Teensy 4.x and communication

The voice controller card needs to communicate both with the master controller and with its peripherials.

Master controller should use the voice card as slave and needs a hardware SPI to do so properly. Hopefully, one master may control multiple voice cards simultaneously if we just buffer the SPI output.

The voice card controller needs to control these three:

The CV DAC

Currently, this is done in software, at 50MHz. This eats up a lot of clock cycles. Instead, we should look into using DMA backed SPI - that would let us send 24bit in the background without blocking the controller:

 https://forum.pjrc.com/threads/67247-Teensy-4-0-DMA-SPI

Update: 

Using SPI.transfer((void *)txBuffer, nullptr, 8, callbackHandlerFast); sends a whole buffer in the background and may be exactly what we need.

The DCOs

These are connected to the hardware SPI today, but runs at max 4MHz, which means moving them to software would block for even longer periods than the DACs. Not entirely sure what to do here, perhaps some timer based scheme, or perhaps we can change the SPI speed and use the same SPI as for DAC?

I should really look into SPI transactions for this.

https://forum.pjrc.com/threads/57754-Teensy-4-0-SPI-Clock-Speed

Update: 

Switching transfer speed is painless and costs next to nothing. Using 

SPI.beginTransaction(fastSettings);

where fastSettings sets speed etc: 

SPISettings     fastSettings(8000000, MSBFIRST, SPI_MODE0);

This means that DAC and DCOs can share SPI bus.

Port expanders

Right now I use SPI for this, but I will move these to PCA95xx, I2C chips. We don't need high speed here so freeing up SPI from this task is good. Port expander changes will always (?) be on-demand so it doesn't matter that much if they block for a tiny amount of time.

Audio daughterboard

I have yet to try this, but it runs from a combo of I2S and I2C. Both are available and there is a separate I2C that can be used if I don't want to combine it with the port expanders even though the bus is just that, a bus.

Hardware

My current voice card uses a Teensy 4.1. It has SPI (SPI0) and SPI1 exposed as pins, and two I2C-busses readily available. SPI2 is used for SD card and memory on the teensy itself. I may be able to solder stuff directly to the pads but not sure how easy it will be to use them. 

The Teensy 4.0 only exposes SPI (0) as a pin, but adding SPI1 is not too hard. SPI2 is also available but requires soldering wires. It can be done but I would prefer not to.

Going with the Teensy 4.0 would save significant physical space on the voice card, and it is also about 50% cheaper than the 4.1 (with ethernet). I would however lose the posibility of using on board PSRAM and extra flash memory which MAY be nice if using an audio daughterboard?