Description

The PIC 12F629 and 12F675 devices have an internal 4Mhz oscillator that enables the devices to be used without an external crystal or RC network.  This frees up one or two pins for I/O use and allows the device to be built into minimum component count designs.

Problems arise if by accident or otherwise, the factory programmed oscillator calibration word, located at program memory address 0x3FF is erased or over written.  If application code tries to read the calibration word and it has been erased, the code will normally crash.

It turns out that erased calibration words on the 12F629/675 PIC are the cause of almost all queries relating to code on my website that uses a 12F629/675.  People using JDM type programmers and associated software seem to have this problem more than most.  This is, in part I suspect, due to the free software / cheap hardware used by the JDM programmer that attracts inexperienced users.

My advise to people with problems is to put the PIC into their programmer hardware and read out the program memory to see if the calibration word is present.  However, experience again shows that many users just become even more confused so I've written this application to give a quick Good/Bad test of the calibration word.  There's also an optional bit of hardware that will display the value of the calibration word if it's present.

How it works

If the calibration word has been erased, when the application code tries to read it, instead of returning with the calibration value, it wraps back round to the start of program memory as if it had been reset.  When the PIC starts after a Power-on-reset (POR)  certain internal registers are initialised to known values.  If the code wraps back round to the reset vecotr because of a missing calibration word, unlike the Power-on-reset, these registers are not initialised.

The application use the fact that this wrap-around doesn't affect the internal registers so if we alter the value in a specific register after a power-on-reset we can tell if it has had a Power-on-reset or the code has wrapped back round due to a missing calibration value.

The code indicates the result of the test as Good/Bad by blinking an LED; green LED meaning the calibration word is present, and red LED if it has been erased.

With some extra hardware around a 74HC595 it can also display the value of the calibration word, if present, as a binary number using eight LEDs.  This is optional and the good/bad test works without it being present.

Hardware & Schematic

click for full size

For the basic calibration good / bad test, you only need to build the circuit on the left of the dotted line in the schematic. I deliberately kept it simple so you can quickly and easily build the test circuit.

The circuit around 74HC595 is only used to display the binary value of the calibration word. I added it for a bit of fun, you don't need it if you just want to test for the presence of the calibration word.

Software

Download HEX code  (for use with either 12F629 or 12F675)

Source code ASM (if you want to see how it works)

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Running the test

Disclaimer. This test will normally correctly identify an erased calibration word. However, I have to say this to cover myself; it is possible that for some peculiar issues it just might return an incorrect result.

1. Program the PIC to be tested with this code
   Download HEX code (for use with either 12F629 or 12F675)
    

2. Insert into the test circuit and apply power
    

3. Result:
   If the Green LED blinks the calibration word is present
   If the Red LED blinks, the calibration word is missing

   	a Okay. Calibration word is present, no problem.
   	r  Bad. Calibration word missing, you must correct this.       Recalibrate the PIC using the code here

    

   The photo below shows a PIC being tested with a good calibration word. The actual calibration value is displayed in binary on the 8 LEDs.  The calibration value is unique to each PIC so the devices you test will probably show a different value to this example. If you're having programmer trouble then I suggest you look at the Microchip PICkit2 programmer, or a hardware clone of the same.  The Microchip software used with this programmer makes erasing the calibration word by accident almost impossible. If you do erase it then it also makes fixing the problem easy too!

   Screendump of the same PIC being programmed with the test code in my PICkit2 programmer. You can see the PICkit2 software has read the OSSCAL word from the PIC (circled red).  34 is the RETLW instruction and 70 is the value being returned.