How to change the Read Out Protection on STM32F7?
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Introduction
At level 0, no read-out protection is enabled, and all read and write operations on the flash memory and backup SRAM are possible in all boot configurations. Option bytes on the microcontroller can also be changed.
Level 1 sets read protection for the Flash memory. Access to protected memories is only allowed when booting from User Flash memory, otherwise a system hard fault is generated, blocking all code execution until the next power-on reset. Option bytes are still configurable at this level, making it possible to revert the read-out protection to level 0 either via system bootloader mode or regular SWD/JTAG interface.
Level 2 provides the same protection as level 1, but with permanent protection by locking the option bytes. Once level 2 protection is set, it cannot be undone, and the microcontroller will remain permanently protected, the system bootloader is also unreachable in this mode.
Regressing from level 1 to level 0 protection causes a mass erase of the Flash memory and backup SRAM, only option bytes will not be erased in this process.
In this article we will showcase this functionality using the firmware to modify it. You will be able to configure any other option bytes using this method as well, but the focus will be on read out protection.
- Microcontroller Configuration
After opening the software, click on File -> New -> STM32 Project:
After the menu loads, search for STM32F746ZG, you can easily locate it with the it with the corresponding Nucleo board and press ‘next’.
Name your project (avoid using special characters and spaces) and click finish with the default settings.
Now we will setup the microcontroller peripherals the logic will use, including the SWD interface, USART and GPIO:
To setup the microcontroller’s SWD go to System Core > SYS and enable the Debug Serial Wire, we can use the default Timebase source as SysTick
Next, we will configure USART3, which is connected on this Nucleo board to the on-board ST-LINK. With the help of ST-Link’s virtual COM port, we will then use this link to relay messages. Go to Connectivity > USART3 and set it to Asynchronous mode. The default basic parameters under configuration tab should work, but feel free to adjust it to your preferences, just remember that this will be used later when configuring your terminal.
In the nucleo 144 boards the USART3 is wired to the pins PD9 and PD8, thus you must manually reassign them since the Cube MX software will configure RX and TX pins to PB11 and PB10. You need to left click the PD9 pin in your microcontroller and select the USART3_RX option. That must also be done for the PD8 pin and the USART3_TX.
For the button pin and LED, we’ll use the hardware available in the NUCLEO board, this means that PC13 will be the push button and we can configure it by clicking and selecting ‘EXTI_13’. After that, setup a user label for the button doing the following: System Core > GPIO > PC13 and under User Label type ‘B1 [Blue PushButton]’.
As for the LED, we will use the PB0, as it controls the onboard Green LED. On the dropdown menu select GPIO_Output, and similarly to the button pin, go to System Core > GPIO > PB0 and under User Label type ‘LD1 [Green Led]’.
It is now possible to either click on the Device configuration tool code generation icon in the top bar or press Alt+K. This will generate a basic project with all the configuration we selected using the GUI.
- Coding
#include "stdio.h"
With the base code ready, it is time to start adding the application code. First include the stdio.h library in the private includes section so we can easily print to the vcom terminal.
int __io_putchar(int ch)
{
HAL_UART_Transmit(&huart3, (uint8_t *)&ch, 1, 100);
return ch;
}
Under the USER CODE BEGIN 0 section we will create the regression function in a way that it runs from the microcontroller’s RAM instead of Flash
void __attribute__((__section__(".RamFunc"))) RDP_Regression(void) {
__disable_irq();
printf("Mass Erase Start\r\n");
HAL_FLASH_Unlock();
HAL_FLASH_OB_Unlock();
*(__IO uint8_t*) OPTCR_BYTE1_ADDRESS = OB_RDP_LEVEL_0;
HAL_FLASH_OB_Launch();
}
Still under the same comment session, add the simple function to blink the on-board LED at a given time interval.
void ToggleLED(uint16_t n) // Function for toggling the on-board LED
{
if(HAL_GPIO_ReadPin(LD1_GPIO_Port, LD1_Pin))
{
printf("\033[5;32mLED\033[0m \r\n");
HAL_GPIO_WritePin(LD1_GPIO_Port, LD1_Pin, GPIO_PIN_RESET);
}
else
{
printf("\033[1;32mLED\033[0m \r\n");
HAL_GPIO_WritePin(LD1_GPIO_Port, LD1_Pin, GPIO_PIN_SET);
}
HAL_Delay(n);
}
Already inside the main function and under the USER CODE BEGIN 1 label we will define a structure to edit the option bytes.
FLASH_OBProgramInitTypeDef OptionsBytesStruct; //Define a structure to store the Options Bytes configuration for the Flash memory
The next code portion should be placed between the USER CODE BEGIN 3 and END 3. After the RDP level check, the small decision branch happens, if it’s at level 1 the code will blink the led at a 500ms rate until the button is pressed. Upon button press, it will start the regression process by unlocking the flash memory and the Option bytes, and finally call the RDP_Regression function, which was added earlier and executed from RAM memory. It is important to remember that upon RDP regression, a full mass erase is performed.
printf("\033[0m");
HAL_FLASHEx_OBGetConfig(&OptionsBytesStruct); // Get the current Options Bytes configuration for the Flash memory
// Check if the RDP level is set to Level 1
if (OptionsBytesStruct.RDPLevel == OB_RDP_LEVEL_1) {
printf("RDP LVL1 \r\n");
printf("wait BT1 to be pressed\r\n");
// Loop until button BT1 is pressed
while (HAL_GPIO_ReadPin(B1_GPIO_Port, B1_Pin) != GPIO_PIN_SET) {
ToggleLED(500);
}
printf("wait BT1 to be released\r\n");
// Loop until button BT1 is released
while (HAL_GPIO_ReadPin(B1_GPIO_Port, B1_Pin) != GPIO_PIN_RESET) {
;
}
printf("RDP regression LV0 init\r\n");
// Call RDP_Regression() function
RDP_Regression();
}
Then the code performs a check every 2 seconds to decide when the microcontroller’s RDP level should be set to 1. While it waits for a button press and release, the code blinks the led at a 2s rate. When the button is released the RDP level is set to 1.
//if RDP is set to 0, wait for a button press to set it to level 1
printf("\033[96mPress BT1 to change Option Bytes\033[0m \r\n");
if (HAL_GPIO_ReadPin(B1_GPIO_Port, B1_Pin) != GPIO_PIN_RESET) {
printf("BT1 pressed, please release it to proceed\r\n");
while (HAL_GPIO_ReadPin(B1_GPIO_Port, B1_Pin) != GPIO_PIN_RESET)
;
while (HAL_FLASH_Unlock() != HAL_OK) {
printf("Waiting Flash Unlock\r\n");
}
while (HAL_FLASH_OB_Unlock() != HAL_OK) {
printf("Waiting OB Unlock\r\n");
}
OptionsBytesStruct.OptionType = OPTIONBYTE_RDP; //Configure the RDP
OptionsBytesStruct.RDPLevel = OB_RDP_LEVEL_1;
while (HAL_FLASHEx_OBProgram(&OptionsBytesStruct) != HAL_OK) {
printf("Waiting OB Program\r\n");
}
printf("RDP will now be set to level 1\r\n");
printf("OBLaunch\r\n");
HAL_FLASH_OB_Launch();
} else {
ToggleLED(2000);
}
}
/* USER CODE END 3 */
- Testing
After this, the NUCLEO board must be disconnected from the USB’s computer and the CN4 jumpers removed from the ST-Link as shown:
Now the board must be plugged back in, which will power up the board and restart program execution.
Note: The reason behind removing the ST-Link jumpers is to prevent the debug connection by the ST-Link (which automatically happens at board power up) from interfering with the RDP entry/exit.
To confirm this, you can open a terminal, such as tera term and configure it to match the USART2 configuration we setup earlier, just to recall, it was: 115200/8/N/1. The terminal should display the following:
Now press and hold the blue button until the following message appears:
Release the blue button to change the RDP level to 1. The MCU will reset itself and display the new RDP level message with a faster LED blink rate.
Press and release the blue button again to restore the RDP level to 0, causing a mass erase of the MCU.
At this point the Flash memory is automatically mass erased.
Warning: This may take up to 30 seconds, if you reset the microcontroller before the FLASH memory is fully erased the RDP byte will stay locked at Level 1 and your program will be deleted, so it is very important so stand still while the FLASH is being mass erased.
To confirm the RDP regression, fit the CN2 jumpers back in (no need to power cycle) and use the STM32CubeProg (GUI or CLI) to read the memory. Here it is demonstrated using the CLI to read the option byte.
Open your windows command prompt and type the following commands as shown
cd C:\Program Files\STMicroelectronics\STM32Cube\STM32CubeProgrammer\bin
STM32_Programmer_CLI.exe -c port=swd -ob displ
If the RDP regression worked, you’ll see a message like the following:
By using the command ‘STM32_Programmer_cli.exe -c port=swd mode=HOTPLUG -r32 0x08000000 1000’ you can see the contents of the flash memory, which as we expected, are all FFFF FFFF:
In order to see the RDP protection taking place, you can re-do the steps (reprogram your code using the CubeIDE, remove CN2 jumpers, power cycle, press the button to set RDP level 1) and fit the CN2 jumpers after it was set to level 1. Now, you can reissue the option byte command ‘STM32_Programmer_CLI.exe -c port=swd -ob displ’ to view the RDP status. Be aware that this will trigger the read-out protection and after the execution of this command your microcontroller will halt completely and recover only after a power cycle.
To further explore, you can try to check the Flash contents again using the command:
STM32_Programmer_cli.exe -c port=swd mode=HOTPLUG -r32 0x08000000 1000, but this time you’ll receive an error message as the microcontroller has protected the Flash content, and again the micro will halt until it is power cycled.
Note: You can follow the same process to set RDP to level 2, but keep in mind that this action is irreversible and should only be done at your final production. If you need to update your firmware after RDP level 2, you should first implement your own bootloader (see IAP bootloader example https://www.st.com/content/ccc/resource/technical/document/application_note/27/38/37/58/c2/8c/40/07/DM00161366.pdf/files/DM00161366.pdf/jcr:content/translations/en.DM00161366.pdf ) because RDP level 2 will disable the built-in System Bootloader.
- Conclusion