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Improve comments and name of preprocessor symbol

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Add a note in the code comments that SysTick requests an interrupt when
decrementing from 1 to 0, so that's why stopping SysTick on zero is a
special case.  Readers might unknowingly assume that SysTick requests
an interrupt when wrapping from 0 back to the load-register value.

Reconsider new "_SETTING" suffix since "_CONFIG" suffix seems more
descriptive.  The code relies on *both* of these preprocessor symbols:

portNVIC_SYSTICK_CLK_BIT
portNVIC_SYSTICK_CLK_BIT_CONFIG  **new**

A meaningful suffix is really helpful to distinguish the two symbols.
This commit is contained in:
Jeff Tenney 2020-06-23 10:36:30 -07:00
parent 316b656326
commit 18222af90a
1 changed files with 14 additions and 11 deletions
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25
portable/GCC/ARM_CM4F/port.c
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@ -100,11 +100,11 @@ configuration register. */
#ifndef configSYSTICK_CLOCK_HZ
#define configSYSTICK_CLOCK_HZ configCPU_CLOCK_HZ
/* Ensure the SysTick is clocked at the same frequency as the core. */
#define portNVIC_SYSTICK_CLK_BIT_SETTING ( portNVIC_SYSTICK_CLK_BIT )
#define portNVIC_SYSTICK_CLK_BIT_CONFIG ( portNVIC_SYSTICK_CLK_BIT )
#else
/* Select the option to clock SysTick not at the same frequency as the core.
The clock used is often a divided version of the core clock. */
#define portNVIC_SYSTICK_CLK_BIT_SETTING ( 0 )
#define portNVIC_SYSTICK_CLK_BIT_CONFIG ( 0 )
#endif
/* Let the user override the pre-loading of the initial LR with the address of
@ -544,12 +544,13 @@ void xPortSysTickHandler( void )
is accounted for as best it can be, but using the tickless mode will
inevitably result in some tiny drift of the time maintained by the
kernel with respect to calendar time. */
portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT_SETTING | portNVIC_SYSTICK_INT_BIT );
portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT_CONFIG | portNVIC_SYSTICK_INT_BIT );
/* Use the SysTick current-value register to determine the number of
SysTick decrements remaining until the next tick interrupt. If the
current-value register is zero, then there are actually
ulTimerCountsForOneTick decrements remaining, not zero. */
ulTimerCountsForOneTick decrements remaining, not zero, because the
SysTick requests the interrupt when decrementing from 1 to 0. */
ulSysTickDecrementsLeft = portNVIC_SYSTICK_CURRENT_VALUE_REG;
if( ulSysTickDecrementsLeft == 0 )
{
@ -620,7 +621,7 @@ void xPortSysTickHandler( void )
be, but using the tickless mode will inevitably result in some tiny
drift of the time maintained by the kernel with respect to calendar
time*/
portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT_SETTING | portNVIC_SYSTICK_INT_BIT );
portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT_CONFIG | portNVIC_SYSTICK_INT_BIT );
/* Determine whether the SysTick has already counted to zero. */
if( ( portNVIC_SYSTICK_CTRL_REG & portNVIC_SYSTICK_COUNT_FLAG_BIT ) != 0 )
@ -656,7 +657,7 @@ void xPortSysTickHandler( void )
number of SysTick decrements remaining until the expected idle
time would have ended. */
ulSysTickDecrementsLeft = portNVIC_SYSTICK_CURRENT_VALUE_REG;
#if( portNVIC_SYSTICK_CLK_BIT_SETTING != portNVIC_SYSTICK_CLK_BIT )
#if( portNVIC_SYSTICK_CLK_BIT_CONFIG != portNVIC_SYSTICK_CLK_BIT )
{
/* If the SysTick is not using the core clock, the current-
value register might still be zero here. In that case, the
@ -667,7 +668,7 @@ void xPortSysTickHandler( void )
ulSysTickDecrementsLeft = ulReloadValue + 1UL;
}
}
#endif /* portNVIC_SYSTICK_CLK_BIT_SETTING */
#endif /* portNVIC_SYSTICK_CLK_BIT_CONFIG */
/* Work out how long the sleep lasted rounded to complete tick
periods (not the ulReload value which accounted for part
@ -693,11 +694,13 @@ void xPortSysTickHandler( void )
portNVIC_SYSTICK_CURRENT_VALUE_REG = 0UL;
portNVIC_SYSTICK_CTRL_REG = portNVIC_SYSTICK_CLK_BIT | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT;
portNVIC_SYSTICK_LOAD_REG = ulTimerCountsForOneTick - 1UL;
#if( portNVIC_SYSTICK_CLK_BIT_SETTING != portNVIC_SYSTICK_CLK_BIT )
#if( portNVIC_SYSTICK_CLK_BIT_CONFIG != portNVIC_SYSTICK_CLK_BIT )
{
portNVIC_SYSTICK_CTRL_REG = portNVIC_SYSTICK_CLK_BIT_SETTING | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT;
/* The temporary usage of the core clock has served its purpose,
as described above. Resume usage of the other clock. */
portNVIC_SYSTICK_CTRL_REG = portNVIC_SYSTICK_CLK_BIT_CONFIG | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT;
}
#endif /* portNVIC_SYSTICK_CLK_BIT_SETTING */
#endif /* portNVIC_SYSTICK_CLK_BIT_CONFIG */
/* Step the tick to account for any tick periods that elapsed. */
vTaskStepTick( ulCompleteTickPeriods );
@ -731,7 +734,7 @@ __attribute__(( weak )) void vPortSetupTimerInterrupt( void )
/* Configure SysTick to interrupt at the requested rate. */
portNVIC_SYSTICK_LOAD_REG = ( configSYSTICK_CLOCK_HZ / configTICK_RATE_HZ ) - 1UL;
portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT_SETTING | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT );
portNVIC_SYSTICK_CTRL_REG = ( portNVIC_SYSTICK_CLK_BIT_CONFIG | portNVIC_SYSTICK_INT_BIT | portNVIC_SYSTICK_ENABLE_BIT );
}
/*-----------------------------------------------------------*/