A PIC-BASED DIGITAL VOM

THE MAGAZINE FOR COMPUTER APPLICATIONS Circuit Cellar Online offers articles illustrating creative solutions and unique applications through complete projects, practical tutorials, and useful design techniques.

	A PIC-BASED DIGITAL VOM
by Duane Perkins Start ı The Circuit ı Construction ı Testing ı Program Code ı Sources and PDF CONSTRUCTION A heat sink for VR1 is optional. A Radio Shack 270-216 enclosure is ideal. Use ahesive-backed laminating film to protect the panel and cut out the holes with a hobby knife or razor blade. Drill the back panel insert for a power connector. The pinout of the LCD header (J2) is the mirror image of the pinout on the LCD board. This allows the use of a header on the LCD board that matches the header on the VOM board. Make a connector cable with ribbon cable and 16-pin header connectors. Do not mount the LCD in the front panel until later. A vacuum-fluorescent display (VFD) from Digi-Dey is a drop-in replacement for the LCD and gives a bright display that issuperior to even a backlit LCD. PROGRAM ASSEMBLY Every PIC14000 is shipped with preprogrammed calibration constants. Do not erase a new PIC until you have captured the calibration constants. Label the chip with a unique digit. Read the PIC with your programmer and save the code to a hex file named PIC14000.H0n, where "n" is the number of the chip. If the calibration constants are lost after the PIC is erased, they cannot be recovered, but the VOM does not use them so no harm will be done. Use asm.bat to assemble PIC programs. For number 1, specify the chip number on the command line: asm <filename> 1 The asm.bat file will run calcons.exe, which extracts the calibration constants from PIC14000.H0n, and creates calcons.inc and derived.inc. These are not used in any of the supplied programs, but will be available for inclusion in any programs you may develop. Calibration constants for the VOM are in files named calibrat.00n where "n" is the number of the chip. The format for these files is (the numbers are an example): NSHIFT EQU 4187 NRANGE EQU 3919 PSLOPE EQU 1487032226;.001352447931 ı 2⁴⁰ NSLOPE EQU 1487032226;.001352447931 ı 2⁴⁰ VDD EQU 84657550; 5.045983231 ı 2²⁴ PREVIOUS • NEXT Circuit Cellar provides up-to-date information for engineers. Visit www.circuitcellar.com for more information and additional articles. For subscription information, call (860) 875-2199, subscribe@circuitcellar.com or subscribe online. ıCircuit Cellar, the Magazine for Computer Applications. Posted with permission.