finished my superprobe!

i finally finished my superprobe. i even managed to squeeze it into an empty habanero shake-a-salsa bottle for a stylish homebrew aesthetic. i made just a few changes from the mondo-technology design: 1. i put a diode on /mclr and resistors on rb6 and rb7, so i could program the pic in-circuit without frying everything. 2. i used a 16f877a instead of a 16f870. i changed two lines in supmp.asm: <       list      p=16f870               ; list directive to define processor <       #include          ; processor specific variable definitions --- >       list      p=16f877a >       #include but the two parts are similar enough that when i compile both of these with gpasm, the resulting hexes are identical. oddly, these hexes are significantly different from the SUPER.HEX provided by the mondo-technology page, which i assume was compiled from super.src with an unknown compiler. i used the gpasm version and it seems to work just fine. 3. instead of a passive crystal, i used a ECS-2200AX-200 on osc1 and left osc2 flapping in the breeze. so i didn't need the pair of 22pF caps, but i did find that a tiny bit capacitance (<1nF) from clk to gnd helped the osc get started. 4. instead of using power from the circuit, i decided it was much more convenient and a little safer to use a 9v battery. i have so many of those things from changing smoke alarms (and the superprobe takes so little power) that i have practically a limitless supply. it works great, and it has an amazing number of features for such a simple project. there is one glaring omission, however; there is no ohmmeter. were it not for this, i could almost entirely replace my old multimeter with this superprobe. i wonder if i could add such a feature... microchip shows a circuit and code in an512 for an ohmmeter that uses a capacitor for measuring time constants. the problem is, the resistor to be measured needs to be between the cap and gnd, and the superprobe puts the neg probe electrode at gnd. could i just use a simple voltage divider with the existing resistors on rax? the resistance would then be v/(1-v)*r, where v is the ratio of adc voltage to supply and r is the resistance of the feed resistor. by putting ra1-ra5 and rc6-rc7 high, the feed resistance can be as low as 60 ohm, though this probably isn't much better that just using the 100ohm resistor (puts out up to 85mA, too). on the high end, i could use a 100kohm feed resistor which might let me measure up to 750kohm or so. i guess the big question is whether or not there is room on the pic for this. EDIT: maybe an algorithm like this would work. and i could use the 100, 10k, and 100k ohm resistors. that way i could set the decimal location (in kohms) by the resistor that was used. 10.00 to 99.99 for the 100kohm 1.000 to 9.999 for the 10kohm .0000 to .9999 for the 100ohm and i could get up to 1Mohm by moving the decimal for voltage ratios >.5 with the 100kohm 100.0 to 999.9 for the 100kohm, v > .5 i don't think i would get good precision for much higher than 2Mohm, anyway. the max power dissipation would be with a 0omh load, at .25 watt in the 100ohm resistor and 50mA current, which is no problem. the max power dissipation in the measured device would be for a 100ohm load, at .125 watt and 25mA. this could be a problem for smd parts. typically 1206 and bigger resistors are rated at 1/8 watt, but 0805 and smaller are rated lower. maybe i could get away with it if i just used pulses and delayed between measurements, so that the average power is lower. i read somewhere that smd resistors can have ratings as low as 1/20 watt; if a measurement cycles through the three source resistors with the voltage turned off during the computation, the average power should be less than 1/24 watt. so maybe power dissipation is nothing to worry about. hmm. actually it would probably make the code simpler if i used these ranges: 100.0 to 999.9 for the 100kohm 01.00 to 99.99 for the 10kohm .0000 to .9999 for the 100ohm i would lose a digit of precision for the 1k to 9.99k range. but i wouldn't have to do anything to account for the source resistance.