There is a post by Darrel re this but I can't find it. I don't have any PICBasic code to hand but this in Proton might help -

Code:
  ADCON1 = %00100000                  ' ADCS=010 Fosc/32 4usec
  Symbol Go_Done = ADCON0.1
  Symbol ADON = ADCON0.0
  Symbol VP6EN = VRCON.4              ' 0.6V Reference Enable Bit

  Dim Ad_Result As ADRESL.Word        ' Convert the ADRESL register into a WORD variable
  Dim VDD       As Word


  VP6EN = 1                           ' Turn 0.6V reference ON
  ADCON0 = %10110101                  ' right justify, Vdd, select 0.6v ref, ADC on   
  DelayUS 100                         ' Wait for sample/hold capacitors to charge and VP6 to settle
  Go_Done = 1                         ' Start conversion
  While Go_Done = 1 : Wend            ' Poll the GO_DONE flag for completion of conversion
  ADCON0 = %10110100                  ' right justify, Vdd, Select 0.6v ref, ADC off   
  VP6EN = 0                           ' Turn 0.6V reference OFF
  VDD = 6144/Ad_Result                ' convert input reading to VDD voltage *VDD must be a Word
                                      ' 0.6 * 1024 = 614.4 - 6144 To get 1/10th
George