Reading the value of FVR using ADC

[David Stephenson]

Is there a way of reading (16F18426) the fixed voltage reference(FVR) like READad12(FVR).
I know I could do it by setting the register ADCON0.
In case you're wondering why, it's to check the battery voltage level. Wth Vref+ set to Vdd and reading the FVR should give an indication of the voltage level (and charge left).

[kent_twt4]

I know what you mean, but don't see how you get away with that yet. For readability I have used a define constant in this situation for a 16f1827 like:

#define FVR = 31  'there is the ADCON0 part
...
...
Vref = ReadAD(FVR)

[David Stephenson]

I've got something to work I switched to a 16F18326 as the ADC is a little easier to understand (and I don't need 12-bits).
I am a little confused by the datasheet. It suggested to check the FVRRDY of the fvrcon register to make sure that the voltage reference has stabilised yet further on it states that this is always "1" (see attachment). Can anyboby enlighten me as to whether this bit can be used to determine if the FVR is ready.

[kent_twt4]

I don't think you will find a difference. Put a toggle led at the end of the adc conversion, and the adc conversion itself in an endless loop. Test both ways with DVM Hz. You shouldn't see more than about a 4 instruction cycle difference for the FVR ready instructions. Both methods should give valid readings from my experience.

[David Stephenson]

Below is the code. This gives a reading of the battery voltage (higher reading = lower voltage).
I am using a LCD display that needs to have the contrast adjusted as the voltage falls. Good batteries LDC background is black and low battery it is difficult to read.
Another question:
The datasheet uses GOTO $-1 to jump back one instruction yet using this is generally considered "bad form" anybody have a strong view? Should I use GOTO linelabel instead I suppose BRA $-1 would have also worked.

SUB BATLEVEL
'FVRCON=b'00000001' '1.024 V
FVRCON=b'00000010' '2.048 V
ADCON1=b'11000000' 'SET REFS AND Fadc WITH Fosc=4MHz
BAT2=0
BAT1=0
BSF FVRCON,FVREN 'ENABLE FVR
NOP
BTFSS FVRCON,FVRRDY 'wait for FVR to stabilise
GOTO $-1
ADCON0=b'11111100' 'SET adc to read FVR
REPEAT 64
BSF ADCON0,ADON 'adc on
WAIT 20 US 'AQUISITION DELAY
BSF ADCON0,ADGO 'start conversion
NOP
BTFSC ADCON0,ADGO 'check for coversion complete
GOTO $-1
BCF ADCON0,ADON 'turn off adc
BAT1=ADRESL
BAT1_H=ADRESH
BAT2=BAT2+BAT1
END REPEAT
BCF FVRCON,FVREN 'DISABLE FVR
END SUB

[mmotte]

David,
This has nothing about FVR but it is the way i checked battery condition in one project. I made a voltage divider between battery+ and Gnd out of two 10K resistors and sent the middle to ADC0.

Sub LED_BLINK_BAT
          batValue = ReadAD(AN0)  ' 5.79 bat volt = 168
                                    '5.2 v bat = 151  new bat =6.4 =184
                                    '% useful bat = (adc-151)/3
          If batValue > 181 then batValue = 181
          If batValue < 154 then batValue = 154

          If batValue >= 160 then
          percent = (batValue -151)/3
          FlashAMBER(percent)
          end if
          If batValue < 160 then
          percent = (batValue -151)/3
          FlashGRN(percent)
          end if

end sub

sub FlashGRN( times)
    Repeat times
    PulseOut GRNLED, 150 ms
    Wait 400 ms
    End Repeat
end sub

The processor runs two diodes below the battery because I needed the 6vdc for the servos. I mention this because of the numbers being used and the adc using the the full processor voltage for top of scale which moves as the battery wears down.
This is a rough estimate of amount % of battery left and is working fine in my project.

Best regards
Mike

[David Stephenson]

Not sure how that works don't the diodes just drop a fixed amount and your voltage divider halves the ~6vdc.
The problem is (as usual) I am running out of pins so I wanted a solution that did not use up an extra pin.
I've taken to using microchip's LDO voltage regulators (MCP1700) to drop voltage when it is required.

[kent_twt4]

Just a thought on the lcd contrast problem, you could consider a small buck/boost module to maintain the 5V required for the character lcd? Would need a shutdown mechanism or under voltage protection.