嵌入式开发笔记——调试组件SEGGER_HardFaultHandle

一、前言

在使用Cortex-M内核的MCU进行开发时，有时候会因为对内存错误访问等原因造成程序产生异常从而进入HardFaultHandler错误中断。如果程序结构比较复杂，尤其是运行了RTOS时可能短时间内不易定位异常产生的原因。Segger提供了一种分析CortexM内核芯片HardFault的方法，我在项目中使用后感觉该方法比较实用，本文用来记录该异常分析组件的使用。

二、组件添加

在SEGGER官网的Application Notes页面下提供了该组件的源码和文档

下载下来后将源文件添加到工程中，然后将中断处理文件中的void HardFault_Handler(void)函数屏蔽掉（和添加的文件中的函数有冲突）就完成添加了。

三、组件应用

执行以下代码使程序进入HardFault_Handler

volatile unsigned int* p;p = (unsigned int*)0x007FFFFF;// 小于0x8000000的地址在STM32中无效
*p = 0x123456;
SEGGER_RTT_printf(0, "p = %d\r\n", *p);


在调试模式下运行程序会停在SEGGER_HardFaultHandler.c文件中的void HardFaultHandler(unsigned int* pStack)函数中。

/*********************************************************************
*
* HardFaultHandler()
*
* Function description
* C part of the hard fault handler which is called by the assembler
* function HardFault_Handler
*/
void HardFaultHandler(unsigned int* pStack) {SEGGER_RTT_printf(0, "system enter hard-fault\r\n"); //
// In case we received a hard fault because of a breakpoint instruction, we return.
// This may happen when using semihosting for printf outputs and no debugger is connected,
// i.e. when running a "Debug" configuration in release mode.
//
if (NVIC_HFSR & (1u << 31)) {
NVIC_HFSR |= (1u << 31); // Reset Hard Fault status
*(pStack + 6u) += 2u; // PC is located on stack at SP + 24 bytes. Increment PC by 2 to skip break instruction.
return; // Return to interrupted application
}
#if DEBUG
//
// Read NVIC registers
//
HardFaultRegs.syshndctrl.byte = SYSHND_CTRL; // System Handler Control and State Register
HardFaultRegs.mfsr.byte = NVIC_MFSR; // Memory Fault Status Register
HardFaultRegs.bfsr.byte = NVIC_BFSR; // Bus Fault Status Register
HardFaultRegs.bfar = NVIC_BFAR; // Bus Fault Manage Address Register
HardFaultRegs.ufsr.byte = NVIC_UFSR; // Usage Fault Status Register
HardFaultRegs.hfsr.byte = NVIC_HFSR; // Hard Fault Status Register
HardFaultRegs.dfsr.byte = NVIC_DFSR; // Debug Fault Status Register
HardFaultRegs.afsr = NVIC_AFSR; // Auxiliary Fault Status Register
//
// Halt execution
// If NVIC registers indicate readable memory, change the variable value to != 0 to continue execution.
//
_Continue = 0u;
while (_Continue == 0u) {
}
//
// Read saved registers from the stack.
//
HardFaultRegs.SavedRegs.r0 = pStack[0]; // Register R0
HardFaultRegs.SavedRegs.r1 = pStack[1]; // Register R1
HardFaultRegs.SavedRegs.r2 = pStack[2]; // Register R2
HardFaultRegs.SavedRegs.r3 = pStack[3]; // Register R3
HardFaultRegs.SavedRegs.r12 = pStack[4]; // Register R12
HardFaultRegs.SavedRegs.lr = pStack[5]; // Link register LR
HardFaultRegs.SavedRegs.pc = pStack[6]; // Program counter PC
HardFaultRegs.SavedRegs.psr.byte = pStack[7]; // Program status word PSR
//
// Halt execution
// To step out of the HardFaultHandler, change the variable value to != 0.
//
_Continue = 0u;
while (_Continue == 0u) {
}
#else
//
// If this module is included in a release configuration, simply stay in the HardFault handler
//
(void)pStack;
do {
} while (1);
#endif
}


在HardFaultRegs结构体中包含了用来分析异常原因的寄存器

static struct {
struct {
volatile unsigned int r0; // Register R0
volatile unsigned int r1; // Register R1
volatile unsigned int r2; // Register R2
volatile unsigned int r3; // Register R3
volatile unsigned int r12; // Register R12
volatile unsigned int lr; // Link register
volatile unsigned int pc; // Program counter
union {
volatile unsigned int byte;
struct {
unsigned int IPSR : 8; // Interrupt Program Status register (IPSR)
unsigned int EPSR : 19; // Execution Program Status register (EPSR)
unsigned int APSR : 5; // Application Program Status register (APSR)
} bits;
} psr; // Program status register.
} SavedRegs; union {
volatile unsigned int byte;
struct {
unsigned int MEMFAULTACT : 1; // Read as 1 if memory management fault is active
unsigned int BUSFAULTACT : 1; // Read as 1 if bus fault exception is active
unsigned int UnusedBits1 : 1;
unsigned int USGFAULTACT : 1; // Read as 1 if usage fault exception is active
unsigned int UnusedBits2 : 3;
unsigned int SVCALLACT : 1; // Read as 1 if SVC exception is active
unsigned int MONITORACT : 1; // Read as 1 if debug monitor exception is active
unsigned int UnusedBits3 : 1;
unsigned int PENDSVACT : 1; // Read as 1 if PendSV exception is active
unsigned int SYSTICKACT : 1; // Read as 1 if SYSTICK exception is active
unsigned int USGFAULTPENDED : 1; // Usage fault pended; usage fault started but was replaced by a higher-priority exception
unsigned int MEMFAULTPENDED : 1; // Memory management fault pended; memory management fault started but was replaced by a higher-priority exception
unsigned int BUSFAULTPENDED : 1; // Bus fault pended; bus fault handler was started but was replaced by a higher-priority exception
unsigned int SVCALLPENDED : 1; // SVC pended; SVC was started but was replaced by a higher-priority exception
unsigned int MEMFAULTENA : 1; // Memory management fault handler enable
unsigned int BUSFAULTENA : 1; // Bus fault handler enable
unsigned int USGFAULTENA : 1; // Usage fault handler enable
} bits;
} syshndctrl; // System Handler Control and State Register (0xE000ED24) union {
volatile unsigned char byte;
struct {
unsigned char IACCVIOL : 1; // Instruction access violation
unsigned char DACCVIOL : 1; // Data access violation
unsigned char UnusedBits : 1;
unsigned char MUNSTKERR : 1; // Unstacking error
unsigned char MSTKERR : 1; // Stacking error
unsigned char UnusedBits2 : 2;
unsigned char MMARVALID : 1; // Indicates the MMAR is valid
} bits;
} mfsr; // Memory Management Fault Status Register (0xE000ED28) union {
volatile unsigned int byte;
struct {
unsigned int IBUSERR : 1; // Instruction access violation
unsigned int PRECISERR : 1; // Precise data access violation
unsigned int IMPREISERR : 1; // Imprecise data access violation
unsigned int UNSTKERR : 1; // Unstacking error
unsigned int STKERR : 1; // Stacking error
unsigned int UnusedBits : 2;
unsigned int BFARVALID : 1; // Indicates BFAR is valid
} bits;
} bfsr; // Bus Fault Status Register (0xE000ED29)
volatile unsigned int bfar; // Bus Fault Manage Address Register (0xE000ED38) union {
volatile unsigned short byte;
struct {
unsigned short UNDEFINSTR : 1; // Attempts to execute an undefined instruction
unsigned short INVSTATE : 1; // Attempts to switch to an invalid state (e.g., ARM)
unsigned short INVPC : 1; // Attempts to do an exception with a bad value in the EXC_RETURN number
unsigned short NOCP : 1; // Attempts to execute a coprocessor instruction
unsigned short UnusedBits : 4;
unsigned short UNALIGNED : 1; // Indicates that an unaligned access fault has taken place
unsigned short DIVBYZERO : 1; // Indicates a divide by zero has taken place (can be set only if DIV_0_TRP is set)
} bits;
} ufsr; // Usage Fault Status Register (0xE000ED2A) union {
volatile unsigned int byte;
struct {
unsigned int UnusedBits : 1;
unsigned int VECTBL : 1; // Indicates hard fault is caused by failed vector fetch
unsigned int UnusedBits2 : 28;
unsigned int FORCED : 1; // Indicates hard fault is taken because of bus fault/memory management fault/usage fault
unsigned int DEBUGEVT : 1; // Indicates hard fault is triggered by debug event
} bits;
} hfsr; // Hard Fault Status Register (0xE000ED2C) union {
volatile unsigned int byte;
struct {
unsigned int HALTED : 1; // Halt requested in NVIC
unsigned int BKPT : 1; // BKPT instruction executed
unsigned int DWTTRAP : 1; // DWT match occurred
unsigned int VCATCH : 1; // Vector fetch occurred
unsigned int EXTERNAL : 1; // EDBGRQ signal asserted
} bits;
} dfsr; // Debug Fault Status Register (0xE000ED30) volatile unsigned int afsr; // Auxiliary Fault Status Register (0xE000ED3C), Vendor controlled (optional)
} HardFaultRegs;


将HardFaultRegs结构体添加到Watch窗口中，通过一步步向下执行程序可以看到结构体中各参数状态，每个寄存器及寄存器bit位表示什么含义在结构体定义中均有说明。

将_Continue变量也添加到Watch窗口，当执行到以下代码处时在Watch窗口中改变变量值就可以继续向下执行。

//
// Halt execution
// If NVIC registers indicate readable memory, change the variable value to != 0 to continue execution.
//
_Continue = 0u;
while (_Continue == 0u) {
}


通过对寄存器分析可得出产生异常的原因。

四、扩展

Cortex-M故障异常

Cortex-M processors implement different fault exceptions.

HardFault Exception

The HardFault is the default exception, raised on any error which is not associated with another (enabled) exception.

The HardFault has a fixed priority of -1, i.e. it has a higher priority than all other interrupts and exceptions except for NMI. Therefore a HardFault exception handler can always be entered when an error happens in application code, an interrupt, or another exception. The HardFault is exception number 3 in the vector table with IRQ number -13.

MemManage Exception

The MemManage exception is available with the use of a Memory Protection Unit (MPU) to raise an exception on memory access violations.

The MemManage is exception number 4 in the vector table, IRQ Number -12, and has a configurable priority.

BusFault Exception

The BusFault exception is raised on any memory access error. E.g. by illegal read, write, or vector catch.

The BusFault is exception number 5 in the vector table, IRQ number -11, and has configurable priority. BusFaults can explicitly be enabled in the system control block (SCB). When BusFault is not enabled, a HardFault is raised.

UsageFault Exception

The UsageFault exception is raised on execution errors. Unaligned access on load/store multiple instructions are always caught. Exceptions on other unaligned access, as well as division by zero can be additionally enabled in the SCB.

The UsageFault is exception number 6 in the vector table, IRQ number -10, and has configurable priority. When UsageFault is not enabled, a HardFault is raised instead.

故障状态寄存器

The Cortex-M System Control Block (SCB) contains some registers which enable configuration of exceptions and provide information about faults.

HardFault Status Register (HFSR)

The HFSR is in the SCB at address 0xE000ED2C. It is a 32-bit register.

Bitfields:

[31] DEBUGEVT - Reserved for use by debugger/debug probe. Always write 0.
[30] FORCED - If 1, HardFault has been caused by escalation of another exception, because it is disabled or because of priority.
[1] VECTTBL - If 1, a BusFault occurred by reading the vector table for exception processing.


UsageFault Status Register (UFSR)

The UFSR is a 16-bit pseudo-register, part of the Configurable Fault Status Register (CFSR) at address 0xE000ED28. It can also be directly accessed with halfword access to 0xE000ED2A.

Bitfields:

[9] DIVBYZERO - If 1, SDIV or UDIV instruction executed with divisor 0.
[8] UNALIGNED - If 1, LDM, STM, LDRD, STRD on unaligned address executed, or single load or store executed when enabled to trap.
[3] NOCP - If 1, access to unsupported (e.g. not available or not enabled) coprocessor.
[2] INVPC - If 1, illegal or invalid EXC_RETURN value load to PC.
[1] INVSTATE - If 1, execution in invalid state. E.g. Thumb bit not set in EPSR, or invalid IT state in EPSR.
[0] UNDEFINSTR - If 1, execution of undefined instruction.


BusFault Status Register (BFSR) and BusFault Address Register (BFAR)

The BFSR is a 8-bit pseudo-register in the CFSR. It can be directly accessed with byte access ad 0xE000ED29. The BFAR is a 32-bit register at 0xE000ED38.

Bitfields:

[7] BFARVALID - If 1, the BFAR contains the address which caused the BusFault.
[5] LSPERR - 1f 1, fault during floating-point lazy stack preservation.
[4] STKERR - If 1, fault on stacking for exception entry.
[3] UNSTKERR - If 1, fault on unstacking on exception return.
[2] IMPRECISERR - If 1, return address is not related to fault, e.g. fault caused before.
[1] PRECISERR - If 1, return address instruction caused the fault.
[0] IBUSERR - If 1, fault on instruction fetch.


MemManage Fault Status Register (MMFSR) and MemManage fault Address Register (MMFAR)

The MMFSR is a 8-bit pseudo-register in the CFSR. It can be directly accessed with byte access ad 0xE000ED28. The MMFAR is a 32-bit register at 0xE000ED34.

Bitfields:

[7] MMARVALID - If 1, the MMFAR contains the address which caused the MemManageFault.
[5] MLSPERR - 1f 1, fault during floating-point lazy stack preservation.
[4] MSTKERR - If 1, fault on stacking for exception entry.
[3] MUNSTKERR - If 1, fault on unstacking on exception return.
[1] DACCVIOL - If 1, data access violation.
[0] IACCVIOL - If 1, instruction access violation.


Stack Recovery

On exception entry, the exception handler can check which stack has been used when the fault happened. When bit EXC_RETURN[2] is set, MSP has been used, otherwise PSP has been used.

The stack can be used to recover the CPU register values.

CPU Register Recovery

On exception entry, some CPU registers are stored on the stack and can be read from there for error analysis. the following registers are recoverable:

r0 = pStack[0]; // Register R0
r1 = pStack[1]; // Register R1
r2 = pStack[2]; // Register R2
r3 = pStack[3]; // Register R3
r12 = pStack[4]; // Register R12
lr = pStack[5]; // Link register LR
pc = pStack[6]; // Program counter PC
psr.byte = pStack[7]; // Program status word PSR


故障分析示例

The following examples show how/why some faults can be caused, and how to analyze them. A project to test the faults is available here.

BusFault Examples

Illegal Memory Write

/*********************************************************************
*
* _IllegalWrite()
*
* Function description
* Trigger a BusFault or HardFault by writing to a reserved address.
*
* Additional Information
* BusFault is raised some instructions after the write instruction.
* Related registers on fault:
* HFSR = 0x40000000
* FORCED = 1 - BusFault escalated to HardFault (when BusFault is not activated)
* BFSR = 0x00000004
* IMPREISERR = 1 - Imprecise data access violation. Return address not related to fault
* BFARVALID = 0 - BFAR not valid
*/
static int _IllegalWrite(void) {
int r;
volatile unsigned int* p; r = 0;
p = (unsigned int*)0x00100000; // 0x00100000-0x07FFFFFF is reserved on STM32F4
// F44F1380 mov.w r3, #0x00100000
*p = 0x00BADA55;
// 4A03 ldr r2, =0x00BADA55
// 601A str r2, [r3] <- Illegal write is done here
return r;
// 9B00 ldr r3, [sp]
// 4618 mov r0, r3
// B002 add sp, sp, #8 <- Fault might be raised here
// 4770 bx lr
}


Illegal Memory Read

/*********************************************************************
*
* _IllegalRead()
*
* Function description
* Trigger a BusFault or HardFault by reading from a reserved address.
*
* Additional Information
* BusFault is immediately triggered on the read instruction.
* Related registers on fault:
* HFSR = 0x40000000
* FORCED = 1 - BusFault escalated to HardFault
* BFSR = 0x00000082
* PRECISERR = 1 - Precise data access violation
* BFARVALID = 1 - BFAR is valid
* BFAR = 0x00100000 - The address read from
*/
static int _IllegalRead(void) {
int r;
volatile unsigned int* p; p = (unsigned int*)0x00100000; // 0x00100000-0x07FFFFFF is reserved on STM32F4
// F44F1380 mov.w r3, #0x00100000 <- The read address. Will be found in BFAR
r = *p;
// 681B ldr r3, [r3] <- Illegal read happens here and raises BusFault
// 9300 str r3, [sp] return r;
}


Illegal Function Execution

/*********************************************************************
*
* _IllegalFunc()
*
* Function description
* Trigger a BusFault or HardFault by executing at a reserved address.
*
* Additional Information
* BusFault is triggered on execution at the invalid address.
* Related registers on fault:
* HFSR = 0x40000000
* FORCED = 1 - BusFault escalated to HardFault
* BFSR = 0x00000001
* IBUSERR = 1 - BusFault on instruction prefetch
*/
static int _IllegalFunc(void) {
int r;
int (*pF)(void); pF = (int(*)(void))0x00100001; // 0x00100000-0x07FFFFFF is reserved on STM32F4
// F44F1380 mov.w r3, #0x00100001
r = pF();
// 4798 blx r3 <- Branch to illegal address, causes fetch from 0x00100000 and fault exception
return r;
}


UsageFault Examples

Undefined Instruction Execution

/*********************************************************************
*
* _UndefInst()
*
* Function description
* Trigger a UsageFault or HardFault by executing an undefined instruction.
*
* Additional Information
* UsageFault is triggered on execution at the invalid address.
* Related registers on hard fault:
* HFSR = 0x40000000
* FORCED = 1 - UsageFault escalated to HardFault
* UFSR = 0x0001
* UNDEFINSTR = 1 - Undefined instruction executed
*/
static int _UndefInst(void) {
static const unsigned short _UDF[4] = {0xDEAD, 0xDEAD, 0xDEAD, 0xDEAD}; // 0xDEAD: UDF #<imm> (permanently undefined)
int r;
int (*pF)(void); pF = (int(*)(void))(((char*)&_UDF) + 1);
// 4B05 ldr r3, =0x08001C18 <_UDF> <- Load address of "RAM Code" instructions
// 3301 adds r3, #1 <- Make sure Thumb bit is set
r = pF();
// 4798 blx r3 <- Call "RAM Code", will execute UDF instruction and raise exception
// 9000 str r0, [sp]
return r;
}


Illegal State

/*********************************************************************
*
* _NoThumbFunc()
*
* Function description
* Trigger a UsageFault or HardFault by executing an address without thumb bit set.
*
* Additional Information
* UsageFault is triggered on execution at the invalid address.
* Related registers on hard fault:
* HFSR = 0x40000000
* FORCED = 1 - UsageFault escalated to HardFault
* UFSR = 0x0002
* INVSTATE = 1 - Instruction execution with invalid state
*/
static int _NoThumbFunc(void) {
int r;
int (*pF)(void); pF = (int(*)(void))0x00100000; // 0x00100000-0x07FFFFFF is reserved on STM32F4
// F44F1380 mov.w r3, #0x00100000 <- Note that bit [0] is not set.
r = pF();
// 4798 blx r3 <- Branch exchange with mode change to ARM, but Cortex-M only supports Thumb mode.
return r;
}


Division By Zero

/*********************************************************************
*
* _DivideByZero()
*
* Function description
* Trigger a UsageFault or HardFault by dividing by zero.
*
* Additional Information
* UsageFault is triggered immediately on the divide instruction.
* Related registers on hard fault:
* HFSR = 0x40000000
* FORCED = 1 - UsageFault escalated to HardFault
* UFSR = 0x0200
* DIVBYZERO = 1 - Divide-by-zero fault
*/
static int _DivideByZero(void) {
int r;
volatile unsigned int a;
volatile unsigned int b;
a = 1;
// 2301 movs r3, #1 <- Load dividend
b = 0;
// 2300 movs r3, #0 <- Load divisor
r = a / b;
// FBB2F3F3 udiv r3, r2, r3 <- divide by 0 raises fault exception
return r;
}


Unaligned Access

/*********************************************************************
*
* _UnalignedAccess()
*
* Function description
* Trigger a UsageFault or HardFault by an unaligned word access.
*
* Additional Information
* UsageFault is triggered immediately on the read or write instruction.
* Related registers on fault:
* HFSR = 0x40000000
* FORCED = 1 - UsageFault escalated to HardFault
* UFSR = 0x0100
* UNALIGNED = 1 - Unaligned memory access
*/
static int _UnalignedAccess(void) {
int r;
volatile unsigned int* p; p = (unsigned int*)0x20000002;
// 4B04 ldr r3, =0x20000002 <- Not word aligned address
r = *p;
// 681B ldr r3, [r3] <- Load word from unaligned address raises exception
// 9300 str r3, [sp]
return r;
}


HardFault Examples

Illegal Vector Table Fetch

/*********************************************************************
*
* _IllegalVector()
*
* Function description
* Trigger a HardFault by interrupt with illegal vector table.
*
* Additional Information
* Related registers on fault:
* HFSR = 0x00000002
* VECTTBL = 1 - Vector table read fault
*/
static int _IllegalVector(void) {
int r; SCB->VTOR = 0x001000000; // Relocate vector table to illegal address
// 4B09 ldr r3, =0xE000ED00
// F04F7280 mov.w r2, #0x1000000
// 609A str r2, [r3, #8]
SCB->ICSR = SCB_ICSR_PENDSVSET_Msk; // Trigger PendSV exception to read invalid vector
// 4B07 ldr r3, =0xE000ED00
// F04F5280 mov.w r2, #0x10000000
// 605A str r2, [r3, #4]
__ISB();
// F3BF8F6F isb <- PendSV exception is to be executed. PendSV vector is tried to be read from illegal address 0x00100038 causes fault exception
// BF00 nop
__DSB();
// F3BF8F4F dsb sy
// BF00 nop
return r;
}