Doug's STM32 MIDI Synth Proof of Concept 1 - Software

• Author: Douglas P. Fields, Jr.
• Copyright 2024, Douglas P. Fields Jr.
• License: Apache 2.0
• Last updated: 2024-11-10
• Repo Self-Link
• Portions used under other licenses
  • SPI Display - MIT License
  • STM Software / Generated Code - Delegated to LICENSE file, hence Apache 2.0

Software for my STM32 MIDI Synthesizer PoC 1.

Credits

• SPI Display Software by Aleksander Alekseev under MIT License
  • As enhanced by my project

Software

• Windows 11
• STM32CubeIDE
• STM32 HAL libraries

Hardware

• STM32F722RET6
  • STM32F7x2 Overview
  • STM32F722RE Page
  • STM32F722RET6 Part
  • STM32F722 Data Sheet
    • DS11853 Rev 9 July 2022 as of this writing
  • Package: LQFP-64
  • Core: ARM Cortex-M7
• Audio DAC with headphone amplifier
  • DAC: PCM5102A
  • Amplifier: PAM8908JER
• MIDI in/out
  • Opto-isolator:
    • EVT1: 6N137S-TA1
    • EVT2: TLP2362 - much smaller
• Power: USB-C
• Programming: SWD (with 5-pin header, no SWO)

External hardware:

• USB Console: Inland FT232 TTL UART to USB board
  • Similar to this one
• SPI Display: PJRC ILI9341

Pinout

See pinout-with-alternates.csv. This was generated in the IOC editor, Pinout & Configuration → Pinout → Export Pintout with Alt. Functions.

SPI

GPIO Pins

• CS (Chip Select) - PA15
• DC (Data or Command) - PB8
• RESET - PB5

Power (for PJRC ILI9341 Display)

• VCC - 3.3V (works despite spec sheet saying 3.6-5.5V)
• GND - GND
• LED - 3.3V

SPI MISO/MOSI/CLK - wired directly to SPI2 header

SPI2 DMA

• DMA 1 Stream 4
  • Memory auto-increment
  • Mode Normal
  • Priority Medium
• NVIC
  • DMA 1 Stream 4 Global Interrupt
  • Set the priority just behind the audio I2S and ahead of the UART interrupts

SPI2 DMA Callbacks

Setting HAL DMA callbacks doesn't call anything back when using HAL SPI DMA calls.

So, enable SPI callbacks:

• Edit IOC file ->
• Project Manager ->
• Advanced Settings ->
• Register Callback ->
• SPI: Enable

See example.

Now the transfer complete callbacks for SPI transfers in DMA mode works.

DMA Notes for SPI - Flash

DMA directly from Flash to SPI does not work. The HAL DMA transfer never completes and the status just says BUSY the whole time. This was evidenced in the image DMA directly from text (const image data) stored in the flash being unable to be DMA'd to the display.

I don't know if this is supposed to work but I'm just doing it wrong (e.g., using the flash alias address instead of the original flash address, or something).

This was worked around by copying the flash to memory and sending the data that way.

Functionality

The software is adapted from my project Nucleo UART, which has this functionality:

• Decode incoming MIDI
• Create a single triangle wave tone based on last note on received
• Output the tone to the Audio DAC used in this board
• Display on the PJRC ILI9341 SPI display

TODO

• TODO Transfer the memory map over for all the different memory areas
  • And the BSS/Data section initializers

DONE

• DONE Make the LED code toggle the LED on my board
• DONE Make the button reading code read a button on my board
• DONE Set up the clocks
• DONE Set up the UARTS
  • Console: 460,800, Overrun enable, DMA on RX Error enable
  • MIDI: 31,250 Bits/s, 8-N-1, overrun enable, DMA on RX Error enable
• DONE Set up NVIC
  • USARTx global interrupt (3, 6) with preemption priority (2, 3)
  • 3 bits for pre-emption
• DONE Update the text/functionality
• DONE Set up the DMA
  • SPIx_TX -> DMA 1 Stream x (was 3, 5)
  • Circular to Memory
  • Half Word both ways
  • Global interrupt & preemption priority ("Force DMA channels interrupts")
• DONE Set up I2S
  • Master Transmit
  • I2S Philips
  • 16 bits data on 16 bits frame
  • 32 KHz
  • Clock Polarity Low

DMA UART Circular Receive Test

Goal

Turn UART receive into free-running DMA circular buffer receive. Periodically check what the last byte written by the DMA was, and read the bytes since we last read until DMA's last written.

Need to know: Can I get the current write pointer from the DMA system?

Online notes

Reference Steps to be considered (in order):

• Use configuration from CubeMX

• Configure DMA memory & peripheral addresses for UART and your buffer

• Configure DMA length (number of bytes to transmit in your case)

• Enable UART DMA TX request (not done in your example)

• Enable UART TX mode

• Enable UART itself

• Enable DMA channel

• Transmission will start

• Doc that might help.

EVT1 Hardware Test Notes

Serial console

• Works
• Only the Ground pin needs to be connected
  • The 3.3V pin does not need a connection

USART6

Connected this to ubld.it MIDI breakout:

• RX -> Rx
• TX -> Tx
• Power/Ground

Configured as such, MIDI in works fine.

MIDI

MIDI In

sendmidi dev "U2MIDI Pro" on 64 100 +00:00:00.200 off 64 100

• MIDI 1 and 2 IN (UARTs 1 & 3) work
  • WHEN REWIRED externally - see problems below

MIDI Out

receivemidi dev MidiView

• MIDI 1 & 2 OUT work
  • When rewired externally - see problems below

LEDs and Buttons

• All work

EVT1 Hardware Problems Noted

Problems noted exist on both assembled boards. Note one board is missing J5 so easy to differentiate.

Tests remaining

• I2C
• SPI
• GPIO
• The other serial ports
  • Test MIDI with a ubld.it MIDI breakout
  • USART6 RX has been tested
• RTC
• Master clock output
• Wakeup

P0

• MIDI In doesn't seem to be working on either port

  • I miswired the Audio Jack
    • See Helpful Page
    • I got the tip and sleeve reversed
    • Using a patch cable to swap them makes the MIDI 1 work fine
  • This was fixed in EVT2 of the hardware

• Enabling/disabling audio mute while no sound is playing makes an audible clicking sound on headphone out

  • This is also audible on the line out
  • It does not occur when I2S audio out is stopped

P1

• Console is USART2
  • Nucleo boards use UART3 (oops!)
  • (But note the console UART works)

Debugging Notes

MIDI Input Fails after a while

For some unknown reason, the MIDI input now fails after a while.

I tried it with MIDI1 & 2 (USART1 & 3), and my external MIDI adapter on USART6.

It does not show any MIDI overrun errors.

Currently a complete mystery why this is happening.

Seems like it's getting some error condition as these make it work again: LL_USART_ClearFlag_LBD(MIDI1_UART); LL_USART_ClearFlag_PE(MIDI1_UART); LL_USART_ClearFlag_NE(MIDI1_UART); LL_USART_ClearFlag_ORE(MIDI1_UART); LL_USART_ClearFlag_IDLE(MIDI1_UART);

Looks like it's getting an ORE.

The fix: Move to interrupt-driven receiving.

I have moved to interrupt-driven receiving using LL drivers. This does not seem to have fixed the problem. I can still get plenty of overrun errors on MIDI (which is slow, ~3kBps)!

Let me try making the interrupt priority higher. That seems to have done the trick (setting interrupt priority higher), but it interferes with refilling the I2S audio buffer and we get crackling. Doubling the I2S buffer from 128 to 256 samples seems to have helped quite a bit.

Break over Serial Console

Interestingly, if I send a BREAK using Putty over the serial console at 460,800 bps, it resets the Microcontroller!