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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.

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    \$\begingroup\$ 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. \$\endgroup\$ – JYelton Oct 9 '13 at 0:20
  • \$\begingroup\$ @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. \$\endgroup\$ – Alex I Oct 9 '13 at 0:24
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    \$\begingroup\$ The epoxy for COB might end up taking more area than WLCSP. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 9 '13 at 1:55
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    \$\begingroup\$ This sort of survey question is not suited for this site. \$\endgroup\$ – Olin Lathrop Aug 3 '16 at 19:23
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    \$\begingroup\$ @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. \$\endgroup\$ – Alex I Aug 5 '16 at 9:31
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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) 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
  • 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.

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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)

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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.

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  • \$\begingroup\$ And presumably comparable in size to the ATtiny20-UUR that the asker has already found. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 9 '13 at 13:37
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    \$\begingroup\$ @Ignacio: Well, I added that the ARM is 32-bit. The ATtiny is an 8-bit controller. \$\endgroup\$ – Johan.A Oct 9 '13 at 13:38
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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.

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    \$\begingroup\$ So does the ATtiny4/5/9/10. And johnfound has already determined that the ATtiny20-UUR is even smaller. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 9 '13 at 13:10
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    \$\begingroup\$ @IgnacioVazquez-Abrams "some of ..." not the very smallest. There's nothing wrong with alternatives. \$\endgroup\$ – placeholder Oct 9 '13 at 14:18
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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.

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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

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    \$\begingroup\$ From the datasheet: "To find a package drawing, go to freescale.com and perform a keyword search for the drawing’s document number". Really? \$\endgroup\$ – radagast Oct 9 '13 at 13:48
  • \$\begingroup\$ @radagast: Yeah, some manufacturers really hate redundancy, even though it's less convenient for the end user (such as NXP with their 74HC datasheets). \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 9 '13 at 14:09
  • \$\begingroup\$ @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. \$\endgroup\$ – Johan.A Oct 9 '13 at 15:36
  • \$\begingroup\$ @Johan.A: You don't need tens of pages to mention something basic like what the voltage thresholds are. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 9 '13 at 15:45
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    \$\begingroup\$ @Ignacio: Well, the HCT User Guide is 46 pages of interesting engineering information. If you think only thresholds matter, I guess it's your loss. Peace! \$\endgroup\$ – Johan.A Oct 9 '13 at 16:01
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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.

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