I'm still looking for clues as to why the PIC will not do the entire voltage
range with it's A-D.

Clues so far:  Lowpass filtering the voltage to the A-D got me .5 volt
improvment.
(Went from failing at 3.56 volts to 4.07 volts.)

Slowing the conversion by a factor of 4 got me an additional .07 volts.
(It fails at 4.14 volts now.)

Perhaps some one can spot a code error in my source.

;
 RADIX DEC   ; Specify Decimal default Radix.
 list      p=16c73a ; list directive to define processor
 #include <p16c73a.inc> ; processor specific variable definitions
;
;******************************************************************************
; '__CONFIG' directive is used to embed configuration data within .asm file.
; Set the PIC configuration for: CodeProtection OFF, WatchDogTimer OFF,
; BrownOutDetectENable ON, PoWeR-upTimerEnable ON, EXTernal OSCillator enabled.
;******************************************************************************
;
 __CONFIG   _CP_OFF & _WDT_OFF & _BODEN_ON & _PWRTE_ON & _XT_OSC
;
;******************************************************************************
;
 ORG 0x00
 GOTO Restart
;
 ORG 0x04
;************************* Restart Program ************************************
; This Program is called on a power on reset.
; The routine:  Initialize the A-D, PORTs A,B,C, Start the Scheduler.
;******************************************************************************
;
Restart BSF STATUS, RP0 ; Select Bank 1 Registers.
  MOVLW B'00000100' ; Form a byte to signify:
  MOVWF ADCON1 ; Set RA<5> LED Driver.
     ; RA<4:3> unused analog inputs.
     ; RA<2> LED Driver.
     ; RA<1:0> as Sensor Analog inputs.
  BCF STATUS, RP0 ; Select Banks 0 Registers.
  MOVLW B'01000001' ; Form a byte to signify:
  MOVWF ADCON0 ; Set Analog to Digital as:
     ; <7:6> Fosc/8
     ; <5> Read Analog Channel 0.
     ; <2> Stop A-D.
     ; <0> A-D on.
Init_PORTA BSF STATUS, RP0 ; Select Bank 1 Registers.
  MOVLW B'00011011' ; Form I/O pins Definitions.
  MOVWF TRISA  ; Set RA<5> LED Driver.
     ; RA<4:3> unused analog inputs.
     ; RA<2> LED Driver.
     ; RA<1:0> as Sensor Analog inputs.
Init_PORTB BCF STATUS, RP0 ; Select Banks 0 Registers.
  CLRF PORTB       ; Clear all Port B outputs.
  BSF STATUS, RP0 ; Select Bank 1 Registers.
  CLRF TRISB       ; Set Port B all outputs.
      BCF STATUS, RP0 ; Select Banks 0 Registers.
Init_PORTC CLRF PORTC       ; Clear all Port C outputs.
  BSF STATUS, RP0 ; Select Bank 1 Registers.
  CLRF TRISC       ; Set Port C all outputs.
  BSF PIE1, ADIE ; Enable A-D IRQs.
  BCF STATUS, RP0 ; Select Bank 0 Registers.
Clear_IRQs CLRF INTCON ; Clear all Interrupts.
  BCF PIR1, ADIF ; Clear any random A-D IRQ.
Start_A2D BSF ADCON0, GO ; Start the A to D processes.
;
;******************************* Scheduler ************************************
; The Scheduler checks all task STATEs for "Active", (Non zero)
; activates the associated task for each state.
;******************************************************************************
;
Scheduler BTFSC ADCON0,2 ; Is the A-D Ready?
  GOTO Scheduler ; No, Return to scheduler loop.
;******************************* NEW SWR **************************************
; Write the forward and reflected power to 1 ma. panel meters.
;******************************************************************************
;
NEW_SWR BTFSS ADCON0,3 ; Yes, Are we doing the Reflected sensor?
  GOTO Read_F ; No, Go process the Forward sensor.
Read_R BCF ADCON0,3 ; Yes, Set A-D to Read (Forward)Analog Channel 0.
  CALL P_Table ; Convert the value to the output current.
  MOVWF PORTC  ; Send the Reflected power meter current.
  BCF PORTA,5 ; Set the Status LED Red.
  BSF PORTA,2 ; Set the Status LED Red.
  GOTO Read_Exit ; Return to scheduler loop.
;
Read_F BSF ADCON0,3 ; Set A-D to Read (Reflected) Analog Channel 1.
  CALL P_Table ; Convert the value to the output current.
  MOVWF PORTB  ; Send the Forward power meter current.
  BSF PORTA,5 ; Set the Status LED Green.
  BCF PORTA,2 ; Set the Status LED Green.
Read_Exit BSF ADCON0, GO ; Start the A to D processes.
  GOTO Scheduler ; Return to scheduler loop.
;
;******************************************************************************
P_Table MOVF ADRES,W ; Fetch the sensor voltage.
 ADDWF PCL,1 ; Index into the Power table.
 RETLW  0 ;
 RETLW  1 ;

etc...

--
Thomas C. Sefranek  WA1RHP
ARRL Instructor, Technical Specialist, VE Contact.
http://www.harvardrepeater.org
http://hamradio.cmcorp.com/inventory/Inventory.html