In the age of 'music streaming', terrestrial radio listening seems antiquated. In the absence of WLAN or mobile communications, however, it can be an alternative to stay connected to the world, at least while consuming. FM (UKW) transmission was first trial took place in Germany almost 100 years ago (1925). And actually FM was supposed to be discontinued in 2012, but this only became reality in the cable network. To the delight of motorists with conventional analogue radios. The current digital successor DAB+ (the previous, incompatible and not very successful DAB was discontinued) is at least visible on the market and there are a number of inexpensive DAB+ receivers under €30. I decided to try out a TechniSat TECHNIRADIO RDR:

The radio has all the important functions for digital reception in a handy housing. But what if you had a small, programmable FM/DAB+ module that you could use in your own projects? A new project is born…

By the way, I had my first DIY attempt with DAB+ in 2014 with a module (T2-L4A-8650C) from the Taiwanese company KeyStone Semiconductor. Here is the related thread in microcontroller.net (German).

In the hobby area, however, the selection is still very limited. In the pandemic year 2020 I could only find two modules:

• DAB/DAB+/FM Radio Shield for Arduino for ca. £60,-
• uGreen DAB Board for ca. 45,-€

What all of these receivers have in common is that they are based on an IC SI4684-A10 from the US company Skyworks Solutions Inc. (formerly Silicon Labs.) which is available for approx. 15,- € from few US distributors (e.g. Mouser ). The SI4684 is a single-chip FM/DAB/DAB+ radio receiver (Datasheet ).

In order to use the IC for your own projects, you would need the evaluation board, which also includes the necessary firmware CD. However, the price of over €500 leaves the maker a bit perplexed and therefore restless. Unfortunately, the firmware alone is not (anymore) available online. A good source of information is the Rundfunkforum (German).

A look at the receiver H/W revealed that the firmware resides in an external serial FLASH memory (typ. 25L16). Depending on whether it is in FM or DAB+ operation, a different part of it is loaded into the SI4684.

So there is a solution at hand. Simply read out the FLASH and copy it for your own use. The right tool can be obtained in the Far East for less than €10.

Reading it out is easy, but it is more difficult to find out where what is located. After a detailed study of the data sheets, application notes and the evaluation of the SPI traffic between the MCU and SI4684 in the TechniSat radio, the following address locations could be found:

Start      End      Size       Module
=============================================
0x002000 - 0x0036A1 (0x016A1)  Patch016
0x004000 - 0x0056A1 (0x016A1)  Patch016
0x086000 - 0x0FEE69 (0x78E69)  DAB F/W  V6.0.5
0x106000 - 0x187721 (0x81721)  FM F/W  V5.1.0

This information is needed later when the corresponding firmware has to be loaded. But first we need a hardware.

My goal was to develop the smallest possible module, which can then be placed anywhere, similar to the KeyStone solution. The chip manufacturer's Application Note AN851 provides an example schematic and some PCB design rules. All the designers of the boards I have have adhered to this. This inhibits willingness to experiment:

The module contains everything for relaxed FM/DAB+ reception and offers the following connectivity:

• 3.3V/GND - Supply voltage
• RESB - Reset of the SI4684
• INTB - Interrupt output of the SI4684
• ROUT/LOUT/GND - Analog audio output with GND (music comes out here)
• SSB/SCLK/MOSI/MISO - SPI interface to controlling MCU
• SMA - Antenna connection via SMA jack or simple wire

The connections are in a 2.54mm grid and can also be soldered to pin strips. To keep everything small, the passive components are designed in SMD 0402. A great challenge for the manual assembly but with a steady hand it is (mostly) possible.

The PCB always looks more benign after routing than during the design process. For a better representation in the following picture without filled ground areas: