How to use the HLVD module as a low voltage monitor

In hopes this might help someone avoid the time it took me to learn this I will make this contribution of code as a solution to using the HLVD module as a low Vdd voltage monitor. I was able to resolve this with some offline support from Darrel Taylor. This process has been tested and it works!

First, you need to make sure you have setup a DT_INTS-18 interrupt for the HLVD module by inclusion of this code ahead of your main program:

Code:

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ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        ;INT_Handler  USB_Handler
        INT_Handler  HLVD_INT,        _LowVolt, PBP,  yes
    endm
    INT_CREATE                ; Creates the interrupt processor
ENDASM

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Then you need to insert this code in your main program loop to setup the use of the HLVD module.

Code:

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 ' Set registers for using HLVD feature per section 24.2 of the 18F4550 data sheet
          ' Step 1-Disable the module by clearing HLVDCON.4
                        HLVDCON.4 = 0        ' Implement Step 1         
          ' Step2 HLVD3:HLVDL0=1101 'Set the trip point at Vdd=4.33 vdc
          ' Step3 HLVDCON.7=0 'Set VDIRMAG=0 to detect low voltage transition
          ' Step4 HLVDCON.4=1 'Enable the HLVD module
                      HLVDCON = %00011101  'Implements Steps 2-4
          ' Clear the HLVDIF interrupt flag (PIR2<2>)
                        PIR2.2 = 0

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Then you need to create an interrupt service routine (ISR) near the end of your code as follows:

Code:

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LowVolt:
  ' Blink LED_RED 5X to indicate Low Battery voltage detected
        For i = 0 to 4
          HIGH LED_RED
          Pause 500
          LOW LED_RED
          PAUSE 500
        Next
@ INT_DISABLE HLVD_INT      ; This statement very important, else code will lockup on exit
                                            ; from the ISR to main program.
        ' Resume Main Program
@ INT_RETURN
'------------------{ End of Interrupt Handlers }-------------------------

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Then you need to insert this assembler statement in your code wherever you want to test for a low Vdd voltage condition. I insert this in my main loop so that regardless of what other events are occuring, whenever the above ISR completes, the mainloop sets up the HLVD module for another low voltage interrupt.

Code:

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@    INT_ENABLE  HLVD_INT              ; enable HLVD interrupt

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I hope this might help someone! ;)

I cannot get this example code to compile - syntax error after step 2 . I tried several different versions of the statement, but no luck.

This will be expanded to a much larger program, and used with a 18f2520 once I get this simple part to compile.
I am using PBP 2.60

Code:

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      DEFINE OSC 4
      OSCCON = %11101110 ' USE THIS, AND CHANGE CONFIGS FOR INT OSCILLATOR
@ __CONFIG    _CONFIG1H, _OSC_INTIO67_1H  & _FCMEN_OFF_1H & _IESO_OFF_1H

TRISC = %00000000
 
  i var byte

    ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
       
        INT_Handler  HLVD_INT,        _LowVolt, PBP,  yes
    endm
    INT_CREATE                ; Creates the interrupt processor
ENDASM

MAIN:

          ' Step 1-Disable the module by clearing HLVDCON.4
    HLVDCON.4 = 0    ' Implement Step 1         
          ' Step2
    HLVD3:HLVDL0=1101 'Set the trip point at Vdd=4.33 vdc
 
          ' Step3
         
      HLVDCON.7=0 'Set VDIRMAG=0 to detect low voltage transition
          ' Step4 HLVDCON.4=1 'Enable the HLVD module
      HLVDCON = %00011101  'Implements Steps 2-4
          ' Clear the HLVDIF interrupt flag (PIR2<2>)
      PIR2.2 = 0

 PAUSE  1000
    GOTO    MAIN



LowVolt:
  ' Blink LED_RED 5X to indicate Low Battery voltage detected
        For i = 0 to 4
          HIGH PORTC.0
          Pause 500
          LOW PORTC.0
          PAUSE 500
        Next
@ INT_DISABLE HLVD_INT      ; This statement very important, else code will lockup on exit
                                            ; from the ISR to main program.
        ' Resume Main Program
@ INT_RETURN
'------------------{ End of Interrupt Handlers }-------------------------

@    INT_ENABLE  HLVD_INT              ; enable HLVD interrupt

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Sorry - I guess I "Jumped the Gun" so to speak. I commented out step 2, and it works fine. By the way, my listing does not show DT's interrupt includes.
Once again, sorry for not seeing the obvious.

Ken