Can the preenF405 be used standalone

[☺]

I am looking to embded an FM synth in a project, and the preenF405 looks good and simple to use. I want to control it over serial (ideally faster than 31,250 baud)

Is it possible to add the two MCP4922s, a USB for programming (if required) and have this work?

If that is possible, are there any potential hiccoughs I would have to look out for?

Thanks for the help.

[Xavier]

Is it possible to add the two MCP4922s, a USB for programming (if required) and have this work?

Best start would be to have a look at the PCB R5 and see what you want to keep :
https://github.com/Ixox/preenfm2/tree/master/hardware
(Files are done with a very old version of Kicad. Latest version loads the PCB fine, but not the schema).

So that would be preenF405, 2 MCP9422 + 3.3v converter + some resistors and capacitors.
In any case you have to modify the firmware (it won't boot without USB key), so no need for the USB (programming) find a STLink to upload the firmware.

If that is possible, are there any potential hiccoughs I would have to look out for?
Thanks for the help.

Possible ? yes sure.
That's not a small project 

[☺]

Thanks for the quick response and clarifying.

Thankfully you've got a .pdf schematic there which I've had a look at.

I would imagine the USB key is heavily integrated in the code and it would be simpler to just include the USB stick rather than go through and try to remove all references to it.

I've attached a schematic, copied from the R5's, but cut down to just the essentials. Do you think this would work as is without modifiying firmware i.e. would it take a 3.3V MIDI signal from a microcontroller and produce sounds on L and R?

Again, thanks for your help.

[Xavier]

I can  say it looks good.
Now you have to try to see if that works

I never tried direct midi communication between 2 microcontrollers.
First try with the default 31250 b/s then it will be easy to get rid of this limit

[☺]

Thanks. I'll give it a try.

[Xavier]

I realized the output stage of the DAC is not great in your schema.
You miss the opamp and the DC removal part.

Resistors 1.8k and 115k (1.8*64) which adds the output of the 2 internal DACs will make a high output impedance, so i'm afraid the volume will be very low.
Also the output will not be zero centered, dac output is between 0v and 5v which is not great for the speaker/headphone you'll add after.

2 solutions :
. After  L & R , you can add an opamp (MCP6002) in following mode + DC removal as shown on diagram A.
. Or you can replace 1.8k and 115k by lower values (but with a 64 ratio between the 2 values / replace 1.8k by < 200) and directly adds diagram B.

[☺]

As I intend this to be a module, amplification and DC removal are on the main board as per the attached schematic.

Your message does make me think it would also be a good idea to deal with this on a per module basis to give a stronger signal which is hopefully less likely to receive interference as it winds its way along the board. It won't matter for a while though because I still need to sort out the unit itself before I start looking too in depth at modules. I have a lot of programming to do.

Thanks for all your help. If I ever get far enough that I make this into a module, I'll report back.