Second Version - STM32L071KBU

A project log for Tiny Arduino for Hackers

A dime-sized, ARM-based Arduino complete with a programming/development system.

doctekdoctek 12/28/2020 at 23:090 Comments

Using all that I had learned from my initial design, I bravely launched into a revision. It is called "tArmDuino". The STM32L071 seemed like the right family and I expected to use a 28 pin version again. But the 28 pin version only supported 32K of memory! To gain access to the larger flash memories, I would have to use a 32 pin package. Fortunately, the re-design was not difficult and the resulting board is only 1mm longer, and the same width, so no changes to the frame for the programmer were needed. The breadboard version is 26 pins vice 24 pins. Note that pin PB4 is a no-connect, according to the errata sheet. Actually, it's not totally clear to me what versions this applies to, so I'm just not using PB4. The complete part number is STM32L071KBU6 for the 128K part. Change the B to a Z for the 192K part. It's worth noting that there is an STM32L072KBU that has the same footprint and pin-out, and adds two DACs and crystalless USB. I haven't tried this chip yet.

Here are the schematics for the tiny version of the tArmDuino and the small (breadboard) version.

Besides dividing the pin connections into 4 connectors instead of two, the main difference is that the SWD and SWC lines (used for programming and debug) are brought out to the connectors on the debug version, while they are routed to the debug footprint on the tiny version. The pictures of the two boards follow.

The tArmDuino is shown sitting on the debug alignment frame. The breadboard version is shown in use. Details on connecting the STLink programmer will be given in another project log.

Building both boards uses standard process. The boards were produced by OSHPark. The solder paste stencils were cut on my Silhouette Portrait in 2 mil mylar. I stenciled on the paste, placed the parts, and reflowed using my modified toaster oven. Any required touch-up is done by hand.

The KiCAD files are in the Files section for both boards. Also included are the gerbers and the gerber for the solder paste stencil. Note that the stencil has rectangle vice rounded rectangle shapes for all pads. If I use rounded rectangles, then cutting the stencils with the Portrait takes a painfully long time! Since all the KiCAD files are available, a different stencil could be created if desired.