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.

The internal oscillator needs to be calibrated and this is achieved by reading a factory programmed calibration setting and writing it into the OSCCAL register during initialisation of the device by the application software.

The calibration word is located at the last address in the user program memory area, address 0x3FF.  It is in the form of a RETLW instruction and the user code should include a CALL 0x3FF instruction which will return with the calibration setting in the W register. This can then be written into the OSSCAL register.

Problems arise if by accident or otherwise, the program memory at address 0x3FF is erased or over written.  Since the calibration value is unique to each individual PIC there is no way to know what it was, but it is possible to recover it by recalibrating against a known frequency.

That's where this software and circuit come into their own.  Load a PIC with the code on this page and drop it into the circuit described here and within a couple of seconds it will provide a new calibration value to ensure the internal oscillator runs within 1% of 4Mhz.

PICkit 2 update

If you have a PICkit 2 programmer, get version 2.50 (or later) software from the Microchip website.  This includes a menu option to recalibrate and reprogram the OSSCAL setting in one operation.

How it works

In order to calibrate the internal oscillator a known reference frequency is needed. Fortunately we don't need signal generators or calibrated test equipment for this. In fact an accurate reference is available from the AC utility electric supply.  In most parts of the World the utility electricity supply is generated at a frequency of either 50 or 60Hz (many digital clocks take advantage of this fact to keep time)

Using almost any transformer (or Wall Wart) with a 6 to 12 volt RMS AC output we can obtain an accurate reference source to calibrate the PICs oscillator against.

Schematics, Construction, and Code

So let's get on with it.  Construct the circuit shown below using a piece of strip or pad board, or just hook it up on a solderless breadboard.

• (circuit schematic )

The frequency of the internal oscillator in the PIC varies with both changes in temperature and supply voltage.  As the supply voltage increases the frequency decreases slightly.  When switch S1 is open the 5 volt supply to the PIC is dropped across the two diodes to about 3.4 volts.  With S1 closed, the PIC operates at 5 volts.  Normally you will want to calibrate the PIC while running at 3.4 volts, but by closing S1 you can perform the calibration at 5 volts. 

The two 1N4148 diodes provide the voltage drop and the 150R resistor draws sufficient current to ensure the voltage across the diodes remains stable. 

The 100nF decoupling capacitor should be placed as close as possible to the power pins (1 and 8) of the PIC.

The reference signal is supplied using the BC548 transistor, 10K resistor, 1N4148 diode and transformer.  You can use any general purpose NPN transistor here as it is not critical.  The transformer is also not too critical but should output between 6 and 12 volts AC RMS.

It is important that input to the transistor is an AC signal. Don't use DC, not even unfiltered  DC (no smoothing capacitor)

DO NOT UNDER ANY CIRCUMSTANCES CONNECT THIS CIRCUIT DIRECTLY TO THE UTILITY POWER

PIC code for recalibration

Next you need to program the PIC to be recalibrated with the code below. The same code will work with both the PIC 12F629 and 12F675 devices.  Download it now and and program it into the PIC to be calibrated.

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

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Once you have successfully recalibrated the PIC and read back the new calibration value you can reprogram the PIC with your own application code.  This software is only needed to calculate the new calibration value, it doesn't need to remain in the PIC afterwards. (someone will ask believe me!)

Running the calibration

1. Close J1 if the utility power in your area is generated at 60Hz and leave it open if it is 50Hz.  Make sure you get this right or the calibration will either fail or worse, be incorrect.

2. The AC reference signal should be present before powering up the PIC under calibration to ensure the signal is stable and running.

3. Insert the PIC into the socket, open switch S1 and apply 5 volts to the circuit.

4. When the PIC starts both the LEDs will blink once.
    
5. If no reference signal is detected on GP2 (pin 5) the fault LED will light and the 'done' LED will blink until a signal is detected. (If this happens, turn the power to the PIC off and on to ensure it calibrates correctly)
    
6. During the calibration process both LEDs are off.  The calibration will take less than 5 seconds to complete.
    
7. If the calibration failed the red LED will come on and the code will halt.
    
8. If the calibration is successful the green ' done' LED comes on and a 5Khz test signal will be generated on GPIO1.  If you have a frequency meter handy you can use this to verify the calibration.
    
9. Once calibration is complete, you can open and close switch S1 while monitoring the 5KHz test signal to see the change in frequency vs. supply voltage.
    
10. Turn the power off, remove the PIC from the test circuit and put it in your programmer.  Now read back the PIC and inspect the contents of the EEPROM memory

When you read back the EEPROM it will contain one of three pairs of values in addresses 0x00 and 0x01 as follows: ý EEPROM addresses 0x00 and 0x01 contain 0xFF then the code failed to run correctly. ý EEPROM addresses 0x00 and 0x01 contain 0x00 then the code did run but it has failed to set the calibration word correctly (finished with fault LED on).  Check that your reference clock is running at the correct frequency, J1 is set correctly and then retry. þ EEPROM address 0x00 contains 0x34 and address 0x01 contains 0xNN where NN is the new calibration setting. then the code ran successfully (finished with 'done' LED on) and  calibrated the OSCCAL value correctly.  Use the 0xNN value in address 0x01 for the calibration memory.

Using the new calibration word

IMPORTANT: This code will calculate the correct calibration word but it can't save it back into the PICs program memory - You must do this manually using your PIC programmer.    If you calibrate a PIC where you still know the factory calibration value you may find the value produced by this circuit is slightly different from the original factory value.  This is normal and is down to differences in supply voltage and temperature.   You can verify the 5Khz test signal on GP1 with a frequency meter after calibration is complete and it should be within 1% of 5Khz.

If calibration completes successfully, remove the PIC and place it back into your programmer.  Read back the data from the PICs EEPROM memory (not program memory) where the newly calculated value has been saved. (see example below)

Address 0x00 will contain 0x34 and address 0x01 will contain the calibration value.  It is this value that needs to be written back to program memory location 0x3FF using a RETLW instruction.  Some programmer software will allow you to enter the calibration value and program it back into the device for you.  If your programmer doesn't do this then you need to put a RETLW 0xNN instruction at address 0x3FF where 'NN' is the hex calibration value read from the EEPROM.