How to Securely Update STM32 Firmware with SPI NOR Flash and CRC Validation

Implementing a Secure Firmware Update Mechanism for STM32 Microcontrollers Using SPI-Connected External NOR Flash with CRC Validation

In this tutorial, we will walk you through the process of implementing a secure firmware update mechanism for STM32 microcontrollers. We’ll utilize SPI-connected external NOR flash memory to store the firmware, and employ CRC validation to ensure the integrity of the firmware during updates.

Prerequisites

• Basic understanding of STM32 microcontrollers
• Familiarity with SPI communication protocol
• Knowledge of C programming and embedded systems
• Development environment set up with STM32CubeIDE or similar tools
• Access to an STM32 development board

Parts/Tools

• STM32 microcontroller (e.g., STM32F4 series)
• SPI-connected external NOR flash memory chip (e.g., Winbond W25Q series)
• Development environment (STM32CubeIDE)
• Programming tools (ST-LINK, USB-to-serial converter)
• C library for CRC calculations

Steps

1. Setting up the SPI interface
   1. Configure the SPI peripheral in STM32CubeMX:

   
   SPI_HandleTypeDef hspi1;
   hspi1.Instance = SPI1;
   hspi1.Init.Mode = SPI_MODE_MASTER;
   hspi1.Init.Direction = SPI_DIRECTION_2LINES;
   hspi1.Init.DataSize = SPI_DATASIZE_8BIT;
   hspi1.Init.CLKPolarity = SPI_POLARITY_LOW;
   hspi1.Init.CLKPhase = SPI_PHASE_1EDGE;
   hspi1.Init.NSS = SPI_NSS_SOFT;
   hspi1.Init.BaudRatePrescaler = SPI_BAUDRATEPRESCALER_16;
   hspi1.Init.FirstBit = SPI_FIRSTBIT_MSB;
   HAL_SPI_Init(&hspi1);
               

   2. Initialize the GPIO pins for CS (Chip Select), SCK (Clock), MOSI (Master Out Slave In), and MISO (Master In Slave Out).
2. Writing firmware to external NOR flash
   1. Define the commands for writing and reading from the NOR flash:

   
   #define WRITE_ENABLE 0x06
   #define PAGE_PROGRAM 0x02
   #define READ_DATA 0x03
               

   2. Implement the function to write data to the flash:

   
   void writeFlash(uint32_t address, uint8_t* data, uint16_t size) {
       HAL_GPIO_WritePin(GPIOB, CS_PIN, GPIO_PIN_RESET); // Select the flash
       HAL_SPI_Transmit(&hspi1, (uint8_t*)&WRITE_ENABLE, 1, HAL_MAX_DELAY);
       HAL_GPIO_WritePin(GPIOB, CS_PIN, GPIO_PIN_SET);
   
       HAL_GPIO_WritePin(GPIOB, CS_PIN, GPIO_PIN_RESET); // Select the flash
       uint8_t command[4] = {PAGE_PROGRAM, (address >> 16) & 0xFF, (address >> 8) & 0xFF, address & 0xFF};
       HAL_SPI_Transmit(&hspi1, command, 4, HAL_MAX_DELAY);
       HAL_SPI_Transmit(&hspi1, data, size, HAL_MAX_DELAY);
       HAL_GPIO_WritePin(GPIOB, CS_PIN, GPIO_PIN_SET); // Deselect the flash
   }
               

3. Implementing CRC validation
   1. Include a CRC library and define the CRC function:

   
   #include "crc32.h" // Your CRC library
   uint32_t calculateCRC(uint8_t* data, uint32_t length) {
       return crc32(data, length);
   }
               

   2. Calculate the CRC of the firmware before writing and store it in a predefined address in the flash.
4. Reading firmware from external NOR flash
   1. Implement the function to read data from the flash:

   
   void readFlash(uint32_t address, uint8_t* buffer, uint16_t size) {
       HAL_GPIO_WritePin(GPIOB, CS_PIN, GPIO_PIN_RESET); // Select the flash
       uint8_t command[4] = {READ_DATA, (address >> 16) & 0xFF, (address >> 8) & 0xFF, address & 0xFF};
       HAL_SPI_Transmit(&hspi1, command, 4, HAL_MAX_DELAY);
       HAL_SPI_Receive(&hspi1, buffer, size, HAL_MAX_DELAY);
       HAL_GPIO_WritePin(GPIOB, CS_PIN, GPIO_PIN_SET); // Deselect the flash
   }
               

5. Updating firmware securely
   1. Read the stored CRC from flash and compare it with the newly calculated CRC of the firmware.
   2. If the CRCs match, proceed to the firmware update.
   3. If there is a mismatch, abort the update and notify the user.

Troubleshooting

• If you’re unable to read/write from the NOR flash:
  • Check your SPI connections and ensure the CS pin is functioning correctly.
  • Verify the SPI configuration settings in STM32CubeMX.
• If CRC validation fails:
  • Ensure the correct data is being sent for CRC calculation.
  • Check the implementation of your CRC function.

Conclusion

In this tutorial, we covered how to implement a secure firmware update mechanism for STM32 microcontrollers using SPI-connected external NOR flash with CRC validation. By following these steps, you can ensure that your firmware updates are not only effective but also secure against corruption and unauthorized changes.