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Hardware design with the SAMD21E18

by: May 18,2021 1370 Views 0 Comments Posted in PCB Design Tutorial

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Summary:       Perhaps one of the most commonly known chips, and low cost, is the SAMD21E18. It is somewhat similar to the 328P, despite having less I/O pins, they are much more functional, with higher clock, ARM, and more flash + RAM options. They also require very little external components. This article is meant to teach you how to start incorporating the SAM D21E series into your next design.

The SAMD21 is a Cortex M0 offering from Microchip. The E series has 32 pins with a maximum of 26 I/O pins, including built in USB and up to 256 channels for cap touch with no additional hardware. The MCU has a clock speed of 48MHz and can have as much as 256KB Flash and 32KB RAM. The cost of the E18(most RAM + Flash in the E series) is competitive to, if not lower than, the ATMega32u4. However, one drawback is its lack of 5V tolerance. Due to the built-in USB, it's extremely simple to implement the chip in your design with no additional components. Commonly, USB-C is used with the SAMD21 series. It's available in both TQFP32 and VQFN32 packages, depending on the amount of space you have. The most minimal design, with a reset button, and a USB-C, has only 11 components including the button, connector, and chipset.

Here is a KiCAD schematic I have made:

As you can see there are only a minimum amount of components.

Here's a quick description of what's happening:

The USB-C connector is used along with it's 5.1K resistors from CC1/2 to GND as per standard use protocol. There is also a diode from VBUS to 5V for protection.

As mentioned, the SAMD21E18 has only a 3.3V logic level with no 5V support meaning that if you use 5V you will zap the chip and render it dead.

Thus, we must convert USB 5V power to 3.3 to safely power the board. We will be using an AP2112K-3.3V but a MCP1700 or similar should also work well. There are 2 decoupling 10uF capacitors for input and output of the AP2112. In addition there is a 1uF decoupling on VDDCORE(standard usage of E18s) and 1uF between 3.3 and GND.

While the SAMD21s usually do ask for a 32.768KHz crystal, it's not needed necessarily. Thus, this design will work without such crystal that adds cost and complexity.

Due to the low component count, it is very simple to route this!

For example, here is one of the designs I just routed.

For reference, USB-C connector used is a USB-4085, an easy-solder part with through hole I/O.

In addition, I have a few E18 designs on github that may assist you:

The SAMD21s are highly versatile and especially the E18s. The E series(there are variants with different flash+ram options, E18 is the highest) come in TQFP32 and QFN32. TQFP32 is large but has 0.8mm pitch, allowing you to easily solder it with an iron. Of course, to prototype the pcb it would be very inexpensive, as PCBway offers 10 boards that are 100mmx100mm for only 5 dollars.

In addition, you can have a QFN design, to save space, and if you're unable to solder yourself, you can get a stencil from PCBway or spend only 30 dollars to have it assembled, up to 30 boards ;)

Thank you for reading this guide! I hope you find it useful, and always feel free to DM me on twitter(@rpitechguy) in case you need any assistance with your design!

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