Feedback Requested for SAML21 Dev Board design - Page 1
Forum section: electronics / projects

author: cbc02009
author_rank: Regular Contributor
timestamp: December 30, 2018, 02:30:27 pm
content: Hello forum,I used the SAM L21 processor in my senior design project, and I really enjoyed working with the chip, but I don't seem to see it used very often (probably because of it's price point). It's got several quality of life features I really like over the D21, especially with regards to the ADC, that I feel make it much easier to work with for beginners like myself. I also really like Atmel Studio as an IDE (though I don't have much to compare it to). I've tried Microchip's, TI's code composer, and IAR, and I definitely prefer Atmel Studio's design, especially for someone who's maybe just past the arduino, and needs things like debugging, but still just a hobbyist. Because I think it's such a great little chip, I've decided to work on a breakout board for it. (mostly just to give myself something to do until my job starts)So, I present to you the dev board I've been working on. It fits on a normal breadboard (similar size to the adafruit feather), and is powered either by USB, or 3.3 volts from the breadboard. The regulator was specifically selected so that it's safe from reverse current. It has a reset button (I wish I could find one smaller, for as cheap as the one I have...), a power LED, an external crystal oscillator, and is programmed through the ARM cortex debugger, which you can get for ~$20 US.Having never done such a project before, I'd like to request feedback on several things.First off, I'm curious about whether other people have used the chip, and what they think of it, or why they haven't.Second, are there any major features you look for on a dev board that this is missing? I'm aware it's very barebones, but I wanted to keep it cheap (since I'm basically just building it for me). The main one I'm thinking is that this should have a target usb jack, since the chip has USB built in. I could possibly make room for it.Third, I just always like for someone to glance over my work and make sure I didn't do anything super stupid.I've attached pictures of the schematic and board layout, and a zip file with the kicad project. Once I finish going through Digikey I will also attach the full BOM.Thanks in advance for any feedback/advice!

author: cbc02009
author_rank: Regular Contributor
timestamp: December 30, 2018, 04:22:18 pm
content: Here's the BOM, attached. Total cost per unit is $4.34 for parts and ~$.50 for the pcb from SeeedStudio. So under $5 total, which is pretty cool.

author: ataradov
author_rank: Super Contributor
timestamp: December 30, 2018, 09:29:48 pm
content: I would put pull-ups on reset and SWD clock lines. They are technically not necessary, but if you have space on the board, there is no need to take any chances.Otherwise looks like a well designed board to me.

author: floobydust
author_rank: Super Contributor
timestamp: December 30, 2018, 09:37:38 pm
content: I found the PCB thermal-reliefs and ground-pours need some tweaking in your PCB CADD software.The pour is backing off way too much and leaving some odd polygons. Pads 6 GND should have been connected to the GND net and pour at the PCB edge but the pour went around them.The minimum clearance rule seems to not be set, some of your traces are very close and don't need to be if the trace is moved or centered.pic is two examples

author: cbc02009
author_rank: Regular Contributor
timestamp: December 30, 2018, 11:52:58 pm
content: Quote from: floobydust on December 30, 2018, 09:37:38 pmYes, I forgot to set the pad-to-pour clearance to the 8 mils seeed studio specifies, so that's why it looks like that. Thank you very much for picking up on that for me.I also have the track clearance set at the 6 mils specification, and it passes the DRC, so I think they're okay. Kicad does have a tendency to put the tracks as close together as possible, I've noticed.Quote from: ataradov on December 30, 2018, 09:29:48 pmThey have built in pull-up resistors in the chip. would the extras be for redundancy, just in case?

author: ataradov
author_rank: Super Contributor
timestamp: December 30, 2018, 11:55:26 pm
content: Quote from: cbc02009 on December 30, 2018, 11:52:58 pmInternal pull-ups are 60 - 100 kOhm. It will be fine for the bench use, but for any sort of industrial applications you want that reduced to 10 kOhm or so. Why create yourself a pain of catching weird intermittent issues later?

author: cbc02009
author_rank: Regular Contributor
timestamp: December 31, 2018, 12:01:03 am
content: Quote from: ataradov on December 30, 2018, 11:55:26 pmGotcha. Thanks.