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:
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
Then you need to insert this code in your main program loop to setup the use of the HLVD module.
Code:
' 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
Then you need to create an interrupt service routine (ISR) near the end of your code as follows:
Code:
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 }-------------------------
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:
@ INT_ENABLE HLVD_INT ; enable HLVD interrupt
I hope this might help someone!
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