July 1998, Issue 96

A PIC-Based AC Power Meter

DISPLAY CONCERNS

Now that I’ve discussed how the E accumulator is maintained, I want to turn to how the energy accumulation is displayed.

If watt-hour readings are the finest resolution displayed, then the user might have to wait 2 min. to see 0.001 kWh, depending on the load. To solve this problem, I implemented autoranging using three display modes.

Display mode d1 displays readings in the range of 0.000–9.999 Wh. Once 10 Wh are accumulated, display mode automatically switches to d2 mode.

The d2 mode displays readings in the range of 0.010–9.999 kWh. Once 10 kWh are accumulated, the display mode automatically switches to display mode d3, which displays readings in the range 10.00–16.77 kWh.

Display mode d1 is implemented by converting the low-order 16 bits of E (milliwatt-hours) to BCD, then displaying the four least significant digits with the decimal point three places to the left (i.e., 0.000). The move of the decimal point effects a multiply by 1000, causing watt-hours to be displayed.

Display mode d2 requires display of kilowatt-hours, and has a least significant digit of watt-hours. Divide E (in milliwatt-hours) by 1000 to get the result in watt-hours. This is implemented by using the upper 16 bits of E, effecting a divide by 256 operation on E.

Recall that canned math routines support 16-bit math, not 24-bit math. So, by taking the upper 16 bits of E and performing a divide by 4, a divide by 1024 is implemented, which approximates the correct divide by 1000. This result is converted to BCD, and then the four least significant digits are displayed with the decimal point set three places to the left.

Display mode d3 also requires display in kilowatt-hours. The only difference between the d2 and d3 modes is that d3 needs an additional divide by 10 to provide a reading in the range (10.00–16.77). By taking advantage of the fact that the BCD conversion routine returns a five-digit result, a divide by 10 is done just by displaying the four most significant digits and placing the decimal point two places to the left.

The LCD services are designed such that the LCD routines expect data where the BCD math routine deposits its result. Although I don’t elaborate on these routines in detail here, I mention them because they cooperate with the PIC math routines well.