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A PIC-BASED DIGITAL VOM

by Duane Perkins

Start ý The Circuit ý Construction ý Testing ý Program Code ý Sources and PDF

PROGRAM CODE

calibrat.asm and vom.asm are the same except that calibrat.asm displays the ADC count for calibration purposes. The main loop executes about once per second and updates the display. The secondary loop is executed 256 times. It generates a ramp of 13,312 cycles of the 8-MHz oscillator (1664 ýs). As shown in Figure 3, this allows the ramp to reach its maximum voltage well after the highest possible count has been captured.

Using RA1AN1 requires programming the ADC function to shift the input voltage upward by about 0.5 V, thus avoiding inaccuracy that could result in a low count. First, a count for the input voltage plus the shift voltage is captured, then a count for the shift voltage alone is calculated. The latter is subtracted from the former so as to make the adjusted count relative to zero. If the absolute difference between the adjusted count and NSHIFT is found to exceed NRANGE, "Out of Range" is displayed and the main loop is executed. Otherwise, the mean count from the previous main loop execution is subtracted from the current count to cancel out the DC component and the result is squared.

The adjusted count and the squared result are summed in two accumulatorsýa 24-bit accumulator for the counts and a 40-bit accumulator for the squares. After 256 iterations, the mean of the sums is contained in the accumulators, disregarding the low-order byte (effectively a divide by 256). The square root of the mean of the sum of the squares is calculated. NSHIFT is subtracted from the mean count (making it relative to NSHIFT) and the result is multiplied by the slope to convert it to DC voltage.

NSLOPE is used if the count is greater than NSHIFT, otherwise PSLOPE is used. The root of the sum of the squares is always multiplied by PSLOPE to obtain the AC voltage. When measuring resistance, the DC voltage is converted to ohms. The results are displayed and the main loop begins again.

PIC14000.INC provides the basic definitions for the PIC14000. vom.inc provides additional definitions and function for the VOM. Other .inc files provide code for various functions used by vom.asm. You can experiment with programming the PIC14000 by using these files.