Your Arduino SAMD21 ADC Is Lying To You

August 30, 2019 by Ted Yapo 64 Comments

One of the great things about the Arduino environment is that it covers a wide variety of hardware with a common interface. Importantly, this isn’t just about language, but also about abstracting away the gory details of the underlying silicon. The problem is, of course, that someone has to decode often cryptic datasheets to write that interface layer in the first place. In a recent blog post on omzlo.com, [Alain] explains how they found a bug in the Arduino SAMD21 analogRead() code which causes the output to be offset by between 25 mV and 57 mV. For a 12-bit ADC operating with a reference of 3.3 V, this represents a whopping error of up to 70 least-significant-bits!

Excerpt from the SAMD wiring_analog.c file in the Arduino Core repo.

While developing a shield that interfaces to 24 V systems, the development team noticed that the ADC readings on a SAMD21-based board were off by a consistent 35 mV; expanding their tests to a number of different analog pins and SAMD21 boards, they saw offsets between 25 mV and 57 mV. It seems like this offset was a known issue; Arduino actually provides code to calibrate the ADC on SAMD boards, which will “fix” the problem with software gain and offset factors, although this can reduce the range of the ADC slightly. Still, having to correct for this level of error on a microcontroller ADC in 2019 — or even 2015 when the code was written — seems really wrong.

After writing their own ADC read routine that produced errors of only between 1 mV and 5 mV (1 to 6 LSB), the team turned their attention to the Arduino code. That code disables the ADC between measurements, and when it is re-enabled for each measurement, the first result needs to be discarded. It turns out that the Arduino code doesn’t wait for the first, garbage, result to finish before starting the next one. That is enough to cause the observed offset issue.

It seems odd to us that such a bug would go unnoticed for so long, but we’ve all seen stranger things happen. There are instructions on the blog page on how to quickly test this bug. We didn’t have a SAMD21-based Arduino available for testing before press time, but if you’ve got one handy and can replicate these experiments to verify the results, definitely let us know in the comments section below.

If you don’t have an Arduino board with a SAMD21 uC, you can find out more about them here.

This Tiny TFT Pendant Is Digital Jewelry

June 21, 2019 by Lewin Day 9 Comments

Hackers have a multitude of skills, many are well-versed in the ways of all things that blink and flash. These abilities have often be applied to the field of jewelry and human adornment, and many LEDs have been employed in this work. [Deshipu] has been attempting something a touch different however, by constructing a tiny TFT pendant.

The basic idea is not dissimilar from those USB photo keychains of recent history. A SAMD21 Cortex M0+ serves as the brains of the operation, with the tiny microcontroller being soldered to a custom PCB that makes up the body of the pendant. A ST7735S TFT LCD screen is then attached to act as the display. Charging and delivery of images is done over USB, which can be handled natively by the SAMD21.

Currently, the pendant is capable of displaying 16-color BMPs, with the intention to create a converter for animated GIFs in the pipeline. Potential upgrades also involve creating a larger battery pack to sit behind the wearer’s neck, as currently the device has just 8 mAh to work with.

It’s a nicely designed piece, with the pendant appearing barely bigger than the screen itself. It’s not the first time we’ve seen a hacker take on a pendant, and we’re sure it won’t be the last. If you’ve got the goods, be sure to hit up the tip line.

Save Some Steps With This Arduino Rapid Design Board

July 28, 2018 by Dan Maloney 18 Comments

We’re all familiar with the wide variety of Arduino development boards available these days, and we see project after project wired up on a Nano or an Uno. Not that there’s anything wrong with that, of course, but there comes a point where some hobbyists want to move beyond plugging wires into header sockets and build the microcontroller right into their project. That’s when one generally learns that development boards do a lot more than break the microcontroller lines out to headers, and that rolling your own design means including all that supporting circuitry.

To make that transition easier, [Sean Hodgins] has come up with a simple Arduino-compatible module that can be soldered right to a PCB. Dubbed the “HCC Mod” for the plated half-circle castellations that allows for easy soldering, the module is based on the Atmel SAMD21 microcontroller. With 16 GPIO lines, six ADCs, an onboard 3.3 V regulator, and a reset button, the module has everything needed to get started — just design a PCB with the right pad layout, solder it on, and surround it with your circuitry. Programming is done in the familiar Arduino IDE so you can get up and running quickly. [Sean] has a Kickstarter going for the modules, but he’s also releasing it as open source so you’re free to solder up your own like he does in the video below.

It’s certainly not the first dev module that can be directly soldered to a PCB, but we like the design and can see how it would simplify designs. [Sean] as shown us a lot of builds before, like this army of neural net robots, so he’ll no doubt put these modules to good use.

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