Difference between revisions of "Using embOS for Cortex-M with a bootloader"
(Created page with "Introduction This application note describes the usage of embOS within bootloader applications for Cortex-M devices. A bootloader is a program which allows the stand-alone pro...") |
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− | Introduction |
+ | ''Introduction'' |
This application note describes the usage of embOS within bootloader applications for |
This application note describes the usage of embOS within bootloader applications for |
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Cortex-M devices. |
Cortex-M devices. |
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+ | |||
A bootloader is a program which allows the stand-alone programming of application |
A bootloader is a program which allows the stand-alone programming of application |
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code to the device it is running on. The bootloader has to ensure that the CPU is in |
code to the device it is running on. The bootloader has to ensure that the CPU is in |
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the same state as after reset if the application is started. |
the same state as after reset if the application is started. |
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There are no restrictions for building a bootloader using embOS. |
There are no restrictions for building a bootloader using embOS. |
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+ | |||
− | Jump from bootloader to application |
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+ | ''Jump from bootloader to application'' |
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+ | |||
Some steps have to be performed before the main application can be safely accessed |
Some steps have to be performed before the main application can be safely accessed |
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from a task. For the following example, we assume that the bootloader is located at |
from a task. For the following example, we assume that the bootloader is located at |
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address 0x00000000 and the application at 0x00100000. |
address 0x00000000 and the application at 0x00100000. |
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+ | |||
+ | <nowiki> |
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#define THUMB_BIT 1 |
#define THUMB_BIT 1 |
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#define APP_START_ADDR 0x00100000 |
#define APP_START_ADDR 0x00100000 |
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} |
} |
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} |
} |
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+ | </nowiki> |
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− | The following description analyzes the task step by step: |
+ | ''The following description analyzes the task step by step:'' |
+ | |||
+ | <nowiki> |
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#define THUMB_BIT 1 |
#define THUMB_BIT 1 |
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#define APP_START_ADDR 0x00100000 |
#define APP_START_ADDR 0x00100000 |
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#define APP_RESET_PTR (*(volatile OS_U32*)(APP_START_ADDR + 0x04)) |
#define APP_RESET_PTR (*(volatile OS_U32*)(APP_START_ADDR + 0x04)) |
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void (*AppPtr)(void); |
void (*AppPtr)(void); |
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+ | </nowiki> |
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+ | |||
After reset two words, the start value of the stack pointer and the reset vector, will |
After reset two words, the start value of the stack pointer and the reset vector, will |
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be fetched from the beginning of the vector table, which is initially located at address |
be fetched from the beginning of the vector table, which is initially located at address |
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0x00000000. The reset vector is used to start the program execution from the reset |
0x00000000. The reset vector is used to start the program execution from the reset |
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vector address. |
vector address. |
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+ | |||
Both programs, the bootloader and the application, have their own vector table, so it |
Both programs, the bootloader and the application, have their own vector table, so it |
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is neccessary to change the stack pointer value and the reset vector manually before |
is neccessary to change the stack pointer value and the reset vector manually before |
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the application can be started. |
the application can be started. |
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+ | |||
+ | <nowiki> |
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static void HPTask(void) { |
static void HPTask(void) { |
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while (1) { |
while (1) { |
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// |
// |
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OS_IncDI(); |
OS_IncDI(); |
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+ | </nowiki> |
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+ | |||
At first interrupts must be disabled, no interrupt initialized within the bootloader |
At first interrupts must be disabled, no interrupt initialized within the bootloader |
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should affect the start of the application. |
should affect the start of the application. |
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+ | |||
+ | <nowiki> |
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AppPtr = (void (*)(void))(APP_RESET_PTR | THUMB_BIT); |
AppPtr = (void (*)(void))(APP_RESET_PTR | THUMB_BIT); |
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SCB->VTOR = APP_START_ADDR; |
SCB->VTOR = APP_START_ADDR; |
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+ | </nowiki> |
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A function pointer will be used to jump to the new reset vector address. To indicate |
A function pointer will be used to jump to the new reset vector address. To indicate |
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The CPU stores the address of the vector table in the vector table offset register |
The CPU stores the address of the vector table in the vector table offset register |
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(VTOR), it has to be changed accordingly from reset value to the start address of the |
(VTOR), it has to be changed accordingly from reset value to the start address of the |
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− | application. |
+ | application. |
+ | |||
+ | <nowiki> |
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__set_MSP(APP_STACK_PTR); // Set main stack pointer to application |
__set_MSP(APP_STACK_PTR); // Set main stack pointer to application |
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// initial stack value |
// initial stack value |
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__set_CONTROL(0); // Use MSP and Privileged in thread mode |
__set_CONTROL(0); // Use MSP and Privileged in thread mode |
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+ | </nowiki> |
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The main stack pointer must be set to the initial value used for the application. |
The main stack pointer must be set to the initial value used for the application. |
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to switch to the main stack pointer. Furthermore, the thread mode should be set to |
to switch to the main stack pointer. Furthermore, the thread mode should be set to |
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privileged. |
privileged. |
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+ | |||
+ | <nowiki> |
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AppPtr(); // Start the application, we will not return |
AppPtr(); // Start the application, we will not return |
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// from this function |
// from this function |
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} |
} |
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} |
} |
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+ | </nowiki> |
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+ | |||
Calling the function finally starts the application. |
Calling the function finally starts the application. |
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− | Demo project |
+ | ''Demo project'' |
A simple demo project for the emPower board, which demonstrates these steps, can |
A simple demo project for the emPower board, which demonstrates these steps, can |
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be downloaded from http://segger.com. The demo project located at the same web- |
be downloaded from http://segger.com. The demo project located at the same web- |
Revision as of 08:58, 1 April 2020
Introduction This application note describes the usage of embOS within bootloader applications for Cortex-M devices.
A bootloader is a program which allows the stand-alone programming of application code to the device it is running on. The bootloader has to ensure that the CPU is in the same state as after reset if the application is started. There are no restrictions for building a bootloader using embOS.
Jump from bootloader to application
Some steps have to be performed before the main application can be safely accessed from a task. For the following example, we assume that the bootloader is located at address 0x00000000 and the application at 0x00100000.
#define THUMB_BIT 1 #define APP_START_ADDR 0x00100000 #define APP_STACK_PTR (*(volatile OS_U32*)(APP_START_ADDR + 0x00)) #define APP_RESET_PTR (*(volatile OS_U32*)(APP_START_ADDR + 0x04)) void (*AppPtr)(void); static void HPTask(void) { while (1) { // // Start the application // OS_IncDI(); AppPtr = (void (*)(void))(APP_RESET_PTR | THUMB_BIT); SCB->VTOR = APP_START_ADDR; __set_MSP(APP_STACK_PTR); // Set main stack pointer to application // initial stack value __set_CONTROL(0); // Use MSP and Privileged in thread mode AppPtr(); // Start the application, we will not return // from this function } }
The following description analyzes the task step by step:
#define THUMB_BIT 1 #define APP_START_ADDR 0x00100000 #define APP_STACK_PTR (*(volatile OS_U32*)(APP_START_ADDR + 0x00)) #define APP_RESET_PTR (*(volatile OS_U32*)(APP_START_ADDR + 0x04)) void (*AppPtr)(void);
After reset two words, the start value of the stack pointer and the reset vector, will be fetched from the beginning of the vector table, which is initially located at address 0x00000000. The reset vector is used to start the program execution from the reset vector address.
Both programs, the bootloader and the application, have their own vector table, so it is neccessary to change the stack pointer value and the reset vector manually before the application can be started.
static void HPTask(void) { while (1) { // // Start the application // OS_IncDI();
At first interrupts must be disabled, no interrupt initialized within the bootloader should affect the start of the application.
AppPtr = (void (*)(void))(APP_RESET_PTR | THUMB_BIT); SCB->VTOR = APP_START_ADDR;
A function pointer will be used to jump to the new reset vector address. To indicate that the accessed reset handler is thumb code, the LSB of the vector must be set to 1. The CPU stores the address of the vector table in the vector table offset register (VTOR), it has to be changed accordingly from reset value to the start address of the application.
__set_MSP(APP_STACK_PTR); // Set main stack pointer to application // initial stack value __set_CONTROL(0); // Use MSP and Privileged in thread mode
The main stack pointer must be set to the initial value used for the application. Because embOS tasks use the process stack pointer, the control register must be set to switch to the main stack pointer. Furthermore, the thread mode should be set to privileged.
AppPtr(); // Start the application, we will not return // from this function } }
Calling the function finally starts the application.
Demo project A simple demo project for the emPower board, which demonstrates these steps, can be downloaded from http://segger.com. The demo project located at the same web- site where you can download this application note.