DCC Dekoder Attiny402 1

DccFunctionDecoder

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A simple DCC function decoder with 4 AUX channels. Compiles for an ATtiny402

Functionality:

• CV configurable
• 4 individual outputs
• 3 modes available for each output: FADE, BLINK, NEON
• Function mapping for F0...F20 for each output
• all outputs independently dimmable
• direction and/or speed dependent output control
• short and long address
• consist address

Hardware

KiCad files are available in the Hardware-folder

The PCB has a size of approx. 7 by 7 mm. PCB is double-sided and only has standard components in the size of 0603 or larger to enable hand soldering of the decoder.

Software

The software can be compiled and flashed using the Arduino environment.

Usage

Flashing the software

You will require a standard USB-UART adapter (5V compatible). Connect the adapter as shown with the decoder. You MUST NOT connect the decoder's 5V with the UART-adapter. The LDO on the decoder does not like being reverse powered. Switch on the rail's power supply.

Select the appropriate COM port in the Arduino IDE and select Sketch -> Upload using Programmer (or hit CTRL+SHIFT+U). Just hitting the Upload-Button (or pressing CTRL+U) will not work.

First usage of the decoder after flashing

Right after flashing, the Decoder is NOT yet ready for usage, as the ATtiny's EEPROM is still on factory default and the decoder therefore has an unknown address. You can already wire up the decoder as described in the following section. Then, perform a factory reset of the decoder by writing the value 8 to CV8, using the decoder's magic address 10239. In case you forgot your decoder's address, you can always perform CV writes using the magic address 10239. If you are using DecoderPro, proceed as shown below:

Using the decoder in a locomotive or wagon

Connect the LEDs as shown. Use appropriate resistors. The maximum current should be limited to 2 mA per LED. That does sound little but usually it is enough. Otherwise the LDO on the decoder gets too hot.

Programming CVs

The decoder ONLY supports Programming on Main (POM) mode. It cannot generate an ACK. For CVs see the table below. The decoder will confirm each CV write with a short flash of all AUX channels.

Programming the address

Special note on programming the address: If you want to change the decoder's address, you must do so in the following order:

1. program CV3 or CV17 and CV18 as desired.
2. program CV29 = 0 (short address) or CV29 = 32 (long address) to confirm address change (even if you didn't change from short to long address or vice versa)

From then on, the decoder will only accept CV writes or commands to the new address. It will of course still accept commands to its consist address and it will also still be programmable via its magic address 10239.

Decoder reset

The decoder can be reset to its default CV values by writing CV8 = 8. This is confirmed by multiple short flashes of all AUX channels.

CV table

Notes

CV55 and CV56 (forwardOnly and reverseOnly) are evaluated for F1...F20 only. The AUX mapped to F0f/F0r are active in forward/reverse direction only, independently from forwardOnly and reverseOnly

Example for Speed Threshold AUX n

CV113 = 0b0000000: AUX1 active for speed >= 0 (=always active)
CV114 = 0b0010000: AUX2 active for speed >= 64
CV115 = 0b1000000: AUX3 active for speed <= 0 (standstill only)
CV116 = 0b1001000: AUX4 active for speed <= 16

License and Legal

Copyright 2025 by Markus Grün

Software licensed under GNU General Public License v3.0, see license file in Software-folder

Hardware (Schematics, Layout, etc.) licensed under CERN-OHL-P v2 or later, see license file in Hardware-folder

THE SOFTWARE AND SOURCES ARE PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR SOURCES OR THE USE OR OTHER DEALINGS IN THE SOFTWARE OR SOURCES.