]> git.sur5r.net Git - freertos/blobdiff - FreeRTOS/Source/portable/GCC/ARM_CM4F/port.c
Update copyright date ready for tagging V10.1.0.
[freertos] / FreeRTOS / Source / portable / GCC / ARM_CM4F / port.c
index b222f45bab4791d4c64b86b38f4a5f40fa01492b..27a9e667a258cd7547a3a60c2f1ceb8f7fe75245 100644 (file)
@@ -1,67 +1,29 @@
 /*\r
-    FreeRTOS V7.6.0 - Copyright (C) 2013 Real Time Engineers Ltd.\r
-    All rights reserved\r
-\r
-    VISIT http://www.FreeRTOS.org TO ENSURE YOU ARE USING THE LATEST VERSION.\r
-\r
-    ***************************************************************************\r
-     *                                                                       *\r
-     *    FreeRTOS provides completely free yet professionally developed,    *\r
-     *    robust, strictly quality controlled, supported, and cross          *\r
-     *    platform software that has become a de facto standard.             *\r
-     *                                                                       *\r
-     *    Help yourself get started quickly and support the FreeRTOS         *\r
-     *    project by purchasing a FreeRTOS tutorial book, reference          *\r
-     *    manual, or both from: http://www.FreeRTOS.org/Documentation        *\r
-     *                                                                       *\r
-     *    Thank you!                                                         *\r
-     *                                                                       *\r
-    ***************************************************************************\r
-\r
-    This file is part of the FreeRTOS distribution.\r
-\r
-    FreeRTOS is free software; you can redistribute it and/or modify it under\r
-    the terms of the GNU General Public License (version 2) as published by the\r
-    Free Software Foundation >>!AND MODIFIED BY!<< the FreeRTOS exception.\r
-\r
-    >>! NOTE: The modification to the GPL is included to allow you to distribute\r
-    >>! a combined work that includes FreeRTOS without being obliged to provide\r
-    >>! the source code for proprietary components outside of the FreeRTOS\r
-    >>! kernel.\r
-\r
-    FreeRTOS is distributed in the hope that it will be useful, but WITHOUT ANY\r
-    WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS\r
-    FOR A PARTICULAR PURPOSE.  Full license text is available from the following\r
-    link: http://www.freertos.org/a00114.html\r
-\r
-    1 tab == 4 spaces!\r
-\r
-    ***************************************************************************\r
-     *                                                                       *\r
-     *    Having a problem?  Start by reading the FAQ "My application does   *\r
-     *    not run, what could be wrong?"                                     *\r
-     *                                                                       *\r
-     *    http://www.FreeRTOS.org/FAQHelp.html                               *\r
-     *                                                                       *\r
-    ***************************************************************************\r
-\r
-    http://www.FreeRTOS.org - Documentation, books, training, latest versions,\r
-    license and Real Time Engineers Ltd. contact details.\r
-\r
-    http://www.FreeRTOS.org/plus - A selection of FreeRTOS ecosystem products,\r
-    including FreeRTOS+Trace - an indispensable productivity tool, a DOS\r
-    compatible FAT file system, and our tiny thread aware UDP/IP stack.\r
-\r
-    http://www.OpenRTOS.com - Real Time Engineers ltd license FreeRTOS to High\r
-    Integrity Systems to sell under the OpenRTOS brand.  Low cost OpenRTOS\r
-    licenses offer ticketed support, indemnification and middleware.\r
-\r
-    http://www.SafeRTOS.com - High Integrity Systems also provide a safety\r
-    engineered and independently SIL3 certified version for use in safety and\r
-    mission critical applications that require provable dependability.\r
-\r
-    1 tab == 4 spaces!\r
-*/\r
+ * FreeRTOS Kernel V10.1.0\r
+ * Copyright (C) 2018 Amazon.com, Inc. or its affiliates.  All Rights Reserved.\r
+ *\r
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of\r
+ * this software and associated documentation files (the "Software"), to deal in\r
+ * the Software without restriction, including without limitation the rights to\r
+ * use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of\r
+ * the Software, and to permit persons to whom the Software is furnished to do so,\r
+ * subject to the following conditions:\r
+ *\r
+ * The above copyright notice and this permission notice shall be included in all\r
+ * copies or substantial portions of the Software.\r
+ *\r
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR\r
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS\r
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR\r
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER\r
+ * IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN\r
+ * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.\r
+ *\r
+ * http://www.FreeRTOS.org\r
+ * http://aws.amazon.com/freertos\r
+ *\r
+ * 1 tab == 4 spaces!\r
+ */\r
 \r
 /*-----------------------------------------------------------\r
  * Implementation of functions defined in portable.h for the ARM CM4F port.\r
 #define portNVIC_PENDSVCLEAR_BIT                       ( 1UL << 27UL )\r
 #define portNVIC_PEND_SYSTICK_CLEAR_BIT                ( 1UL << 25UL )\r
 \r
+/* Constants used to detect a Cortex-M7 r0p1 core, which should use the ARM_CM7\r
+r0p1 port. */\r
+#define portCPUID                                                      ( * ( ( volatile uint32_t * ) 0xE000ed00 ) )\r
+#define portCORTEX_M7_r0p1_ID                          ( 0x410FC271UL )\r
+#define portCORTEX_M7_r0p0_ID                          ( 0x410FC270UL )\r
+\r
 #define portNVIC_PENDSV_PRI                                    ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 16UL )\r
 #define portNVIC_SYSTICK_PRI                           ( ( ( uint32_t ) configKERNEL_INTERRUPT_PRIORITY ) << 24UL )\r
 \r
 #define portPRIORITY_GROUP_MASK                                ( 0x07UL << 8UL )\r
 #define portPRIGROUP_SHIFT                                     ( 8UL )\r
 \r
+/* Masks off all bits but the VECTACTIVE bits in the ICSR register. */\r
+#define portVECTACTIVE_MASK                                    ( 0xFFUL )\r
+\r
 /* Constants required to manipulate the VFP. */\r
-#define portFPCCR                                      ( ( volatile uint32_t * ) 0xe000ef34 ) /* Floating point context control register. */\r
-#define portASPEN_AND_LSPEN_BITS       ( 0x3UL << 30UL )\r
+#define portFPCCR                                                      ( ( volatile uint32_t * ) 0xe000ef34 ) /* Floating point context control register. */\r
+#define portASPEN_AND_LSPEN_BITS                       ( 0x3UL << 30UL )\r
 \r
 /* Constants required to set up the initial stack. */\r
-#define portINITIAL_XPSR                       ( 0x01000000 )\r
-#define portINITIAL_EXEC_RETURN                ( 0xfffffffd )\r
+#define portINITIAL_XPSR                                       ( 0x01000000 )\r
+#define portINITIAL_EXC_RETURN                         ( 0xfffffffd )\r
 \r
 /* The systick is a 24-bit counter. */\r
 #define portMAX_24_BIT_NUMBER                          ( 0xffffffUL )\r
 \r
+/* For strict compliance with the Cortex-M spec the task start address should\r
+have bit-0 clear, as it is loaded into the PC on exit from an ISR. */\r
+#define portSTART_ADDRESS_MASK         ( ( StackType_t ) 0xfffffffeUL )\r
+\r
 /* A fiddle factor to estimate the number of SysTick counts that would have\r
 occurred while the SysTick counter is stopped during tickless idle\r
 calculations. */\r
 #define portMISSED_COUNTS_FACTOR                       ( 45UL )\r
 \r
 /* Let the user override the pre-loading of the initial LR with the address of\r
-prvTaskExitError() in case is messes up unwinding of the stack in the\r
+prvTaskExitError() in case it messes up unwinding of the stack in the\r
 debugger. */\r
 #ifdef configTASK_RETURN_ADDRESS\r
        #define portTASK_RETURN_ADDRESS configTASK_RETURN_ADDRESS\r
@@ -135,10 +110,6 @@ debugger. */
        #define portTASK_RETURN_ADDRESS prvTaskExitError\r
 #endif\r
 \r
-/* Each task maintains its own interrupt status in the critical nesting\r
-variable. */\r
-static UBaseType_t uxCriticalNesting = 0xaaaaaaaa;\r
-\r
 /*\r
  * Setup the timer to generate the tick interrupts.  The implementation in this\r
  * file is weak to allow application writers to change the timer used to\r
@@ -161,7 +132,7 @@ static void prvPortStartFirstTask( void ) __attribute__ (( naked ));
 /*\r
  * Function to enable the VFP.\r
  */\r
- static void vPortEnableVFP( void ) __attribute__ (( naked ));\r
+static void vPortEnableVFP( void ) __attribute__ (( naked ));\r
 \r
 /*\r
  * Used to catch tasks that attempt to return from their implementing function.\r
@@ -170,10 +141,14 @@ static void prvTaskExitError( void );
 \r
 /*-----------------------------------------------------------*/\r
 \r
+/* Each task maintains its own interrupt status in the critical nesting\r
+variable. */\r
+static UBaseType_t uxCriticalNesting = 0xaaaaaaaa;\r
+\r
 /*\r
  * The number of SysTick increments that make up one tick period.\r
  */\r
-#if configUSE_TICKLESS_IDLE == 1\r
+#if( configUSE_TICKLESS_IDLE == 1 )\r
        static uint32_t ulTimerCountsForOneTick = 0;\r
 #endif /* configUSE_TICKLESS_IDLE */\r
 \r
@@ -181,7 +156,7 @@ static void prvTaskExitError( void );
  * The maximum number of tick periods that can be suppressed is limited by the\r
  * 24 bit resolution of the SysTick timer.\r
  */\r
-#if configUSE_TICKLESS_IDLE == 1\r
+#if( configUSE_TICKLESS_IDLE == 1 )\r
        static uint32_t xMaximumPossibleSuppressedTicks = 0;\r
 #endif /* configUSE_TICKLESS_IDLE */\r
 \r
@@ -189,7 +164,7 @@ static void prvTaskExitError( void );
  * Compensate for the CPU cycles that pass while the SysTick is stopped (low\r
  * power functionality only.\r
  */\r
-#if configUSE_TICKLESS_IDLE == 1\r
+#if( configUSE_TICKLESS_IDLE == 1 )\r
        static uint32_t ulStoppedTimerCompensation = 0;\r
 #endif /* configUSE_TICKLESS_IDLE */\r
 \r
@@ -198,7 +173,7 @@ static void prvTaskExitError( void );
  * FreeRTOS API functions are not called from interrupts that have been assigned\r
  * a priority above configMAX_SYSCALL_INTERRUPT_PRIORITY.\r
  */\r
-#if ( configASSERT_DEFINED == 1 )\r
+#if( configASSERT_DEFINED == 1 )\r
         static uint8_t ucMaxSysCallPriority = 0;\r
         static uint32_t ulMaxPRIGROUPValue = 0;\r
         static const volatile uint8_t * const pcInterruptPriorityRegisters = ( const volatile uint8_t * const ) portNVIC_IP_REGISTERS_OFFSET_16;\r
@@ -220,7 +195,7 @@ StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t px
 \r
        *pxTopOfStack = portINITIAL_XPSR;       /* xPSR */\r
        pxTopOfStack--;\r
-       *pxTopOfStack = ( StackType_t ) pxCode; /* PC */\r
+       *pxTopOfStack = ( ( StackType_t ) pxCode ) & portSTART_ADDRESS_MASK;    /* PC */\r
        pxTopOfStack--;\r
        *pxTopOfStack = ( StackType_t ) portTASK_RETURN_ADDRESS;        /* LR */\r
 \r
@@ -231,7 +206,7 @@ StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t px
        /* A save method is being used that requires each task to maintain its\r
        own exec return value. */\r
        pxTopOfStack--;\r
-       *pxTopOfStack = portINITIAL_EXEC_RETURN;\r
+       *pxTopOfStack = portINITIAL_EXC_RETURN;\r
 \r
        pxTopOfStack -= 8;      /* R11, R10, R9, R8, R7, R6, R5 and R4. */\r
 \r
@@ -241,6 +216,8 @@ StackType_t *pxPortInitialiseStack( StackType_t *pxTopOfStack, TaskFunction_t px
 \r
 static void prvTaskExitError( void )\r
 {\r
+volatile uint32_t ulDummy = 0;\r
+\r
        /* A function that implements a task must not exit or attempt to return to\r
        its caller as there is nothing to return to.  If a task wants to exit it\r
        should instead call vTaskDelete( NULL ).\r
@@ -249,7 +226,16 @@ static void prvTaskExitError( void )
        defined, then stop here so application writers can catch the error. */\r
        configASSERT( uxCriticalNesting == ~0UL );\r
        portDISABLE_INTERRUPTS();\r
-       for( ;; );\r
+       while( ulDummy == 0 )\r
+       {\r
+               /* This file calls prvTaskExitError() after the scheduler has been\r
+               started to remove a compiler warning about the function being defined\r
+               but never called.  ulDummy is used purely to quieten other warnings\r
+               about code appearing after this function is called - making ulDummy\r
+               volatile makes the compiler think the function could return and\r
+               therefore not output an 'unreachable code' warning for code that appears\r
+               after it. */\r
+       }\r
 }\r
 /*-----------------------------------------------------------*/\r
 \r
@@ -266,7 +252,7 @@ void vPortSVCHandler( void )
                                        "       msr     basepri, r0                                     \n"\r
                                        "       bx r14                                                  \n"\r
                                        "                                                                       \n"\r
-                                       "       .align 2                                                \n"\r
+                                       "       .align 4                                                \n"\r
                                        "pxCurrentTCBConst2: .word pxCurrentTCB                         \n"\r
                                );\r
 }\r
@@ -274,12 +260,19 @@ void vPortSVCHandler( void )
 \r
 static void prvPortStartFirstTask( void )\r
 {\r
+       /* Start the first task.  This also clears the bit that indicates the FPU is\r
+       in use in case the FPU was used before the scheduler was started - which\r
+       would otherwise result in the unnecessary leaving of space in the SVC stack\r
+       for lazy saving of FPU registers. */\r
        __asm volatile(\r
                                        " ldr r0, =0xE000ED08   \n" /* Use the NVIC offset register to locate the stack. */\r
                                        " ldr r0, [r0]                  \n"\r
                                        " ldr r0, [r0]                  \n"\r
                                        " msr msp, r0                   \n" /* Set the msp back to the start of the stack. */\r
+                                       " mov r0, #0                    \n" /* Clear the bit that indicates the FPU is in use, see comment above. */\r
+                                       " msr control, r0               \n"\r
                                        " cpsie i                               \n" /* Globally enable interrupts. */\r
+                                       " cpsie f                               \n"\r
                                        " dsb                                   \n"\r
                                        " isb                                   \n"\r
                                        " svc 0                                 \n" /* System call to start first task. */\r
@@ -297,6 +290,12 @@ BaseType_t xPortStartScheduler( void )
        See http://www.FreeRTOS.org/RTOS-Cortex-M3-M4.html */\r
        configASSERT( configMAX_SYSCALL_INTERRUPT_PRIORITY );\r
 \r
+       /* This port can be used on all revisions of the Cortex-M7 core other than\r
+       the r0p1 parts.  r0p1 parts should use the port from the\r
+       /source/portable/GCC/ARM_CM7/r0p1 directory. */\r
+       configASSERT( portCPUID != portCORTEX_M7_r0p1_ID );\r
+       configASSERT( portCPUID != portCORTEX_M7_r0p0_ID );\r
+\r
        #if( configASSERT_DEFINED == 1 )\r
        {\r
                volatile uint32_t ulOriginalPriority;\r
@@ -330,6 +329,24 @@ BaseType_t xPortStartScheduler( void )
                        ucMaxPriorityValue <<= ( uint8_t ) 0x01;\r
                }\r
 \r
+               #ifdef __NVIC_PRIO_BITS\r
+               {\r
+                       /* Check the CMSIS configuration that defines the number of\r
+                       priority bits matches the number of priority bits actually queried\r
+                       from the hardware. */\r
+                       configASSERT( ( portMAX_PRIGROUP_BITS - ulMaxPRIGROUPValue ) == __NVIC_PRIO_BITS );\r
+               }\r
+               #endif\r
+\r
+               #ifdef configPRIO_BITS\r
+               {\r
+                       /* Check the FreeRTOS configuration that defines the number of\r
+                       priority bits matches the number of priority bits actually queried\r
+                       from the hardware. */\r
+                       configASSERT( ( portMAX_PRIGROUP_BITS - ulMaxPRIGROUPValue ) == configPRIO_BITS );\r
+               }\r
+               #endif\r
+\r
                /* Shift the priority group value back to its position within the AIRCR\r
                register. */\r
                ulMaxPRIGROUPValue <<= portPRIGROUP_SHIFT;\r
@@ -364,7 +381,10 @@ BaseType_t xPortStartScheduler( void )
        /* Should never get here as the tasks will now be executing!  Call the task\r
        exit error function to prevent compiler warnings about a static function\r
        not being called in the case that the application writer overrides this\r
-       functionality by defining configTASK_RETURN_ADDRESS. */\r
+       functionality by defining configTASK_RETURN_ADDRESS.  Call\r
+       vTaskSwitchContext() so link time optimisation does not remove the\r
+       symbol. */\r
+       vTaskSwitchContext();\r
        prvTaskExitError();\r
 \r
        /* Should not get here! */\r
@@ -380,24 +400,20 @@ void vPortEndScheduler( void )
 }\r
 /*-----------------------------------------------------------*/\r
 \r
-void vPortYield( void )\r
-{\r
-       /* Set a PendSV to request a context switch. */\r
-       portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;\r
-\r
-       /* Barriers are normally not required but do ensure the code is completely\r
-       within the specified behaviour for the architecture. */\r
-       __asm volatile( "dsb" );\r
-       __asm volatile( "isb" );\r
-}\r
-/*-----------------------------------------------------------*/\r
-\r
 void vPortEnterCritical( void )\r
 {\r
        portDISABLE_INTERRUPTS();\r
        uxCriticalNesting++;\r
-       __asm volatile( "dsb" );\r
-       __asm volatile( "isb" );\r
+\r
+       /* This is not the interrupt safe version of the enter critical function so\r
+       assert() if it is being called from an interrupt context.  Only API\r
+       functions that end in "FromISR" can be used in an interrupt.  Only assert if\r
+       the critical nesting count is 1 to protect against recursive calls if the\r
+       assert function also uses a critical section. */\r
+       if( uxCriticalNesting == 1 )\r
+       {\r
+               configASSERT( ( portNVIC_INT_CTRL_REG & portVECTACTIVE_MASK ) == 0 );\r
+       }\r
 }\r
 /*-----------------------------------------------------------*/\r
 \r
@@ -412,37 +428,6 @@ void vPortExitCritical( void )
 }\r
 /*-----------------------------------------------------------*/\r
 \r
-__attribute__(( naked )) uint32_t ulPortSetInterruptMask( void )\r
-{\r
-       __asm volatile                                                                                                          \\r
-       (                                                                                                                                       \\r
-               "       mrs r0, basepri                                                                                 \n" \\r
-               "       mov r1, %0                                                                                              \n"     \\r
-               "       msr basepri, r1                                                                                 \n" \\r
-               "       bx lr                                                                                                   \n" \\r
-               :: "i" ( configMAX_SYSCALL_INTERRUPT_PRIORITY ) : "r0", "r1"    \\r
-       );\r
-\r
-       /* This return will not be reached but is necessary to prevent compiler\r
-       warnings. */\r
-       return 0;\r
-}\r
-/*-----------------------------------------------------------*/\r
-\r
-__attribute__(( naked )) void vPortClearInterruptMask( uint32_t ulNewMaskValue )\r
-{\r
-       __asm volatile                                                                                                  \\r
-       (                                                                                                                               \\r
-               "       msr basepri, r0                                                                         \n"     \\r
-               "       bx lr                                                                                           \n" \\r
-               :::"r0"                                                                                                         \\r
-       );\r
-\r
-       /* Just to avoid compiler warnings. */\r
-       ( void ) ulNewMaskValue;\r
-}\r
-/*-----------------------------------------------------------*/\r
-\r
 void xPortPendSVHandler( void )\r
 {\r
        /* This is a naked function. */\r
@@ -460,16 +445,17 @@ void xPortPendSVHandler( void )
        "       vstmdbeq r0!, {s16-s31}                         \n"\r
        "                                                                               \n"\r
        "       stmdb r0!, {r4-r11, r14}                        \n" /* Save the core registers. */\r
-       "                                                                               \n"\r
        "       str r0, [r2]                                            \n" /* Save the new top of stack into the first member of the TCB. */\r
        "                                                                               \n"\r
-       "       stmdb sp!, {r3}                                         \n"\r
+       "       stmdb sp!, {r0, r3}                                     \n"\r
        "       mov r0, %0                                                      \n"\r
        "       msr basepri, r0                                         \n"\r
+       "       dsb                                                                     \n"\r
+       "       isb                                                                     \n"\r
        "       bl vTaskSwitchContext                           \n"\r
        "       mov r0, #0                                                      \n"\r
        "       msr basepri, r0                                         \n"\r
-       "       ldmia sp!, {r3}                                         \n"\r
+       "       ldmia sp!, {r0, r3}                                     \n"\r
        "                                                                               \n"\r
        "       ldr r1, [r3]                                            \n" /* The first item in pxCurrentTCB is the task top of stack. */\r
        "       ldr r0, [r1]                                            \n"\r
@@ -492,7 +478,7 @@ void xPortPendSVHandler( void )
        "                                                                               \n"\r
        "       bx r14                                                          \n"\r
        "                                                                               \n"\r
-       "       .align 2                                                        \n"\r
+       "       .align 4                                                        \n"\r
        "pxCurrentTCBConst: .word pxCurrentTCB  \n"\r
        ::"i"(configMAX_SYSCALL_INTERRUPT_PRIORITY)\r
        );\r
@@ -505,7 +491,7 @@ void xPortSysTickHandler( void )
        executes all interrupts must be unmasked.  There is therefore no need to\r
        save and then restore the interrupt mask value as its value is already\r
        known. */\r
-       ( void ) portSET_INTERRUPT_MASK_FROM_ISR();\r
+       portDISABLE_INTERRUPTS();\r
        {\r
                /* Increment the RTOS tick. */\r
                if( xTaskIncrementTick() != pdFALSE )\r
@@ -515,15 +501,15 @@ void xPortSysTickHandler( void )
                        portNVIC_INT_CTRL_REG = portNVIC_PENDSVSET_BIT;\r
                }\r
        }\r
-       portCLEAR_INTERRUPT_MASK_FROM_ISR( 0 );\r
+       portENABLE_INTERRUPTS();\r
 }\r
 /*-----------------------------------------------------------*/\r
 \r
-#if configUSE_TICKLESS_IDLE == 1\r
+#if( configUSE_TICKLESS_IDLE == 1 )\r
 \r
        __attribute__((weak)) void vPortSuppressTicksAndSleep( TickType_t xExpectedIdleTime )\r
        {\r
-       uint32_t ulReloadValue, ulCompleteTickPeriods, ulCompletedSysTickDecrements, ulSysTickCTRL;\r
+       uint32_t ulReloadValue, ulCompleteTickPeriods, ulCompletedSysTickDecrements;\r
        TickType_t xModifiableIdleTime;\r
 \r
                /* Make sure the SysTick reload value does not overflow the counter. */\r
@@ -549,7 +535,9 @@ void xPortSysTickHandler( void )
 \r
                /* Enter a critical section but don't use the taskENTER_CRITICAL()\r
                method as that will mask interrupts that should exit sleep mode. */\r
-               __asm volatile( "cpsid i" );\r
+               __asm volatile( "cpsid i" ::: "memory" );\r
+               __asm volatile( "dsb" );\r
+               __asm volatile( "isb" );\r
 \r
                /* If a context switch is pending or a task is waiting for the scheduler\r
                to be unsuspended then abandon the low power entry. */\r
@@ -568,7 +556,7 @@ void xPortSysTickHandler( void )
 \r
                        /* Re-enable interrupts - see comments above the cpsid instruction()\r
                        above. */\r
-                       __asm volatile( "cpsie i" );\r
+                       __asm volatile( "cpsie i" ::: "memory" );\r
                }\r
                else\r
                {\r
@@ -591,29 +579,47 @@ void xPortSysTickHandler( void )
                        configPRE_SLEEP_PROCESSING( xModifiableIdleTime );\r
                        if( xModifiableIdleTime > 0 )\r
                        {\r
-                               __asm volatile( "dsb" );\r
+                               __asm volatile( "dsb" ::: "memory" );\r
                                __asm volatile( "wfi" );\r
                                __asm volatile( "isb" );\r
                        }\r
                        configPOST_SLEEP_PROCESSING( xExpectedIdleTime );\r
 \r
-                       /* Stop SysTick.  Again, the time the SysTick is stopped for is\r
-                       accounted for as best it can be, but using the tickless mode will\r
-                       inevitably result in some tiny drift of the time maintained by the\r
-                       kernel with respect to calendar time. */\r
-                       ulSysTickCTRL = portNVIC_SYSTICK_CTRL_REG;\r
-                       portNVIC_SYSTICK_CTRL_REG = ( ulSysTickCTRL & ~portNVIC_SYSTICK_ENABLE_BIT );\r
-\r
-                       /* Re-enable interrupts - see comments above the cpsid instruction()\r
-                       above. */\r
-                       __asm volatile( "cpsie i" );\r
-\r
-                       if( ( ulSysTickCTRL & portNVIC_SYSTICK_COUNT_FLAG_BIT ) != 0 )\r
+                       /* Re-enable interrupts to allow the interrupt that brought the MCU\r
+                       out of sleep mode to execute immediately.  see comments above\r
+                       __disable_interrupt() call above. */\r
+                       __asm volatile( "cpsie i" ::: "memory" );\r
+                       __asm volatile( "dsb" );\r
+                       __asm volatile( "isb" );\r
+\r
+                       /* Disable interrupts again because the clock is about to be stopped\r
+                       and interrupts that execute while the clock is stopped will increase\r
+                       any slippage between the time maintained by the RTOS and calendar\r
+                       time. */\r
+                       __asm volatile( "cpsid i" ::: "memory" );\r
+                       __asm volatile( "dsb" );\r
+                       __asm volatile( "isb" );\r
+\r
+                       /* Disable the SysTick clock without reading the\r
+                       portNVIC_SYSTICK_CTRL_REG register to ensure the\r
+                       portNVIC_SYSTICK_COUNT_FLAG_BIT is not cleared if it is set.  Again,\r
+                       the time the SysTick is stopped for is accounted for as best it can\r
+                       be, but using the tickless mode will inevitably result in some tiny\r
+                       drift of the time maintained by the kernel with respect to calendar\r
+                       time*/\r
+                       portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT | portNVIC_SYSTICK_INT_BIT );\r
+\r
+                       /* Determine if the SysTick clock has already counted to zero and\r
+                       been set back to the current reload value (the reload back being\r
+                       correct for the entire expected idle time) or if the SysTick is yet\r
+                       to count to zero (in which case an interrupt other than the SysTick\r
+                       must have brought the system out of sleep mode). */\r
+                       if( ( portNVIC_SYSTICK_CTRL_REG & portNVIC_SYSTICK_COUNT_FLAG_BIT ) != 0 )\r
                        {\r
                                uint32_t ulCalculatedLoadValue;\r
 \r
-                               /* The tick interrupt has already executed, and the SysTick\r
-                               count reloaded with ulReloadValue.  Reset the\r
+                               /* The tick interrupt is already pending, and the SysTick count\r
+                               reloaded with ulReloadValue.  Reset the\r
                                portNVIC_SYSTICK_LOAD_REG with whatever remains of this tick\r
                                period. */\r
                                ulCalculatedLoadValue = ( ulTimerCountsForOneTick - 1UL ) - ( ulReloadValue - portNVIC_SYSTICK_CURRENT_VALUE_REG );\r
@@ -628,11 +634,9 @@ void xPortSysTickHandler( void )
 \r
                                portNVIC_SYSTICK_LOAD_REG = ulCalculatedLoadValue;\r
 \r
-                               /* The tick interrupt handler will already have pended the tick\r
-                               processing in the kernel.  As the pending tick will be\r
-                               processed as soon as this function exits, the tick value\r
-                               maintained by the tick is stepped forward by one less than the\r
-                               time spent waiting. */\r
+                               /* As the pending tick will be processed as soon as this\r
+                               function exits, the tick value maintained by the tick is stepped\r
+                               forward by one less than the time spent waiting. */\r
                                ulCompleteTickPeriods = xExpectedIdleTime - 1UL;\r
                        }\r
                        else\r
@@ -649,22 +653,19 @@ void xPortSysTickHandler( void )
 \r
                                /* The reload value is set to whatever fraction of a single tick\r
                                period remains. */\r
-                               portNVIC_SYSTICK_LOAD_REG = ( ( ulCompleteTickPeriods + 1 ) * ulTimerCountsForOneTick ) - ulCompletedSysTickDecrements;\r
+                               portNVIC_SYSTICK_LOAD_REG = ( ( ulCompleteTickPeriods + 1UL ) * ulTimerCountsForOneTick ) - ulCompletedSysTickDecrements;\r
                        }\r
 \r
                        /* Restart SysTick so it runs from portNVIC_SYSTICK_LOAD_REG\r
                        again, then set portNVIC_SYSTICK_LOAD_REG back to its standard\r
-                       value.  The critical section is used to ensure the tick interrupt\r
-                       can only execute once in the case that the reload register is near\r
-                       zero. */\r
+                       value. */\r
                        portNVIC_SYSTICK_CURRENT_VALUE_REG = 0UL;\r
-                       portENTER_CRITICAL();\r
-                       {\r
-                               portNVIC_SYSTICK_CTRL_REG |= portNVIC_SYSTICK_ENABLE_BIT;\r
-                               vTaskStepTick( ulCompleteTickPeriods );\r
-                               portNVIC_SYSTICK_LOAD_REG = ulTimerCountsForOneTick - 1UL;\r
-                       }\r
-                       portEXIT_CRITICAL();\r
+                       portNVIC_SYSTICK_CTRL_REG |= portNVIC_SYSTICK_ENABLE_BIT;\r
+                       vTaskStepTick( ulCompleteTickPeriods );\r
+                       portNVIC_SYSTICK_LOAD_REG = ulTimerCountsForOneTick - 1UL;\r
+\r
+                       /* Exit with interrpts enabled. */\r
+                       __asm volatile( "cpsie i" ::: "memory" );\r
                }\r
        }\r
 \r
@@ -678,7 +679,7 @@ void xPortSysTickHandler( void )
 __attribute__(( weak )) void vPortSetupTimerInterrupt( void )\r
 {\r
        /* Calculate the constants required to configure the tick interrupt. */\r
-       #if configUSE_TICKLESS_IDLE == 1\r
+       #if( configUSE_TICKLESS_IDLE == 1 )\r
        {\r
                ulTimerCountsForOneTick = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ );\r
                xMaximumPossibleSuppressedTicks = portMAX_24_BIT_NUMBER / ulTimerCountsForOneTick;\r
@@ -686,8 +687,12 @@ __attribute__(( weak )) void vPortSetupTimerInterrupt( void )
        }\r
        #endif /* configUSE_TICKLESS_IDLE */\r
 \r
+       /* Stop and clear the SysTick. */\r
+       portNVIC_SYSTICK_CTRL_REG = 0UL;\r
+       portNVIC_SYSTICK_CURRENT_VALUE_REG = 0UL;\r
+\r
        /* Configure SysTick to interrupt at the requested rate. */\r
-       portNVIC_SYSTICK_LOAD_REG = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ ) - 1UL;;\r
+       portNVIC_SYSTICK_LOAD_REG = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ ) - 1UL;\r
        portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT );\r
 }\r
 /*-----------------------------------------------------------*/\r
@@ -715,7 +720,7 @@ static void vPortEnableVFP( void )
        uint8_t ucCurrentPriority;\r
 \r
                /* Obtain the number of the currently executing interrupt. */\r
-               __asm volatile( "mrs %0, ipsr" : "=r"( ulCurrentInterrupt ) );\r
+               __asm volatile( "mrs %0, ipsr" : "=r"( ulCurrentInterrupt ) :: "memory" );\r
 \r
                /* Is the interrupt number a user defined interrupt? */\r
                if( ulCurrentInterrupt >= portFIRST_USER_INTERRUPT_NUMBER )\r
@@ -761,7 +766,7 @@ static void vPortEnableVFP( void )
                devices by calling NVIC_SetPriorityGrouping( 0 ); before starting the\r
                scheduler.  Note however that some vendor specific peripheral libraries\r
                assume a non-zero priority group setting, in which cases using a value\r
-               of zero will result in unpredicable behaviour. */\r
+               of zero will result in unpredictable behaviour. */\r
                configASSERT( ( portAIRCR_REG & portPRIORITY_GROUP_MASK ) <= ulMaxPRIGROUPValue );\r
        }\r
 \r