Best protocol for daisy chain communication between many small microcontrollers

Question

I'm working on a project where I'd like a large number of small microcontrollers (100+, something like an 8-bit PIC, but flexible on what I use) to each perform audio synthesis behavior defined by an 1-4 byte shift register. The idea is to program each micro as an oscillator and have frequency, pulse width, and phase each controlled by the register data and to have one master device pass data to all the smaller devices in a daisy chain configuration.

Think of it as the audio equivalent of a WS LED strip.

For this to work properly, I need a protocol that can spit out high-speed data to all the micros, allow each micro in the chain to pass data on to the next micro as it is being clocked in, and then update each micro simultaneously once the data write is complete.

It also needs to be a protocol that can survive going some distance over a wire run, I will almost certainly add an input / output buffer to the PCBs, but assume wire runs between PCBs can be up to 10 ft.

What am I looking for to do this properly based on the parameters I have described?

Answer 1

That's literally what SPI is: clocking bits into the shift register in each microcontroller, while clocking out the ones already stored in there.

Since any reasonable MCU already has an SPI peripheral, there's nothing you need to invent.

Distance here is mostly a matter of speed, and you can be as slow as your application allows. ("High-speed" sadly tells me nothing? Is this an easy 1 Mbd, or a hard 50 MBd? Who knows.) Should you need higher speed, the standard way is just to add a differential line driver at each MCU's output, and to just buffer the differential bit clock, as well.

8 bit PICs are currently overly expensive for their capabilities, so you'd probably fare cheaper with an stm32c0 or similar. Also makes the audio stuff much easier, as you get far far far superior timer/PWM units. Plus, programming equipment for cortex-m0 MCUs like these is much cheaper, and the compilers much better, so that this would really be a no-brainer for me.

Answer 2

This sounds like a job for DMX512. DMX512 uses RS-485 with simple serial coding, well within the reach of an 8-bit micro (some Microchip devices even have DMX hardware support, see below.)

Up to 512 8-bit DMX nodes per ‘universe’ are supported, at a refresh rate of 44Hz. Data is sent as entire frames at fixed time intervals.

DMX can physically support up to 32 devices in a daisy chain. Stage setups will often break up DMX into several branches using repeaters, or even break them out as separate universes if a higher refresh rate is needed.

DMX512 devices are electrically daisy-chained using twisted-pair 'balanced' audio cables terminated with XLR plugs. Termination is 120 ohms.

Some DMX implementers will use CAT3 type twisted pair cable instead of audio cables. You could choose that and save the cost.

Finally, there are wireless and Ethernet bridging options for DMX512.

More about DMX512 here, from Microchip.

Answer 3

I worked on a project like this and ended up using the RP2040 with two RS-485 ICs, one for "upstream" messages toward the controller and one for "downstream" messages, using both RP2040 UART buses. Then, when the final boards were in sequence, they all communicated by incrementing the ID in a JSON message. This allowed the firmware on each board to be identical with no dipswitches or other addressing requirements, and the RS-485 protocol allowed for a highly variable distance between each board.

In the final version, broadcasts could be sent from the controller to all boards, or messages could be targeted to any individual board in the array.