6
\$\begingroup\$

I found a project I'm interested in on the Internet ( http://linnix.com/udip/ ). However, I'm a total beginner to electronics, and I'm not sure about the values of the elements as described in the schematics (see below):

  1. Does a "R220" resistor here mean 22 ohms, or 0.22 ohms? [edit: solved, 22 ohm +/- 5%, see below]
  2. Does a "C180" capacitor mean 18pF? In some articles I read that a capacitor symbol contains a letter after the number, meaning tolerance of the capacitor. Here, I don't see such a letter — is it important? Or does the "C" letter mean it here (+/- 0.25pF)? [edit: solved, 18pF, "+/-10% should be fine", see below]
  3. Similarly, "C105" = 1µF? (and what about the tolerance?) [edit: solved, 1µF +/- 10%, see below]
  4. I believe I can assume the board is powered by the UVCC line (from USB), so VCC doesn't need power, only the capacitor to GND? [edit: partially solved, "yes"; but what about GND vs. UVSS?]

I'd be very grateful for help!

UDIP project schematics

Edit: Wrote some answers above, based on answer by @markrages, and on some more info from atmega32u2's reference (20.3.3 Design guidelines).

Edit 2: Moved questions 5, 7 into: "how to ground?" and "which variant of bus-powered?". Also, deleted question 6, I hope I'll be able to solve it myself later.

\$\endgroup\$
  • 2
    \$\begingroup\$ I, for one, hate the notation "220" = 22 Ω, even if it's logical alongside "222" = 2200 Ω. Write "22 Ω" ideally, or "22R" or "22" if you can't. :( I hate ambiguity. \$\endgroup\$ – endolith Oct 18 '11 at 1:50
12
\$\begingroup\$

First of all, this is a nonstandard and unhelpful way of calling out component values. Don't do this on schematics you draw! (In normal practice, "R220" means "resistor number 220" and the value is shown separately.)

But somebody with experience might be able to figure it what's going on. Let's see how I do...

C180 must mean 18 pF. Those are crystal load capacitors, which are commonly in the 10 pF - 20 pF range.

C105 means 1 uF. These are bypass capacitors and 1 uF is a common value and the datasheet-recommended value.

R220 probably means 22 ohms, from the example circuits in the 32u2 datasheet.

I found this drawing in the datasheet:

enter image description here

Also notice the supply connections. The micro has its own internal regulator, so the Vcc doesn't need to be connected to anything else (except its bypass capacitor.)

None of the components are super critical. 10% tolerance should be fine.

Once again, don't use this as example of how to draw schematics.

(edited to revise 0.1uF guess to 1 uF.)

\$\endgroup\$
  • 4
    \$\begingroup\$ Im guessing that these values are basically what appear on SMT (surface mount) components. The first two digits are the significant values and the last is the exponent of 10 of the multiplier. So R220 is read as a resistor with value 22 x 10 to the power of 0. So 22 x 1 or 22 ohms. Capacitors are written in terms of pico-farads (pF) C180 is a capacitor of 18 x 10^0 or 18pF and C105 is 10 x 10^5 or 1000000 pF or 1uF \$\endgroup\$ – martyvis Oct 9 '11 at 4:57
  • \$\begingroup\$ +1, thanks a lot, some subquestions (1, 3) now have a strong answer; but (5.) and (6.) still not, with (2.) and (4.) I still have doubts, and as a result I added (7.) (please see edited question, as it'd be too long a comment to put here). Also thanks for pointing out XTAL1 and XTAL2 names, I didn't notice they weren't named on the original pic! \$\endgroup\$ – akavel Oct 9 '11 at 13:34
  • \$\begingroup\$ Thanks for all help, and for the edition, which clarified some more. My question grew a bit, so now I've come to think it might be good idea to promote some sub-questions to separate normal questions, and be able to accept your answer. Thank you! \$\endgroup\$ – akavel Oct 27 '11 at 17:01
4
\$\begingroup\$

As Mark said, this is a irresponsibly drawn schematic. That casts doubts on other parts of the schematic and anything else that author has to say. Someone being sloppy in one area always seems to carry over to other areas.

To answer your question directly, it seems like the author is using 3 digit floating point to specify values. This is the same scheme used on some capacitors and to define the color bands on 5% resistors. The first two digits are the mantissa, and specify a value from 10 to 99. The third digit is the exponent of 10 to apply to the mantissa. You have to guess the units, but it looks like the author is assuming Ohms and picoFarads.

Here are the values from the schematic expanded out:

C180 = 18 x 10^0 pF = 18 pF
R220 = 22 x 10^0 Ω = 22 Ω
C105 = 10 x 10^5 pF = 1000000 pF = 1.0 µF

A little sanity check adds credence to this interpretation. C180 are crystal load caps, and 18pF is right in the expected range. Actually the right load cap values are dependent on the crystal, not the schematic, but 18pF will be good enough with most crystals unless you need the last bit of accuracy. Certainly 1.8pF and 180pF are out of range, so the pF interpretation for capacitance is pretty much confirmed.

I'm not familiar with this chip, but the caps on VCC and UCAP are either for bypass or to hold up a internally generated supply. 1µF for those is quite plausible.

The R220 resistors appear to be in series with USB lines (guessing from the signal names). You certainly wouldn't want more than 22Ω in series with those. The USB chips I'm familiar with shouldn't have any deliberate resistance in series. Check the datasheet.

Now let's look at the circuit at bit. Since there is nothing else on VCC and UCAP, that's not where the power is coming from. This micro appears to be powered directly from the USB power, between UVCC and UGND, which the signal names also hint at. If so, there certainly should be a bypass cap accross those close to the micro, and a larger cap can be a bit further away, probably right at the USB connector. The USB spec allows up to 10µF capacitance between its power and ground. Use at least a good fraction of that.

What is the ground connected to, and how does that relate to UGND? I expect those to be tied together at one point, but there is no evidence of that here. Note that the two switches are between a micro pin and ground. That's OK as long as those pins have internal pullups. Check the datasheet.

So why couldn't this guy draw the schematic like everyone else would and be clear about the part values? Good question. He's either being cutesy, stupid, or has some axe to grind. Either way, it demonstrates poor engineering. Don't you do this. Also, I'd look elsewhere for whatever you are trying to find. You have no way of knowing what you can trust and what you can't from this author.

\$\endgroup\$
  • \$\begingroup\$ I've worked with USB devices that specify external resistance around this range. Definitely 22 ohm. \$\endgroup\$ – endolith Oct 18 '11 at 2:03

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.