When I was brainstorming for this project I debated using a dedicated DTMF decoder IC or decoding tones using software and digital signal processing. The microcontroller I'll be using for this project is an Arduino Nano clone which is based on an ATmega328P microcontroller. This microcontroller has a couple of libraries available for DTMF decoding, but I can't find any such libraries that include tests or evidence that they work reliably in the real world. I decided to take the easy way out and buy a few DTMF decoder modules that use the MT8870D decoder IC.
I ordered the modules from Amazon, but the same blue module seems to be available from a variety of online sellers. For some reason all the modules I've seen have the pins installed "upside down". I'm not sure what the logic behind this was, but it makes prototyping a bit more difficult. I corrected this issue by removing the pins and installing them on the bottom of the boards so I can easily use the modules on breadboard and perfboard. To the left is a module as it is assembled, to the right is a module that I modified.
I was curious about what all the extra interface circuitry was on the module, so I looked up a schematic for the module and hit up the chip's datasheet. I found the schematic for the module by Googling and ended up at this product page. It turns out the MT8870D can be configured for a wide variety of use cases that can affect the timing of DTMF digits. The module utilizes an example circuit exactly as it appears in the datasheet.
The table at the end of the datasheet indicates that the example configuration should accept 40ms mark and space durations and reject any durations that are less than 20ms. I plugged the module into a breadboard and verified this was the case using and Android app called ToneDef. After I wired up the module I created a firmware repository on GitLab and hacked together a simple driver to echo received digits over the serial line.
I have only done basic testing, but the MT8870 module seems to be working very well. I may try to release a version of the firmware in the future where DTMF decoding is done in software, but for now the low cost and high accuracy of the decoder modules is very attractive.