[time-nuts] uC ADC resolution (was: Poor man's oven)

Poul-Henning Kamp phk at phk.freebsd.dk
Sat Jun 10 15:06:29 EDT 2017

In message <3897C09A-D76C-474C-8907-9EA25F8C3451 at n1k.org>, Bob kb8tq writes:

>The “limited range” part of it is why the op-amp makes so much
>sense. If the ADC can “see” +/-10C that’s way more than will ever be useful.

Most uC's have a pile of mux'ed ADC inputs, so do all of the above:

AI0 = Full range

AI1 = +/- 10C

AI2 = +/- 2C

A big upside to this is that you will not need to invent heuristics
for clamped inputs in your PI(D) controller, something which is a bit
harder than most people realize.

Assuming a 10-bit ADC, the code will look something like this:


		T = read_AI2();
		if (T > 50 && T < 975) {
			T += T2_offset;
			T *= T2_scale;
			return (T);
		T = read_AI1();
		if (T > 50 && T < 975) {
			T += T1_offset;
			T *= T1_scale;
			return (T);
		T = read_AI0();
		if (T > 50 && T < 975) {
			T += T0_offset;
			T *= T0_scale;
			return (T);
		abort("You have problems...");

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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