Changes to the AVR demo's. IAR demo updated with new critical section method.

This commit is contained in:
Richard Barry 2006-08-28 09:35:31 +00:00
parent 41b142bae4
commit 97a570fa10
17 changed files with 866 additions and 318 deletions

View file

@ -51,8 +51,8 @@ static portTickType xCoRoutineTickCount = 0;
#define corINITIAL_STATE ( 0 )
/*
* Place the co-routine represented by pxCRCB into the appropriate ready queue
* for the priority. It is inserted at the end of the list.
* Place the co-routine represented by pxCRCB into the appropriate ready queue
* for the priority. It is inserted at the end of the list.
*
* This macro accesses the co-routine ready lists and therefore must not be
* used from within an ISR.
@ -81,20 +81,20 @@ static void prvInitialiseCoRoutineLists( void );
static inline void prvCheckPendingReadyList( void );
/*
* Macro that looks at the list of co-routines that are currently delayed to
* Macro that looks at the list of co-routines that are currently delayed to
* see if any require waking.
*
* Co-routines are stored in the queue in the order of their wake time -
* meaning once one co-routine has been found whose timer has not expired
* Co-routines are stored in the queue in the order of their wake time -
* meaning once one co-routine has been found whose timer has not expired
* we need not look any further down the list.
*/
static inline void prvCheckDelayedList( void );
/*-----------------------------------------------------------*/
portBASE_TYPE xCoRoutineCreate( crCOROUTINE_CODE pxCoRoutineCode, unsigned portBASE_TYPE uxPriority, unsigned portBASE_TYPE uxIndex )
signed portBASE_TYPE xCoRoutineCreate( crCOROUTINE_CODE pxCoRoutineCode, unsigned portBASE_TYPE uxPriority, unsigned portBASE_TYPE uxIndex )
{
portBASE_TYPE xReturn;
signed portBASE_TYPE xReturn;
corCRCB *pxCoRoutine;
/* Allocate the memory that will store the co-routine control block. */
@ -125,8 +125,8 @@ corCRCB *pxCoRoutine;
vListInitialiseItem( &( pxCoRoutine->xGenericListItem ) );
vListInitialiseItem( &( pxCoRoutine->xEventListItem ) );
/* Set the co-routine control block as a link back from the xListItem.
This is so we can get back to the containing CRCB from a generic item
/* Set the co-routine control block as a link back from the xListItem.
This is so we can get back to the containing CRCB from a generic item
in a list. */
listSET_LIST_ITEM_OWNER( &( pxCoRoutine->xGenericListItem ), pxCoRoutine );
listSET_LIST_ITEM_OWNER( &( pxCoRoutine->xEventListItem ), pxCoRoutine );
@ -190,7 +190,7 @@ portTickType xTimeToWake;
static inline void prvCheckPendingReadyList( void )
{
/* Are there any co-routines waiting to get moved to the ready list? These
are co-routines that have been readied by an ISR. The ISR cannot access
are co-routines that have been readied by an ISR. The ISR cannot access
the ready lists itself. */
while( !listLIST_IS_EMPTY( &xPendingReadyList ) )
{
@ -244,9 +244,9 @@ corCRCB *pxCRCB;
portDISABLE_INTERRUPTS();
{
/* The event could have occurred just before this critical
/* The event could have occurred just before this critical
section. If this is the case then the generic list item will
have been moved to the pending ready list and the following
have been moved to the pending ready list and the following
line is still valid. Also the pvContainer parameter will have
been set to NULL so the following lines are also valid. */
vListRemove( &( pxCRCB->xGenericListItem ) );
@ -310,17 +310,17 @@ unsigned portBASE_TYPE uxPriority;
vListInitialise( ( xList * ) &xDelayedCoRoutineList2 );
vListInitialise( ( xList * ) &xPendingReadyList );
/* Start with pxDelayedCoRoutineList using list1 and the
/* Start with pxDelayedCoRoutineList using list1 and the
pxOverflowDelayedCoRoutineList using list2. */
pxDelayedCoRoutineList = &xDelayedCoRoutineList1;
pxOverflowDelayedCoRoutineList = &xDelayedCoRoutineList2;
}
/*-----------------------------------------------------------*/
portBASE_TYPE xCoRoutineRemoveFromEventList( const xList *pxEventList )
signed portBASE_TYPE xCoRoutineRemoveFromEventList( const xList *pxEventList )
{
corCRCB *pxUnblockedCRCB;
portBASE_TYPE xReturn;
signed portBASE_TYPE xReturn;
/* This function is called from within an interrupt. It can only access
event lists and the pending ready list. */

View file

@ -34,8 +34,8 @@
#include "list.h"
/* Used to hide the implementation of the co-routine control block. The
control block structure however has to be included in the header due to
/* Used to hide the implementation of the co-routine control block. The
control block structure however has to be included in the header due to
the macro implementation of the co-routine functionality. */
typedef void * xCoRoutineHandle;
@ -61,14 +61,14 @@ typedef struct corCoRoutineControlBlock
unsigned portBASE_TYPE uxIndex
);</pre>
*
* Create a new co-routine and add it to the list of co-routines that are
* Create a new co-routine and add it to the list of co-routines that are
* ready to run.
*
* @param pxCoRoutineCode Pointer to the co-routine function. Co-routine
* functions require special syntax - see the co-routine section of the WEB
* @param pxCoRoutineCode Pointer to the co-routine function. Co-routine
* functions require special syntax - see the co-routine section of the WEB
* documentation for more information.
*
* @param uxPriority The priority with respect to other co-routines at which
* @param uxPriority The priority with respect to other co-routines at which
* the co-routine will run.
*
* @param uxIndex Used to distinguish between different co-routines that
@ -118,13 +118,13 @@ typedef struct corCoRoutineControlBlock
for( uxIndex = 0; uxIndex < 2; uxIndex++ )
{
xCoRoutineCreate( vFlashCoRoutine, 0, uxIndex );
}
}
}
</pre>
* \defgroup xCoRoutineCreate xCoRoutineCreate
* \ingroup Tasks
*/
portBASE_TYPE xCoRoutineCreate( crCOROUTINE_CODE pxCoRoutineCode, unsigned portBASE_TYPE uxPriority, unsigned portBASE_TYPE uxIndex );
signed portBASE_TYPE xCoRoutineCreate( crCOROUTINE_CODE pxCoRoutineCode, unsigned portBASE_TYPE uxPriority, unsigned portBASE_TYPE uxIndex );
/**
@ -133,14 +133,14 @@ portBASE_TYPE xCoRoutineCreate( crCOROUTINE_CODE pxCoRoutineCode, unsigned portB
void vCoRoutineSchedule( void );</pre>
*
* Run a co-routine.
*
*
* vCoRoutineSchedule() executes the highest priority co-routine that is able
* to run. The co-routine will execute until it either blocks, yields or is
* preempted by a task. Co-routines execute cooperatively so one
* preempted by a task. Co-routines execute cooperatively so one
* co-routine cannot be preempted by another, but can be preempted by a task.
*
* If an application comprises of both tasks and co-routines then
* vCoRoutineSchedule should be called from the idle task (in an idle task
* If an application comprises of both tasks and co-routines then
* vCoRoutineSchedule should be called from the idle task (in an idle task
* hook).
*
* Example usage:
@ -152,7 +152,7 @@ portBASE_TYPE xCoRoutineCreate( crCOROUTINE_CODE pxCoRoutineCode, unsigned portB
vCoRoutineSchedule();
}
// Alternatively, if you do not require any other part of the idle task to
// Alternatively, if you do not require any other part of the idle task to
// execute, the idle task hook can call vCoRoutineScheduler() within an
// infinite loop.
void vApplicationIdleHook( void )
@ -231,7 +231,7 @@ void vCoRoutineSchedule( void );
#define crEND() }
/*
* These macros are intended for internal use by the co-routine implementation
* These macros are intended for internal use by the co-routine implementation
* only. The macros should not be used directly by application writers.
*/
#define crSET_STATE0( xHandle ) ( ( corCRCB * )xHandle)->uxState = (__LINE__ * 2); return; case (__LINE__ * 2):
@ -251,8 +251,8 @@ void vCoRoutineSchedule( void );
* @param xHandle The handle of the co-routine to delay. This is the xHandle
* parameter of the co-routine function.
*
* @param xTickToDelay The number of ticks that the co-routine should delay
* for. The actual amount of time this equates to is defined by
* @param xTickToDelay The number of ticks that the co-routine should delay
* for. The actual amount of time this equates to is defined by
* configTICK_RATE_HZ (set in FreeRTOSConfig.h). The constant portTICK_RATE_MS
* can be used to convert ticks to milliseconds.
*
@ -292,33 +292,33 @@ void vCoRoutineSchedule( void );
/**
* <pre>
crQUEUE_SEND(
xCoRoutineHandle xHandle,
xQueueHandle pxQueue,
void *pvItemToQueue,
portTickType xTicksToWait,
portBASE_TYPE *pxResult
crQUEUE_SEND(
xCoRoutineHandle xHandle,
xQueueHandle pxQueue,
void *pvItemToQueue,
portTickType xTicksToWait,
portBASE_TYPE *pxResult
)</pre>
*
* The macro's crQUEUE_SEND() and crQUEUE_RECEIVE() are the co-routine
* equivalent to the xQueueSend() and xQueueReceive() functions used by tasks.
* The macro's crQUEUE_SEND() and crQUEUE_RECEIVE() are the co-routine
* equivalent to the xQueueSend() and xQueueReceive() functions used by tasks.
*
* crQUEUE_SEND and crQUEUE_RECEIVE can only be used from a co-routine whereas
* crQUEUE_SEND and crQUEUE_RECEIVE can only be used from a co-routine whereas
* xQueueSend() and xQueueReceive() can only be used from tasks.
*
* crQUEUE_SEND can only be called from the co-routine function itself - not
* from within a function called by the co-routine function. This is because
* co-routines do not maintain their own stack.
*
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* co-routines.
*
* @param xHandle The handle of the calling co-routine. This is the xHandle
* parameter of the co-routine function.
*
* @param pxQueue The handle of the queue on which the data will be posted.
* The handle is obtained as the return value when the queue is created using
* @param pxQueue The handle of the queue on which the data will be posted.
* The handle is obtained as the return value when the queue is created using
* the xQueueCreate() API function.
*
* @param pvItemToQueue A pointer to the data being posted onto the queue.
@ -326,15 +326,15 @@ void vCoRoutineSchedule( void );
* created. This number of bytes is copied from pvItemToQueue into the queue
* itself.
*
* @param xTickToDelay The number of ticks that the co-routine should block
* @param xTickToDelay The number of ticks that the co-routine should block
* to wait for space to become available on the queue, should space not be
* available immediately. The actual amount of time this equates to is defined
* by configTICK_RATE_HZ (set in FreeRTOSConfig.h). The constant
* available immediately. The actual amount of time this equates to is defined
* by configTICK_RATE_HZ (set in FreeRTOSConfig.h). The constant
* portTICK_RATE_MS can be used to convert ticks to milliseconds (see example
* below).
*
* @param pxResult The variable pointed to by pxResult will be set to pdPASS if
* data was successfully posted onto the queue, otherwise it will be set to an
* data was successfully posted onto the queue, otherwise it will be set to an
* error defined within ProjDefs.h.
*
* Example usage:
@ -362,7 +362,7 @@ void vCoRoutineSchedule( void );
// Increment the number to be posted onto the queue.
xNumberToPost++;
// Delay for 100 ticks.
crDELAY( xHandle, 100 );
}
@ -391,43 +391,43 @@ void vCoRoutineSchedule( void );
/**
* croutine. h
* <pre>
crQUEUE_RECEIVE(
xCoRoutineHandle xHandle,
xQueueHandle pxQueue,
void *pvBuffer,
portTickType xTicksToWait,
portBASE_TYPE *pxResult
crQUEUE_RECEIVE(
xCoRoutineHandle xHandle,
xQueueHandle pxQueue,
void *pvBuffer,
portTickType xTicksToWait,
portBASE_TYPE *pxResult
)</pre>
*
* The macro's crQUEUE_SEND() and crQUEUE_RECEIVE() are the co-routine
* equivalent to the xQueueSend() and xQueueReceive() functions used by tasks.
* The macro's crQUEUE_SEND() and crQUEUE_RECEIVE() are the co-routine
* equivalent to the xQueueSend() and xQueueReceive() functions used by tasks.
*
* crQUEUE_SEND and crQUEUE_RECEIVE can only be used from a co-routine whereas
* crQUEUE_SEND and crQUEUE_RECEIVE can only be used from a co-routine whereas
* xQueueSend() and xQueueReceive() can only be used from tasks.
*
* crQUEUE_RECEIVE can only be called from the co-routine function itself - not
* from within a function called by the co-routine function. This is because
* co-routines do not maintain their own stack.
*
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* co-routines.
*
* @param xHandle The handle of the calling co-routine. This is the xHandle
* parameter of the co-routine function.
*
* @param pxQueue The handle of the queue from which the data will be received.
* The handle is obtained as the return value when the queue is created using
* @param pxQueue The handle of the queue from which the data will be received.
* The handle is obtained as the return value when the queue is created using
* the xQueueCreate() API function.
*
* @param pvBuffer The buffer into which the received item is to be copied.
* The number of bytes of each queued item is specified when the queue is
* created. This number of bytes is copied into pvBuffer.
*
* @param xTickToDelay The number of ticks that the co-routine should block
* @param xTickToDelay The number of ticks that the co-routine should block
* to wait for data to become available from the queue, should data not be
* available immediately. The actual amount of time this equates to is defined
* by configTICK_RATE_HZ (set in FreeRTOSConfig.h). The constant
* available immediately. The actual amount of time this equates to is defined
* by configTICK_RATE_HZ (set in FreeRTOSConfig.h). The constant
* portTICK_RATE_MS can be used to convert ticks to milliseconds (see the
* crQUEUE_SEND example).
*
@ -437,7 +437,7 @@ void vCoRoutineSchedule( void );
*
* Example usage:
<pre>
// A co-routine receives the number of an LED to flash from a queue. It
// A co-routine receives the number of an LED to flash from a queue. It
// blocks on the queue until the number is received.
static void prvCoRoutineFlashWorkTask( xCoRoutineHandle xHandle, unsigned portBASE_TYPE uxIndex )
{
@ -483,31 +483,31 @@ void vCoRoutineSchedule( void );
/**
* croutine. h
* <pre>
crQUEUE_SEND_FROM_ISR(
xQueueHandle pxQueue,
void *pvItemToQueue,
crQUEUE_SEND_FROM_ISR(
xQueueHandle pxQueue,
void *pvItemToQueue,
portBASE_TYPE xCoRoutinePreviouslyWoken
)</pre>
*
* The macro's crQUEUE_SEND_FROM_ISR() and crQUEUE_RECEIVE_FROM_ISR() are the
* co-routine equivalent to the xQueueSendFromISR() and xQueueReceiveFromISR()
* functions used by tasks.
* The macro's crQUEUE_SEND_FROM_ISR() and crQUEUE_RECEIVE_FROM_ISR() are the
* co-routine equivalent to the xQueueSendFromISR() and xQueueReceiveFromISR()
* functions used by tasks.
*
* crQUEUE_SEND_FROM_ISR() and crQUEUE_RECEIVE_FROM_ISR() can only be used to
* pass data between a co-routine and and ISR, whereas xQueueSendFromISR() and
* pass data between a co-routine and and ISR, whereas xQueueSendFromISR() and
* xQueueReceiveFromISR() can only be used to pass data between a task and and
* ISR.
*
* crQUEUE_SEND_FROM_ISR can only be called from an ISR to send data to a queue
* that is being used from within a co-routine.
*
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* co-routines.
*
* @param xQueue The handle to the queue on which the item is to be posted.
*
* @param pvItemToQueue A pointer to the item that is to be placed on the
*
* @param pvItemToQueue A pointer to the item that is to be placed on the
* queue. The size of the items the queue will hold was defined when the
* queue was created, so this many bytes will be copied from pvItemToQueue
* into the queue storage area.
@ -515,9 +515,9 @@ void vCoRoutineSchedule( void );
* @param xCoRoutinePreviouslyWoken This is included so an ISR can post onto
* the same queue multiple times from a single interrupt. The first call
* should always pass in pdFALSE. Subsequent calls should pass in
* the value returned from the previous call.
* the value returned from the previous call.
*
* @return pdTRUE if a co-routine was woken by posting onto the queue. This is
* @return pdTRUE if a co-routine was woken by posting onto the queue. This is
* used by the ISR to determine if a context switch may be required following
* the ISR.
*
@ -537,7 +537,7 @@ void vCoRoutineSchedule( void );
// Wait for data to become available on the queue. This assumes the
// queue xCommsRxQueue has already been created!
crQUEUE_RECEIVE( xHandle, xCommsRxQueue, &uxLEDToFlash, portMAX_DELAY, &xResult );
// Was a character received?
if( xResult == pdPASS )
{
@ -561,8 +561,8 @@ void vCoRoutineSchedule( void );
{
// Obtain the character from the UART.
cRxedChar = UART_RX_REG;
// Post the character onto a queue. xCRWokenByPost will be pdFALSE
// Post the character onto a queue. xCRWokenByPost will be pdFALSE
// the first time around the loop. If the post causes a co-routine
// to be woken (unblocked) then xCRWokenByPost will be set to pdTRUE.
// In this manner we can ensure that if more than one co-routine is
@ -580,39 +580,39 @@ void vCoRoutineSchedule( void );
/**
* croutine. h
* <pre>
crQUEUE_SEND_FROM_ISR(
xQueueHandle pxQueue,
void *pvBuffer,
crQUEUE_SEND_FROM_ISR(
xQueueHandle pxQueue,
void *pvBuffer,
portBASE_TYPE * pxCoRoutineWoken
)</pre>
*
* The macro's crQUEUE_SEND_FROM_ISR() and crQUEUE_RECEIVE_FROM_ISR() are the
* co-routine equivalent to the xQueueSendFromISR() and xQueueReceiveFromISR()
* functions used by tasks.
* The macro's crQUEUE_SEND_FROM_ISR() and crQUEUE_RECEIVE_FROM_ISR() are the
* co-routine equivalent to the xQueueSendFromISR() and xQueueReceiveFromISR()
* functions used by tasks.
*
* crQUEUE_SEND_FROM_ISR() and crQUEUE_RECEIVE_FROM_ISR() can only be used to
* pass data between a co-routine and and ISR, whereas xQueueSendFromISR() and
* pass data between a co-routine and and ISR, whereas xQueueSendFromISR() and
* xQueueReceiveFromISR() can only be used to pass data between a task and and
* ISR.
*
* crQUEUE_RECEIVE_FROM_ISR can only be called from an ISR to receive data
* crQUEUE_RECEIVE_FROM_ISR can only be called from an ISR to receive data
* from a queue that is being used from within a co-routine (a co-routine
* posted to the queue).
*
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* See the co-routine section of the WEB documentation for information on
* passing data between tasks and co-routines and between ISR's and
* co-routines.
*
* @param xQueue The handle to the queue on which the item is to be posted.
*
*
* @param pvBuffer A pointer to a buffer into which the received item will be
* placed. The size of the items the queue will hold was defined when the
* queue was created, so this many bytes will be copied from the queue into
* pvBuffer.
*
* @param pxCoRoutineWoken A co-routine may be blocked waiting for space to become
* available on the queue. If crQUEUE_RECEIVE_FROM_ISR causes such a
* co-routine to unblock *pxCoRoutineWoken will get set to pdTRUE, otherwise
* available on the queue. If crQUEUE_RECEIVE_FROM_ISR causes such a
* co-routine to unblock *pxCoRoutineWoken will get set to pdTRUE, otherwise
* *pxCoRoutineWoken will remain unchanged.
*
* @return pdTRUE an item was successfully received from the queue, otherwise
@ -620,7 +620,7 @@ void vCoRoutineSchedule( void );
*
* Example usage:
<pre>
// A co-routine that posts a character to a queue then blocks for a fixed
// A co-routine that posts a character to a queue then blocks for a fixed
// period. The character is incremented each time.
static void vSendingCoRoutine( xCoRoutineHandle xHandle, unsigned portBASE_TYPE uxIndex )
{
@ -636,7 +636,7 @@ void vCoRoutineSchedule( void );
{
// Send the next character to the queue.
crQUEUE_SEND( xHandle, xCoRoutineQueue, &cCharToTx, NO_DELAY, &xResult );
if( xResult == pdPASS )
{
// The character was successfully posted to the queue.
@ -651,7 +651,7 @@ void vCoRoutineSchedule( void );
// from the queue and send it.
ENABLE_RX_INTERRUPT();
// Increment to the next character then block for a fixed period.
// Increment to the next character then block for a fixed period.
// cCharToTx will maintain its value across the delay as it is
// declared static.
cCharToTx++;
@ -691,11 +691,11 @@ void vCoRoutineSchedule( void );
/*
* This function is intended for internal use by the co-routine macros only.
* The macro nature of the co-routine implementation requires that the
* prototype appears here. The function should not be used by application
* The macro nature of the co-routine implementation requires that the
* prototype appears here. The function should not be used by application
* writers.
*
* Removes the current co-routine from its ready list and places it in the
* Removes the current co-routine from its ready list and places it in the
* appropriate delayed list.
*/
void vCoRoutineAddToDelayedList( portTickType xTicksToDelay, xList *pxEventList );
@ -707,7 +707,7 @@ void vCoRoutineAddToDelayedList( portTickType xTicksToDelay, xList *pxEventList
* Removes the highest priority co-routine from the event list and places it in
* the pending ready list.
*/
portBASE_TYPE xCoRoutineRemoveFromEventList( const xList *pxEventList );
signed portBASE_TYPE xCoRoutineRemoveFromEventList( const xList *pxEventList );
#endif /* CO_ROUTINE_H */

View file

@ -52,6 +52,12 @@
#define portBYTES_USED_BY_RETURN_ADDRESS ( 2 )
/*-----------------------------------------------------------*/
/* Stores the critical section nesting. This must not be initialised to 0.
It will be initialised when a task starts. */
#define portNO_CRITICAL_NESTING ( ( unsigned portBASE_TYPE ) 0 )
unsigned portBASE_TYPE uxCriticalNesting = 0x50;
/*
* Perform hardware setup to enable ticks from timer 1, compare match A.
*/
@ -221,6 +227,9 @@ portSTACK_TYPE *pxTopOfHardwareStack;
pxTopOfStack--;
*pxTopOfStack = ( portSTACK_TYPE ) 0x031; /* R31 */
pxTopOfStack--;
*pxTopOfStack = portNO_CRITICAL_NESTING; /* Critical nesting is zero when the task starts. */
/*lint +e950 +e611 +e923 */
return pxTopOfStack;
@ -315,6 +324,22 @@ unsigned portCHAR ucHighByte, ucLowByte;
vTaskIncrementTick();
}
#endif
/*-----------------------------------------------------------*/
void vPortEnterCritical( void )
{
portDISABLE_INTERRUPTS();
uxCriticalNesting++;
}
/*-----------------------------------------------------------*/
void vPortExitCritical( void )
{
uxCriticalNesting--;
if( uxCriticalNesting == portNO_CRITICAL_NESTING )
{
portENABLE_INTERRUPTS();
}
}

View file

@ -70,15 +70,13 @@ Changes from V1.2.3
/*-----------------------------------------------------------*/
/* Critical section management. */
#define portENTER_CRITICAL() asm( "in r15, 3fh" ); \
asm( "cli" ); \
asm( "st -y, r15" )
extern void vPortEnterCritical( void );
extern void vPortExitCritical( void );
#define portENTER_CRITICAL() vPortEnterCritical()
#define portEXIT_CRITICAL() vPortExitCritical()
#define portEXIT_CRITICAL() asm( "ld r15, y+" ); \
asm( "out 3fh, r15" )
#define portDISABLE_INTERRUPTS() asm( "cli" );
#define portENABLE_INTERRUPTS() asm( "sti" );
#define portDISABLE_INTERRUPTS() asm( "cli" )
#define portENABLE_INTERRUPTS() asm( "sei" )
/*-----------------------------------------------------------*/
/* Architecture specifics. */

View file

@ -288,7 +288,7 @@ unsigned portLONG ulOutput;
/* Setup the 8245 to tick at the wanted frequency. */
portOUTPUT_BYTE( usPIT_MODE, us8254_CTR0_MODE3 );
ulOutput = ulPIT_CONST / ulTickRateHz;
portOUTPUT_BYTE( usPIT0, ( unsigned portSHORT )( ulOutput & ( unsigned portLONG ) 0xff ) );
ulOutput >>= 8;
portOUTPUT_BYTE( usPIT0, ( unsigned portSHORT ) ( ulOutput & ( unsigned portLONG ) 0xff ) );