• XTAL tunning

    Daniel Zilinec01/09/2022 at 22:57 6 comments

    I had some complains about the clock is not very accurate so I started to investigate it. A friend of mine borrowed me a HP 5385A frequency counter. It was first time I used one and we weren't sure if it measures correctly.

    ATmega328p that is the MCU in Agora clock has no option to tune the RTC XTAL frequency in firmware. It has 32.768 Hz XTAL connected to internal counter that drives the clock. I hooked this counter's output to a GPIO. It had to divide the frequency first so I was looking for precisely 64 Hz frequency on the GPIO where I connected the counter.

    Then I started to tune the load capacitors of the 32.768 Hz XTAL to get as closer as possible to 64 Hz at my GPIO.

    So I realized that a made a big mistake. I used two XTALs with different load capacitance - the original had 9 pF a the second had one 12.5 pF. That happened in the time when the chip shortage started and I was not aware of the consequences.

    Now with the frequency counter I measured the differences and tuned the load capacitors

    XTAL capacitanceC13 [pF]C14 [pF]ppm
    9 pF - original (mark TBs18)3.34.738
    9 pf - tuned (mark TBs18)2.210.23
    12.5 - original (mark TAV06)3.34.7-31
    12.5 - tuned (mark TAV06)8.26.8-0.2

    Next I wanted to know if I can trust the frequency meter. So I made a cardboard with 4 tuned Agora clock boards. I set the exact time on the clock and prepared to let them go for a month

    OK, so I had now 4 clocks that a wanted to watch for.

    Then I got an idea to turn them into the kitchen clock. My wife was surprisingly not against it.

    So I let the clock to go for a month and then I measured the differences.

    After a month it was 2678400 passed seconds. So I did a little math and the results are here:

    XTALHP counter ppmreal ppm
    9 pF0.232
    12.5 pf0.20-2


    Now I know how important it is to have the RTC frequency of all clocks checked. Because the ATmega328p has no firmware feature to tune the clock I have the only option to tune capacitors on every clock to have good time accuracy.

  • Bed of nails fixture

    Daniel Zilinec12/04/2021 at 23:13 0 comments

    I made a test fixture for faster programming and testing. All parts are 3D printed

  • Menu - new photos

    Daniel Zilinec01/13/2020 at 20:59 0 comments

    The menu is implemented in software a while now, but I haven't published any photos yet.

    Menu with time and alarm settings:

    Time setup:

    Status screen with battery voltage, temperature, time from last charge and total uptime :

  • Improved 3D models

    Daniel Zilinec01/13/2020 at 20:52 0 comments

    I have made some minor changes on the clock's body. But it's still compatible with the electronics from older version.

    New models printed on textured sheet:

    Buttons have symbols now:

    On the bottom there are new holes for the alarm buzzer:

  • Maker Faire 2019 Prague

    Daniel Zilinec06/26/2019 at 11:32 1 comment

    My stand on Maker Faire 2019 in Prague.

  • Aroga PCB v1.1 released

    Daniel Zilinec06/01/2019 at 18:53 0 comments


    New PCB version released and first prototypes is working.


    - new charger IC; charging current is limited to 100 mA and the watch is charged in 2 hours

    - simplified bill of materials

    - 4 backlight LEDs instead of 2

    - THT pads for buttons; easier connection of PCB with buttons for the clock

    New schematic: