Doing Audio playback through CS43L22 and SDIO with the STM32F4Discovery Board

There are so many examples of audio playback using the on board CS43L22 audio DAC for the STM32F4Discovery available online. The official demo uses the USB Host functionality to read a raw audio file from a USB Flash Drive. Then there are examples that use SD cards in the SPI mode. The STM32F407 microcontroller on the STM32F4Discovery does pack in an SDIO bus for native interface with SD cards, however it turns out that the CS43L22 connects to the STM32F407 using two signal pins PC10 and PC12 that are required by the SDIO bus [SDIO_D2 and SDIO_CK], further, those signals are non-remappable. Therefore it seems it is not possible to get both the DAC and the SDIO interface working together.

I wondered if it was possible to physically remap the I2S3 pins that connects to the CS43L22, instead of the SDIO pins. A check at the datasheet confirmed that I2S3 signals I2S3_CK (initially at PC10, also SDIO_DAT2) and I2S3_SD (initially at PC12, also SDIO_CK) may be remapped through software to appear instead at pins PB3 and PB5 respectively.

I also had the following alternatives in mind:

  • Using the on-board DAC on the STM32(pins PA4 and PA5) with a headphone amplifier.
  • Now that the signals have been remapped, using an external audio DAC / codec to play back the audio. I felt this somehow defeated the purpose of having a complete audio DAC setup onboard.
  • Building a full custom design. This is a good long-term solution, and is in progress.

Since I also had a LCD connected via FSMC to my set-up, the LCD RD pin was mapped to FSMC_NOE (PD4), which also goes to the RESET pin on the CS43L22. This issue had been taken care of by making the LCD write-only [the pin was configured as general push-pull, not AF mode].

I however decided to solve the problem in situ by modding my STM32F4Discovery and rewiring the I2S3 signals onto PB3 and PB5. Armed with the CS43L22 and STM32F4Discovery datasheets and following the traces on the bottom layer of the PCB, I was able to cut the traces and then reroute them onto the PB3 and PB5. It was so simple, I wonder why it hadn’t been like that in the first place.


The F4 Discovery board undergoes the knife

However getting the modified board play back audio via the microSD card took longer than expected. This was due to my inability to perform simultaneous I2S and SDIO data transactions to enable streaming audio playback after the traces were cut and rerouted. I was able to play wave files though by sending the stream to the CS43L22 Audio DAC and waiting for the transfer to complete before requesting another chunk of audio data from the SD Card. This naturally led to significant stuttering in playback.

Debugging led me to the SDIO low level driver, which used to fail after the first frame was read and sent, with the error flag SDIO_STA_DCRCFAIL, meaning data corruption (CRC on received data failed). But how? The traces had been cut properly, there was no sign of electrical continuity.

My instinct was to try out SDIO in 1-bit bus mode. Circular audio playback worked perfectly this time. Therefore it could have something to do with the cut traces (PC10 and PC12). The traces were quite close and one of the cut ends now carried a 48 MHz clock signal.

Next was tried addition of series resistors (100 ohm) on the SDIO lines going from the card to the board. I targeted PC10 and PC12 specifically, and the issue seemed to be resolved even with 4-bit mode now. I was now successfully able to play back streaming audio from the SD Card.

Post-Script

Through further trials, I found out that:

  • Keeping 100 ohm resistors between the SD Card both PC10 (SDIO_CK) and PC12 (SDIO_D3), circular audio playback works
  • Keeping 100 ohm between SD Card and PC10 (SDIO_CK) but not on PC12 (SDIO_D3), it still works
  • Keeping 100 ohm between SD Card and PC12 (SDIO_D3) but connecting PC10 (SDIO_CK) directly to the SD card worked but locked up after a few seconds or so.

The SDIO_CK line needs suitable termination, especially with mine being a wire-wrap breakout board that puts everything together.

The sources will be made available soon.
The sources are now available at https://github.com/abhishek-kakkar/STM32F4SDIOAudio . The code is provided “as-is” as a reference that might be useful, compilation may break with the latest versions of ChibiOS and uGFX.

An expansion board for the STM32F4Discovery

The STM32F4Discovery from STMicroelectronics is one of the mature, extremely affordable, and yet capable development boards available in the market [I say mature because it has been around for quite a while; since Q4 2011]. The board is equipped with a STM32F407VGT6 ARM Cortex-M4 Core with embedded FPU running at 168 MHz, 1 MB of Flash and 192 KB of SRAM, and adds an accelerometer, a CS43L22 Stereo Audio DAC (with a headphone jack), an MEMS digital microphone and USB OTG support. The demos provided by ST demonstrate its audio playback and recording capabilities, which I was quite impressed with when I tested it for the first time.

The STM32F407 Microcontroller also packs in support for seamless LCD interfacing (via FSMC), SDIO, DCMI and Ethernet, hence I built a circuit to augment the STM32F4Discovery board capabilities by adding support for attachment of:

  • TFT LCD module with a touchscreen. These are cheaply available via eBay.
  • a microSD card [power to the card controllable via a PMOS switch].
  • Camera module with a 8-bit HSYNC/VSYNC/PCLK interface.
  • [Planned] Ethernet support, either via ENC28J60 or a DP83848 Ethernet PHY

The circuit was initially wired in Summer 2013 and recently modified to support audio playback alongside SDIO [see article here]

Here’s a pic of the board up and running:

BoardCeption - a board within a board within a board :P
BoardCeption – a board within a board within a board :P

Hardware

A double sided PTH dot matrix board is used for construction. The board was just in size to accomodate the STM32F4Discovery board on one side and the LCD with its 40-pin connector on the other making it a stacked 3-layer design. 0.1mm Magnet wire is used for all electrical connections.

The LCD side also has a 3.3V LDO for power distribution, a connector and charging circuit for a 1-cell Li-Ion battery. It also has a connector for a 10-DOF IMU for expansion.

Two 2.5cm spacers and two 6 cm metal studs mounted on the 4 mounting holes on the PCB act as an inclined stand for the assembly to be kept on a table top.

LCD

The LCD used is a 3.5″ HVGA LCD display [320×480 pixels] with a 4-wire touchscreen, mounted on a breakout board. The module used in it is a TRULY TFT1P7134-E, which uses HFFS (High Fringe Field Switching) technology, which is somewhat similar to In-Plane switching (IPS) and gives true-to-life colors. There is no color distortion noticed in the image, even when viewed at almost 180degree angles from any direction. It uses the Renesas R61581 TFT controller, which is an enhancement of the ILITek ILI9481 originally used in such displays (the instruction set is nearly compatible).

3.5" HVGA, resistive touch
The LCD module – 3.5″ HVGA

It connects to the 16-bit FSMC bus on the STM32F407 Microcontroller, which allows the LCD to be accessible as simply external memory, and enables DMA usage for data transfer to save CPU utilization. The module reset pin is connected to the MCU reset pin. The backlight is connected through a PMOS to TIM5 on the STM32F407. The R61581 supports in built LED backlight control, but I have disabled it and gone for the direct control in order to support more compatible displays.

The LCD breakout board contains a XPT2046 (ADS7846 compatible) touchscreen controller that interfaces the 4-wire touch panel via the SPI bus to the microcontroller. The PENIRQ output provides feedback on screen touch.

Almost any LCD from eBay which comes with a 40-pin connector will be able to connect to this connector and can be supported with suitable changes to firmware.

The uGFX library is used to interface to the LCD and touchscreen. It also provides a widget toolkit for UI design. The R61581/ILI9481 drivers adapted for this board have been contributed by me to the uGFX project.

microSD Card

microSD Card socket
microSD Card socket
The microSD card connects to the SDIO bus on the STM32F407. 4-bit bus mode is supported, and performance is stable at 48 MHz operation after recent addition of 100 Ohm termination on SDIO_CK (the clock line).

FatFS is used on the software side for file operations on the card. Read speeds of up to 9MB/s (theoretical maximum is 12MB/s) have been achieved with large buffer sizes.

Card detection is currently not implemented, but will be taken care of in future hardware revisions.

Camera Module

The STM32F407 contains a Digital Camera Interface (DCMI) bus that captures data sent in by a digital camera in the 8-bit format with external/inframe synchronization. Here only the HW synchronization via HSYNC/VSYNC/PCLK is used. The connector on the board was designed to be compatible with a OV7670 camera module, like the one available here

To drive the camera module, the XCLK is fed from PA8, which is a master clock output of the STM32F407. The MCO1 is configured to output a 16MHz clock using the HSI.

The control bus for the camera is SCCB, which does work with standard 400kHz I2C signals, with modifications in the way data is read, and delay between subsequent transfers.

I did try interfacing with a OV7670 camera module but I was largely dissatisfied with the results, they were terribly off-color.

IMU

10DOF IMU
The 10DOF IMU
The board also has a connector for a 10 DOF IMU board available on eBay. It consists of an ADXL345 accelerometer, L3G4200D gyroscope, HMC5883L compass and BMP085 pressure sensor, everything accessible via the I2C port through pins PB6 and PB9.

Software

The board runs ChibiOS and uGFX.

ChibiOS is different from many other real time kernels in the fact that it offers a tightly integrated hardware abstraction layer (HAL), which is very well written and easy to use. The HAL provides, among other things, off-the-shelf support for SD cards via both SDIO and SPI and integration with the FatFS library, GPIO configuration and asynchronous transfers on SPI and I2C and serial ports, with built-in support for DMA usage to offload transfers and save the CPU for computation. ChibiOS 2.6.3 is currently being used.

uGFX offers support for rich graphics and touch input. It also offers a GUI toolkit which can be used with a Windows/Linux simulator to develop GUI on embedded devices.

Both ChibiOS and uGFX are available under a dual license for free non-commercial usage and commercial licenses once used in production purposes.

A custom board file was created off the reference STM32F4Discovery board.h and board.c .
Drivers have been written for the CS43L22 using the ChibiOS STM32 HAL. Since I2S support is currently not implemented in the STM32 HAL, I had to implement I2S configuration & circular transfer using DMA.

The software development takes place using an Eclipse-based development environment on Windows 8, with GNU Tools for ARM Embedded Processors . OpenOCD is used to debug in-circuit using the onboard ST-LINK/V2.

This summarizes the complete development platform.
Schematics would be coming soon.