Freeduino 4809

This is an Arduino Uno Wifi2 derivative (atmega4809) with the WiFi and other "extra" features deleted.

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Basically, an ATmega4809 breakout board in the Uno form-factor, and matching the Uno Wifi2 pinouts.
It makes use of cheap AliExpress modules for power and USB, simplifying construction and (hopefully) improving functionality, and uses very conservative design rules and relatively large SMT components (0805) (except for the ATmega4809 chip itself, alas.)

Design Decisions:

  1. Pin mapping (Digital, Analog, I2C, SPI) identical to Uno Wifi 2.
  2. Designed for "amateur" assembly.  0805 SMT component size.
  3. Use COTS modules for power, USB.
  4. Allow for partial builds.
  5. careful layout of analog paths.
  6. break out most unused pins.  Somewhere.
  7. allow for UPDI debug connection.  But maintain the SPIness of the ICSP connector (sigh.)
  8. use a serial bootloader for upload.  Probably at the end of memory so that apps load at 0 (like old AVRs and Uno WiFi2), even though that's inconsistent with the "bootloader" features of the chip.
  9. Move Aref down next to the Analog pins.

EAGLE design files for v0.1 (6-Jun-02019)

Zip Archive - 76.69 kB - 06/06/2019 at 02:08



Printable Schematic for v0.1 (6-Jun-02019)

Adobe Portable Document Format - 101.58 kB - 06/06/2019 at 02:08


  • Uno-style board Blinking.

    WestfW08/06/2019 at 09:14 2 comments

    A "Freeduino 4809", the board with the Uno pinout, has now also been assembled, runs a Blinky and Hello World program (on an entirely different pin/USART than the DIP version.)

    Pay no attention to the capacitors that don't match the PCB footprints.  They fit, mostly, and are what I have in the junk box!

    The TX and RX pins on this board are also swapped from how they need to be.  :-(  I'm going to start putting a set of crossover pads on any PCB where they'll fit!

  • DIP Boards have swapped Rx/Tx

    WestfW07/31/2019 at 05:52 4 comments

    Sigh.  The DIP boards have the RX and TX pins swapped at the ATmega side.  A crossover works ("Hello World!" !!), and it's less relevant if you're not using particular USB/Serial module I'm using, but it's annoying.  I could swear I had checked that carefully  :-(

    void uart_init() {
        VPORTA.DIR |= 1<<4;  // set TX pin to output
        VPORTA.OUT |= 1<<4;  // and "1" as per datasheet
        PORTMUX.USARTROUTEA = 1;  // alternate pinout to use PA4/5
        USART0.BAUD = (64ULL * F_CPU)/(16UL*BAUDRATE);
        USART0.DBGCTRL = 1;  // run during debug
        USART0.CTRLC = (USART_CHSIZE_gm & USART_CHSIZE_8BIT_gc);  // Async, Parity Disabled, 1 StopBit
        USART0.CTRLA = 0;  // Interrupts: all off

  • Hmmph. Board is short.

    WestfW07/05/2019 at 08:09 0 comments

    My intent was to reject the "Italian Style" Arduino board shape and use a simple rectangle the same size as my previous "Freeeduino" board (as sold by NKCElectronics.)  But on comparison, while I removed the funny shape, I forgot to move the left edge of the board, and it's about 6mm shorter than the Freeduino.  That's not a big deal, but I'm annoyed with myself, particularly since it would have made it easier to fit those power supply modules...

  • Boards have arrived

    WestfW06/23/2019 at 05:14 1 comment

    Boards have arrived.   I made some errors on the bottom silkscreen, and there are some other things I don't like WRT which vias I had tented, but they look pretty good.  (Even the slots of the power connector look rightl the advantage of starting with someone else's part definition, or perhaps clever engineers at PCBWay...)

    Now to try to muster the courage to solder those 0.5mm-pitch TQFPs :-(

    (The bottom boards are 4809 DIP "completers"...)

  • PCBs are being made

    WestfW06/11/2019 at 09:33 0 comments

    Sent out for PCB fab (at

  • Silkscreen?

    WestfW05/30/2019 at 08:05 0 comments

  • Getting closer

    WestfW05/24/2019 at 07:12 0 comments

    I did it - Got rid of Aref. and connected PB3 to that connector instead.  Brought out the Aref signal next to the analog pins...

    I've inflicted the "WestfW Style" on the layout :-)

    Next step, silkscreens...

    (The USB module isn't really on the bottom.  It's on the top, but upside down.  I guess I should make that a package option.(

  • Fiddling.

    WestfW05/23/2019 at 01:47 0 comments

    The board is designed to be partially populated.  The USB module has power and data LEDs, so the pwr LED is sort-of "underneath" that, if you're not using USB.  The 3.3V regulator could be omitted, with the 3.3v pad connected to the USB module (if you only need as much current as the ch340e can provide (which I assume is even less than the 50mA you can get from an FTDI.)

    Thinking of moving or removing Aref; it's on the wrong side of the board (physically and electrically), I don't think anyone ever uses it, and the 4809 has more Aref options internally.

    Added pads for most of the "unused" signals.  Not very big pads, but better than trying to attach wires directly to the lqfp, if  you really need one extra signal...

    Those SMPS modules are tiny.   Theoretically, the board top layer is left blank underneath them, so that if desired one could replace them with "cheaper" linear regulation circuits.  But there isn't enough room for a full-sized TO220 footprint.  Also, the pins on the modules I have do not seem to be on any particular grid :-(

    Added extra bypass caps on the bottom side of the board.  There is plenty of room, so...

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Travis Ayres wrote 06/26/2019 at 05:20 point

Are those Bezier traces? Did you use Topor to route those, or has Eagle had some routing mode I wasn't aware of? Looks super slick, I dig it.

  Are you sure? yes | no

WestfW wrote 07/05/2019 at 07:51 point

The top has "curved" traces of whatever it is that EAGLE uses for curved traces (manually routed; AFAIK, there's no way to get the autorouter to generate them.)
It's my "useless style" to make the top traces curvy, but not the bottom.  I'm happy that you like it!

  Are you sure? yes | no

Paul Stoffregen wrote 06/23/2019 at 08:53 point

For hand soldering 0.5 mm pitch, I use Kester 331 solder and Kester 2331-ZX liquid flux, and a magnifier and bright light.  The 331/2331 flux makes an incredible difference.

But the downside is the 331 & 2331 flux absolutely must be thoroughly washed off the PCB with hot water.  Usually baking the PCB in an oven an low temperature (150-175 Fahrenheit) for 10-20 minutes is needed to get it fully dry.

If that 0.5 mm soldering give you trouble, I highly recommend trying the Kester 2331 flux and 331 solder.  The liquid flux comes in a small pen size, which sometimes isn't mentioned where it's sold in huge containers.  I believe Mouser has the pens in stock.

  Are you sure? yes | no

WestfW wrote 07/05/2019 at 07:57 point

Wire solder?  I have some random-brand paste and quikchip flux, I was hoping for the "apply an excess of flux and paste and surface tension will do magic" technique.

  Are you sure? yes | no

ronald.sutherland wrote 06/17/2019 at 22:58 point

Serial Boot-loader: Is it true that the boot-loader section is in low memory on this chip?

  Are you sure? yes | no

WestfW wrote 07/05/2019 at 07:52 point

Yes. :-(

I think I can use the "virtual boot section" code (from Optiboot) and stick the bootloader at the end of memory (giving up protection.)  This is the same thing that happens on chips like the ATtiny841 that have self-progamming but no explicit bootloader support.
This will avoid needing to have bootloader-based and non-bootloader binaries linked differently.  (The Arduino 4809-based boards all use a programmer chip - no bootloader at all.  I don't know whether to growl, or say "it's about time!")

  Are you sure? yes | no

WestfW wrote 06/06/2019 at 00:12 point

Yeah, I will.
The module I use (and have used before) includes the USB/Serial chip (CH340E in MSOP-10), microB usb connector,  power/activity LEDs, and costs less than $1 ($0.53, currently.)  That makes it pretty tough to get excited about a new chip...

  Are you sure? yes | no

Hans wrote 06/05/2019 at 15:45 point

Would you mind uploading the schematic?
You should have a look at the new CH330N USB to serial chip. They sell it at LCSC, and it's in a SOIC-8 package. Would fit this board nicely!

  Are you sure? yes | no

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