdl]

Surely there are still 7106/7107 based meters available? I figured those chips were copied years ago. I have some of those small, auto-ranging type. I think mine use some kind of Holtek chip but I'm not sure.

[KRISTOFFER]

Thank you for your replies. It will be no suprise to you that they always fit a chip with no numbers, probably to stop rival companies copying it. However always a 14 or 16 pin chip. I can recall coming across a webpage on the internet years ago, covering the topic in some detail. From what I can remember it started off as someones pic project idea and very soon marketed. Hence why I have always assumed they were pic's.  The person who posted their webpage encluded the actual program and commented that there was not much you could do with it to make it any better or worse since it basically did the job. He also mentioned the PCB pads on the boards to program the device after manufacure which he used himself to lift the program. I will have a dig around and see if I can find it to post on here. Just out of interest, you can modify them easily enough. If you take out the decimal point resistor you have a DVM that reads X10.
Even better you can alter the input voltage divider. It is normally set to divide by 30 plus a bit either side if a pot is fitted. EG change it to divide by 300 and you have a 999 volt meter. And so forth. The problem then exists when it has the auto range program. Hope to be positing the circuit diagram and prgram as soon as I find it. 

[jpanhalt]

Thank you for the update.  I wouldn't put too much faith in the assumption the commercial product is a direct copy of what you found on the web.

Just guessing that it is an 8-bit PIC.  If that's the case, there will always be 3 pads for programming (DAT, CK, MCLR).  Usually 2 more (VDD, VSS) and perhaps a 6th for PGM.  How many pads do you see?  VCC, VSS are usually predictable (except some older versions) and MCLR is relatively predictable.  On a breakout, they can be in any order, as one can modify the attachment cable.  The usual order is MCLR, VCC, GND, DAT and CLK. That would be easy to trace out on the PCB you have.

[KRISTOFFER]

Hands up, I got it wrong and you are right. STM8S003f3 typically. I backtracked but could only come up with this site.
http://smokedprojects.blogspot.com/2013/08/i2c-led-display-from-hacked-voltmeter.html.
Currently chasing through CD's and DVD's to find what I saved years ago, hopefully the program, and will post ASAP.

[KRISTOFFER]

And this which is probably where the confusion started all those years back.
https://www.allaboutcircuits.com/projects/make-a-digital-voltmeter-using-the-arduino/

[KRISTOFFER]

AND this, even more confusing.
https://www.zl2pd.com/PeakReadDVM.html