How can I detect the voltage of VDD with only internal resources?

Update:  contrast not contract.

Bob,

That was very helpful.  Although it is not an ADC (more like a comparator), I got it to work with some minor caveats.

Code snippets:

//*****************************************************************

//

uint8  vdd_voltage(void)

{

result=CY_LVD_THRESHOLD_2_50_V; //set to lowest expected VDD value

     for(result=CY_LVD_THRESHOLD_2_50_V; (result<=CY_LVD_THRESHOLD_4_50_V); result++)

     {

          CySysLvdEnable(result);

          CyDelay(10);

          if(CySysLvdGetInterruptSource() == CY_SYS_LVD_INT)

          {

          break; // If int set, break out.

          }

     }

     CySysLvdClearInterrupt();

     return(result);

}

lcd_contrast[] =

{

100, // CY_LVD_THRESHOLD_2_50_V

100, // CY_LVD_THRESHOLD_2_60_V

100, // CY_LVD_THRESHOLD_2_70_V

100, // CY_LVD_THRESHOLD_2_80_V

90, // CY_LVD_THRESHOLD_2_90_V

80, // CY_LVD_THRESHOLD_3_00_V

75, // CY_LVD_THRESHOLD_3_20_V

65, // CY_LVD_THRESHOLD_4_50_V

40, // > CY_LVD_THRESHOLD_4_50_V (16u)

};

//*****************************************************************

//

void main(void)

{

uint8  vdd_idx;

     CyGlobalIntEnable; /* Enable global interrupts. */

     all_LEDs(1);  /* Add load to VDD */

     LCD_Seg_Start();

     vdd_idx = vdd_voltage() - CY_LVD_THRESHOLD_2_50_V;

     LCD_Seg_SetContrast(lcd_contrast[vdd_idx]);         /* Set the contrast for VDD voltage compensation */

     all_LEDs(0);    /* Remove load from VDD */

...

}

CAVEATS!!!

• The LVD thresholds aren't linear. Most are 0.1V to 0.2V steps.  The last one jumps from 3.2V to 4.5V.
• VDD needs time to settle.  I received lower threshold/voltage values if I quickly cycle the power on and off.  That's why my code snippet includes turning on and off the LEDs (higher current draw on VDD) before and after the test.

VSRS,

Thanks for your input.  I've created designs with battery voltage monitoring.  Sadly with this design, I have no spare inputs.  Yuk!  Oh well!

Len