Back in 2016 I had made a thread https://forum.pjrc.com/threads/40982...ynthesizer-DSP and ended up doing some analog projects instead. Decided to give digital another go and saw the 4.0 was coming so cooked up a new design.

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You can see my design here http://www.sinphi.com/synths/humours/humours.html, some of which I will repeat here. My intention is to focus on reverb algorithms, but I imagine I will have some fun with all aspects of the library.

1. Which pin should be used for non-blocking WS2812/neopixel communication? See my pin out list below*.
2. Will quad I2S work with shared pins like on the 3.2 with the WM8731? I am assuming this on hasn't been tested yet with the 4.0. I see some people are still figuring things out with the WM8731, so keeping an eye on it.
3. Is the SPI buffer chip for SRAM needed, the 74LCX126? Currently only using the 74LCX138 to mux chip select.
4. Is any additional filtering on fast edges required? I have left pads for SPI and I2C to have it as necessary. Only on the MCLK of the I2S.
5. Any foreseen issues with the MAX1300, or recommendations on alternative 8 input DAC for DC input?

Schematic for reference:
http://www.sinphi.com/synths/humours/Humours_IO.pdf
http://www.sinphi.com/synths/humours/Humours_Main.pdf

Pin Assignment:

Max 1300 ADC
0 - SPI CS
1 - SPI MISO
26 - SPI MOSI
27 - SPI SCK

WM8731 Codec x2
16 - I2C SCL
17 - I2C SDA
7 - I2S Out Digital 1
8 - I2S In Digital 1
2 - I2S Out Digital 2
5 - I2S In Digital 2
20 - I2S LRCLK
21 - I2S BCLK
23 - I2S MCLK

74LSX138 for SRAM
28 - A2
30 - A1
32 - A0

SRAM
11 - SPI MOSI
12 - SPI MISO
13 - SPI SCK

I2C Expander
18 - I2C SDA
19 - I2C SCL

8051 Mux x3
3 - B
4 - A
6 - C
10 - Common Button
25 - ADC 1
22 - ADC 2

9 - Sun Button
15 - Sun Gate

Neopixel
???