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I am looking for a microcontroller to use in a project which is very space-limited. What microcontrollers have some of the smallest packages? This does not need to be high performance, most microcontrollers are more powerful than is necessary in this case, but it does need to be tiny. Atmel's ATtiny20 comes in a WLCSP-12 package which is 1.55x1.4x0.53mm, this is not bad. What other options are there along those lines or smaller? (From any family, doesn't have to be Atmel)

Update: with at least 4 I/O pins.

Alex I
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    This depends entirely on your application. The "smallest" microcontroller might not have enough pins for whatever you need it for. For example the ATtiny13 comes in a SSOP-8 package which is quite small, but you only get 6 I/O pins. – JYelton Oct 09 '13 at 00:20
  • @JYelton: I only need 4 I/O pins. You're right, I should have specified pin count, but I assume basically anything has 4 or more. SSOP-8 is actually a huge package compared to what I'm looking for. – Alex I Oct 09 '13 at 00:24
  • Are you able to do chip-on-board assembly? – The Photon Oct 09 '13 at 00:28
  • @ThePhoton: Maybe :) Would like to explore WLCSP package first, but please do recommend a COB solution if you have one in mind. Also: what assembly houses can do COB? – Alex I Oct 09 '13 at 00:40
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    The epoxy for COB might end up taking more area than WLCSP. – Ignacio Vazquez-Abrams Oct 09 '13 at 01:55
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    This sort of survey question is not suited for this site. – Olin Lathrop Aug 03 '16 at 19:23
  • @AlexI this site is not for discussion based questions see http://electronics.stackexchange.com/help/how-to-ask AND http://electronics.stackexchange.com/help/on-topic – Voltage Spike Aug 04 '16 at 18:48
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    @laptop2d: I don't care to get into a big discussion about what is and isn't on topic here, I think people are expressing a pretty clear opinion with their votes. This is clearly useful; it is my second-highest rated question and answer. My highest rated answer is of the same type. – Alex I Aug 05 '16 at 09:31
  • @Alex If you haven't read this, http://electronics.stackexchange.com/help/on-topic its a good read. I'm just recommending it if you haven't read it before. – Voltage Spike Aug 08 '16 at 17:25

7 Answers7

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Thanks to everyone who pitched in with suggestions, I have tried to collect all the info in the same answer.

The smallest microcontrollers from each family:

  • Atmel ATtiny20-UUR: 1.55 x 1.40 x 0.53 mm, WLCSP-12, 10 gpio, 0.4mm pitch (the winner)
  • (new) Cypress CY8C4014FNI-421: 1.56 x 1.45 x 0.4 mm, WLCSP-16, 12 gpio, 0.35mm pitch
  • (new) Maxim MAX32660GWE: 1.55 x 1.57 x 0.64 mm, WLCSP-16, 10 GPIO, .35mm pitch
  • (new) Freescale Kinetis KL03 MKL03Z32CAF4RTR: 2.00 x 1.61 x 0.56 mm, BGA-20, 18 gpio, 0.4mm pitch
  • Freescale Kinetis KL02 MKL02Z32CAF4R: 2.00 x 1.95 X 0.56 mm, BGA-20, 18 gpio, 0.4mm pitch
  • NXP LPC1102UK: 2.32 x 2.17 x 0.60 mm, WLCSP-16, 11 gpio, 0.5mm pitch
  • Ambiq Apollo2 2.5 x 2.5 x 0.45 mm, WLCSP-49, 34 gpio, 0.35mm
  • ST STM32F042T6Y6: 2.70 x 2.60 x 0.55 mm, WLCSP-36, 36 pins, 0.40mm pitch
  • TI MSP430G2252: 2.65 x 2.55 x 0.28 mm, bare die, 22 pads
  • TI MSP430G2x01: 4.00 x 4.00 x 0.90 mm, QFN-16, 16 pins, 0.65mm pitch
  • Microchip PIC12LF1552T: 3.00 x 2.00 x 0.50 mm, QFN-8, 5 gpio, 0.5mm pitch

Sometimes there are other options in the same family which use the same package, but I haven't found anything with a smaller package. This doesn't take into account how much supporting circuitry each chip needs, of course.

Community
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Alex I
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ATtiny4, ATtiny5, ATtiny9 and ATtiny10 are packed in different 6pin packages some of them very small. They all have 4 I/O pins. Some of them ADC. Check for details on ATMEL site.

EDIT: After some more search. Well, ATtiny20 in WLCSP package is even smaller. So, it becomes the smallest available Atmel microcontroller. (only in ATtiny20-UUR variant)

johnfound
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The NXP LPC11A00 is an ARM Cortex-M0 controller that comes in a 20-pins WLCSP package:

enter image description here

Comparable in size to SOT-23, but those (AVR and PIC) only have 6 pins. ARM is also 32-bits.

Johan.A
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The smallest microcontrollers I know about are the Microchip PIC 10Fxxx series. These come in SOT-23 packages:

Added:

I should have mentioned that these things also come in a 2x3 mm DFN package, but I didn't have a cool picture of those available.

Olin Lathrop
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You can also obtain microcontrollers as dice - the actual semiconductor after cutting from the silicon wafer, but before packaging. These can be used as Chip On Board (COB) assemblies and can be much, much smaller than a packaged part.

Some companies also offer Chip Scale Packaging (CSP), which is only slightly bigger than that actual silicon dice inside the package. These are often under 0.5mm high, and between 1mm and 2mm in length and width.

Adam Davis
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You can add to your list the new CY8C4014FNI-421 , with a 16 pin WLCSP at 1.56 x 1.45 x 0.4 mm, only slightly larger than the attiny, and has 12 gpio, 16k flash and 2k sram.

dmsc
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I can think of the freescale kinetis KL02 1.9mm X 2mm its a ARM Cortex-M0+ 20 pin, its a BGA though here is the package info

Kvegaoro
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    From the datasheet: "To find a package drawing, go to freescale.com and perform a keyword search for the drawing’s document number". Really? – radagast Oct 09 '13 at 13:48
  • @radagast: Yeah, some manufacturers really hate redundancy, even though it's less convenient for the end user (such as NXP with their 74HC datasheets). – Ignacio Vazquez-Abrams Oct 09 '13 at 14:09
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    @Ignacio: I think you're referring to the HCMOS handbook? That avoids duplicating *tens of pages* of common information in every datasheet. A mechanical drawing is just 1 page. Also, at least NXP has hyperlinks in their PDFs, something Freescale never heard of. – Johan.A Oct 09 '13 at 15:36
  • @Johan.A: You don't need tens of pages to mention something basic like what the voltage thresholds are. – Ignacio Vazquez-Abrams Oct 09 '13 at 15:45
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    @Ignacio: Well, the [HCT User Guide](http://www.nxp.com/documents/user_manual/HCT_USER_GUIDE.pdf) is 46 pages of interesting engineering information. If you think only thresholds matter, I guess it's your loss. Peace! – Johan.A Oct 09 '13 at 16:01
  • @Ignacio: BTW, I saw your comments to the question and the other answers. Are you always that negative? – Johan.A Oct 09 '13 at 16:03