How to use the SD card of the STM32F407VET6 board with Arduino IDE

 

This interesting and economical board mounts an ARM Cortex M4 MCU type STM32F407VET6 produced by ST Microelectronics and is also called “black board” or “black pill” due to the dark color of the pcb. There are several versions, the one I used is shown in figure 1.

Figure 1 -The Black Board

The MCU has 100 pins (LQFP100 package), so it is suitable for those applications where a lot of I/O and also a remarkable speed of execution are required. Its main features are: 168MHz clock, 512KB of Flash memory, 128KB of RAM, 82 GPIO (many of which 5 volt tolerant), 16 12-bit analog channels, 2 12-bit DAC channels, a USB OTG port and many other things. Among these, a 32768 Hz crystal RTC and 4kB of RAM rendered non-volatile by the backup stack.

I bought this interesting and inexpensive card not only for the reasons mentioned above but also because it had an micro SD card connector. I had already done several projects with the STM32F103C8T6 board, also called “bluepill”, compiling with Arduino IDE after loading the package and I wanted to experiment on bigger systems too.

The SD card

It is not connected to the SPI bus but to the SDIO bus and the Arduino libraries use the SPI. To be able to use the more performing SDIO interface, you need to change the development environment, but I didn't want to do this.

To overcome this obstacle I connected the SD to an SPI bus with three external wires connected to the pins available on the various connectors. This is easily achieved by connecting the pins with the wires headed with female Dupont connectors the card is equipped with. The original connections do not go into conflict as I initialize them as a pull-up input, therefore, inserting the resistors that were missing.

SPI pin	J2		SD pin	J3	name
PA7	30	«	PD2	24	SPI1 MOSI
PA6	29	«	PC8	37	SPI1 MISO
PA5	28	«	PC12	27	SPI1 SCK

Table 1 – Wiring to do for SPI bus

As you can see in the table, the pins connected to the SD are all on the J3 connector and the pins of the SPI bus are all on the J2 connector. Pin 1 of the connectors is always indicated by a square pad, on the soldered side. Pin PC11, connected to the SD (DAT3 = CS) is fine, just indicate it in the initialization of the SD.

At the top of the program, among other instructions, these should be placed:

#include <SPI.h>
#include <SD.h>
#define SD_MOSI PA7
#define SD_MISO PA6
#define SD_CLK PA5
#define SD_CS PC11
boolean SDok;

 

The following instructions must be entered in the setup () function:

  pinMode(PD2, INPUT_PULLUP);// SDIO pin
  pinMode(PC8, INPUT_PULLUP);// SDIO pin
  pinMode(PC12, INPUT_PULLUP);// SDIO pin
  SPI.setMOSI(SD_MOSI);
  SPI.setMISO(SD_MISO);
  SPI.setSCLK(SD_CLK);
  if (!SD.begin(SD_CS)) {
    SDok = false;
  } else {
   SDok = true;
  }

The tests gave positive results with the SD.h library and negative with STM32.h.

With this simple trick I could also use the SD card with Arduino IDE.