GCBASIC documentation
Fixed Voltage Reference

Syntax:

    FVRInitialize ( FVR_OFF | FVR_1x | FVR_2x | FVR_4x  )

Command Availability:

Available on all Microchip microcontrollers with the Fixed Voltage Reference (FVR) module.

Explanation:

This subroutine sets the state of the FVR.   

FVR_OFF = Fixed Voltage Reference is set to OFF

FVR_1x = Fixed Voltage Reference is set to 1.024V

FVR_2x = Fixed Voltage Reference is set to 2.048V

FVR_4x = Fixed Voltage Reference is set to 4.096V

Using the 16F1828 device’s datasheet as a general case (datasheet, from the device’s product page at product page), parameter AD06 in table 30-8 on page 359, and the corresponding Note 4, show that the Vref voltage (Vref+ minus Vref-) should not be less than 1.8V, regardless of the reference voltage used, for the ADC module to work within the datasheet specifications.   Also, since Vref- cannot be a negative voltage (voltages below GND), the lowest voltage on it is 0V.  This means an FVR of 1.024V cannot be used as VREF+ for the ADC — only the 2.048V and 4.096V values can.

The 1.024V FVR value still exists for use with other modules, not just the ADC module.

Example:

    '// use FVR 4096 as Reference
    FVRInitialize ( FVR_4x )          ' <<< the FVRInitialize instruction
    wait while FVRIsOutputReady = false
    ADVal = ReadAd(AN0)

    '// Turn off FVR
    FVRInitialize ( FVR_Off )

Key line: FVRInitialize ( FVR_4x ) — switches on the internal Fixed Voltage Reference at 4.096V, ready to be used as the ADC’s voltage reference once FVRIsOutputReady confirms it has stabilised.

See Also: