- Published: Thursday, 07 February 2013
- Written by Jon Chandler
- Hits: 6143
The PICkit 2 has a handy feature to vary the voltage supplied to a connected chip. In addition to being needed to program low voltage parts, the ability to change VDD when using a PICkit 2 is very handy for testing things like the HLVD module. The PICkit 2 must be calibrated to ensure the VDD setting is accurate.
The PICkit 2 is powered from the USB port; the voltage supplied by a USB port is nominally 5 volts but the voltage can be as low as 4.2 volts depending on the USB port used. The PICkit 2 does not use a voltage reference for its ADC or for setting the VDD output voltage. Without calibration, the USB supply voltage is assumed to be 5 volts - if it is not, the voltage setting and detection will be inaccurate.
The calibration procedure is simple, but requires a DC voltmeter. The calibration will be accurate for only the USB port the PICkit 2 is connected to; if the connection is changed, the calibration should be checked and the procedure repeated if necessary. The calibration voltage is stored in EEPROM. Note that the USB supply voltage is the maximum that can be supplied for VDD; it has no ability to increase this voltage.
Note: Not all PICkit 2 clones are created equally. Some clones have no provision to vary VDD. The voltage may be changed in the GUI, but the output voltage will not change.
The calibration procedure requires only a few minutes and a DC voltmeter connected to pin 2 (VDD) and pin 3 (GND).
The following screen shots show the process, starting from the PICkit 2 GUI.
The first dialog box explains the need for calibration and when it should be repeated.
Heed the warning in the next dialog box. Do not proceed if you do not have a volt meter connected.
Next, enter the voltage reading from the voltmeter and click the button to calibrate.
Optionally, you can give the PICkit 2 a name. This is helpful if you have more than one.
That's all there is to the procedure. And the result? When I set my PICkit 2 to provide 5v VDD, I measured 4.933 volts, the maximum possible using my USB connection.