如何优雅的配置寄存器---ZYNQ Cortex-A9定时器配置函数解析XTmrCtr_SetOptions（）

日常废话

北京可太冷了，说好要写一段废话，发生了不少事情，好与丧的都有，丧的占大多数。话不能多说，自己消化就好，多说就泄露了天机。

底层寄存器配置与定时器配置函数的引入

在使用ARM Cortex-A9的中断系统时，阅读到了一段配置定时器的程序，其中包含的一行程序引起了我的注意。

XTmrCtr_SetOptions(&TMRInst, 0, XTC_INT_MODE_OPTION | XTC_AUTO_RELOAD_OPTION);//第三个变量选择了模式选项对寄存器进行了配置

这行程序是对定时器的 Control/Status 寄存器XTC_TCSR_OFFSET进行配置，一般来说配置寄存器是通过修改寄存器的某1bit的值 0,1 来对这一位代表的功能进行设置，或者直接写入一个字节8bit的数来直接将一个寄存器配置完成。

但是这行程序是使用了一个封装更好，更易读的方式对寄存器进行配置的，使用了Xilinx提供的XTmrCtr_SetOptions（）函数对定时器进行模式配置。可以看到函数的第三个变量通过选择option的形式，选择了定时器的不同功能，也就是设置了定时器控制寄存器的不同位的功能，以一个更易读的方式配置了定时器的工作方式，使得程序更易读。下面对此函数的底层进行分析，来看看它是如何工作的。

首先此函数有3个变量，首先要理解这三个变量的含义。

/*****************************************************************************
* @param	InstancePtr is a pointer to the XTmrCtr instance.
* @param	TmrCtrNumber is the timer counter of the device to operate on.
*		Each device may contain multiple timer counters. The timer
*		number is a zero based number with a range of
*		0 - (XTC_DEVICE_TIMER_COUNT - 1).
* @param	Options contains the desired options to be set or cleared.
*		Setting the option to '1' enables the option, clearing the to
*		'0' disables the option. The options are bit masks such that
*		multiple options may be set or cleared. The options are
*		described in xtmrctr.h.
******************************************************************************/
void XTmrCtr_SetOptions(XTmrCtr * InstancePtr, u8 TmrCtrNumber, u32 Options)

Xilinx的函数不仅提供了文档，而且在每个函数都详细备注了函数的功能与变量的意义，这点很棒，就不需要详细解释了，大致含义是InstancePtr是定时器实例，TmrCtrNumber是在有多个定时器的时候指示是第几个定时器，Options是用来设置选项的，当选择了一个选项，该bit就会被置1，未被选择的就被置0.

定时器配置函数的实现文件xtmrctr_options.c

要理解函数是如何工作的，就需要查看这个配置函数实现的文件xtmrctr_options.c，将这个文件的代码贴到最后以便查阅。

文件提供了两个函数接口，一个是配置定时器的控制寄存器函数XTmrCtr_SetOptions，另一个是获得寄存器当前状态的XTmrCtr_GetOptions，两个函数实现的功能类似，只需要看懂一个就能理解另一个。

函数实现如图下，实现核心配置功能的代码是for循环后面。

******************************************************************************/
void XTmrCtr_SetOptions(XTmrCtr * InstancePtr, u8 TmrCtrNumber, u32 Options)
{
	u32 CounterControlReg = 0;
	u32 Index;

	Xil_AssertVoid(InstancePtr != NULL);
	Xil_AssertVoid(TmrCtrNumber < XTC_DEVICE_TIMER_COUNT);
	Xil_AssertVoid(InstancePtr->IsReady == XIL_COMPONENT_IS_READY);

	/*
	 * Loop through the Options table, turning the enable on or off
	 * depending on whether the bit is set in the incoming Options flag.
	 */

	for (Index = 0; Index < XTC_NUM_OPTIONS; Index++) {
		if (Options & OptionsTable[Index].Option) {

			/*
			 * Turn the option on
			 */
			CounterControlReg |= OptionsTable[Index].Mask;
		}
		else {
			/*
			 * Turn the option off
			 */
			CounterControlReg &= ~OptionsTable[Index].Mask;
		}
	}

	/*
	 * Write out the updated value to the actual register
	 */
	XTmrCtr_WriteReg(InstancePtr->BaseAddress, TmrCtrNumber,
			  XTC_TCSR_OFFSET, CounterControlReg);
}

XTC_NUM_OPTIONS的定义在前面有，代表的是有几个可以配置的bit，进行for循环，对每个bit进行配置。而OptionsTable的值在xtmrctr.h和xtmrctr_l.h中有定义，例如option的定义

/**
 * Used to configure the timer counter device.
 * 
 * XTC_CASCADE_MODE_OPTION	Enables the Cascade Mode only valid for TCSRO.
 * XTC_ENABLE_ALL_OPTION	Enables all timer counters at once.
 * XTC_DOWN_COUNT_OPTION	Configures the timer counter to count down from
 *				start value, the default is to count up.
 * XTC_CAPTURE_MODE_OPTION	Configures the timer to capture the timer
 *				counter value when the external capture line is
 *				asserted. The default mode is compare mode.
 * XTC_INT_MODE_OPTION		Enables the timer counter interrupt output.
 * XTC_AUTO_RELOAD_OPTION	In compare mode, configures the timer counter to
 *				reload from the compare value. The default mode
 *				causes the timer counter to hold when the
 *				compare value is hit.
 *				In capture mode, configures the timer counter to
 *				not hold the previous capture value if a new
 *				event occurs. The default mode cause the timer
 *				counter to hold the capture value until
 *				recognized.
 * XTC_EXT_COMPARE_OPTION	Enables the external compare output signal.
 * 
 */
#define XTC_CASCADE_MODE_OPTION		0x00000080UL
#define XTC_ENABLE_ALL_OPTION		0x00000040UL
#define XTC_DOWN_COUNT_OPTION		0x00000020UL
#define XTC_CAPTURE_MODE_OPTION		0x00000010UL
#define XTC_INT_MODE_OPTION			0x00000008UL
#define XTC_AUTO_RELOAD_OPTION		0x00000004UL
#define XTC_EXT_COMPARE_OPTION		0x00000002UL

就假设按照最初调用函数，要配置XTC_INT_MODE_OPTION选项，即开启定时器中断，那么函数的第三个变量Options就会传进来0x00000008UL。对for循环进行分析
index控制配置OptionsTable第几个值。当index从0加到3时

if (Options & OptionsTable[Index].Option) //Options为XTC_INT_MODE_OPTION，OptionsTable[Index].Option与Options相同

if条件内两个变量相与，满足，执行下一句。

CounterControlReg |= OptionsTable[Index].Mask;

当满足时，假设CounterControlReg = 0x00000000

CounterControlReg =0x00000000 | XTC_CSR_ENABLE_INT_MASK(0x00000040)

CounterControlReg 的值就变为了0x00000040。开启定时器中断功能
若没满足，执行

CounterControlReg &= ~OptionsTable[Index].Mask;

假设CounterControlReg = 0x00000040

CounterControlReg =0x00000040 & ~XTC_CSR_ENABLE_INT_MASK(0xFFFFFFBF)

CounterControlReg 的值就变为了0x00000000。只改变了某一bit而其他位并未修改。关闭了定时器中断功能。
最终通过XTmrCtr_WriteReg函数将CounterControlReg 的值写入寄存器中，实现了对定时器控制寄存器的控制。

在获取控制寄存器的函数XTmrCtr_GetOptions()是同理的，理解了set就能理解get。

回头再看代码调用使用的方式

  XTmrCtr_SetOptions(&TMRInst, 0, XTC_INT_MODE_OPTION | XTC_AUTO_RELOAD_OPTION);

在第三个变量通过选择不同的常量OPTION，通过选择的方式开启或者关闭了某项功能，程序可读性极佳，也易于调用者使用，可以不用再了解控制寄存器的不用比特的含义来直接进行控制，封装的也很好。

嗯，就到这里吧，读程序和写解释完全不一样，十几分钟能读完的想说明白可废了老大劲，后面还要多学习。

xtmrctr_options.c代码

/***************************** Include Files *********************************/

#include "xtmrctr.h"
#include "xtmrctr_i.h"

/************************** Variable Definitions *****************************/

/*
 * The following data type maps an option to a register mask such that getting
 * and setting the options may be table driven.
 */
typedef struct {
	u32 Option;
	u32 Mask;
} Mapping;

/*
 * Create the table which contains options which are to be processed to get/set
 * the options. These options are table driven to allow easy maintenance and
 * expansion of the options.
 */
static Mapping OptionsTable[] = {
	{XTC_CASCADE_MODE_OPTION, XTC_CSR_CASC_MASK},
	{XTC_ENABLE_ALL_OPTION, XTC_CSR_ENABLE_ALL_MASK},
	{XTC_DOWN_COUNT_OPTION, XTC_CSR_DOWN_COUNT_MASK},
	{XTC_CAPTURE_MODE_OPTION, XTC_CSR_CAPTURE_MODE_MASK |
	 XTC_CSR_EXT_CAPTURE_MASK},
	{XTC_INT_MODE_OPTION, XTC_CSR_ENABLE_INT_MASK},
	{XTC_AUTO_RELOAD_OPTION, XTC_CSR_AUTO_RELOAD_MASK},
	{XTC_EXT_COMPARE_OPTION, XTC_CSR_EXT_GENERATE_MASK}
};

/* Create a constant for the number of entries in the table */

#define XTC_NUM_OPTIONS   (sizeof(OptionsTable) / sizeof(Mapping))

/*****************************************************************************/
/**
*
* Enables the specified options for the specified timer counter. This function
* sets the options without regard to the current options of the driver. To
* prevent a loss of the current options, the user should call
* XTmrCtr_GetOptions() prior to this function and modify the retrieved options
* to pass into this function to prevent loss of the current options.
*
* @param	InstancePtr is a pointer to the XTmrCtr instance.
* @param	TmrCtrNumber is the timer counter of the device to operate on.
*		Each device may contain multiple timer counters. The timer
*		number is a zero based number with a range of
*		0 - (XTC_DEVICE_TIMER_COUNT - 1).
* @param	Options contains the desired options to be set or cleared.
*		Setting the option to '1' enables the option, clearing the to
*		'0' disables the option. The options are bit masks such that
*		multiple options may be set or cleared. The options are
*		described in xtmrctr.h.
*
* @return	None.
*
* @note		None.
*
******************************************************************************/
void XTmrCtr_SetOptions(XTmrCtr * InstancePtr, u8 TmrCtrNumber, u32 Options)
{
	u32 CounterControlReg = 0;
	u32 Index;

	Xil_AssertVoid(InstancePtr != NULL);
	Xil_AssertVoid(TmrCtrNumber < XTC_DEVICE_TIMER_COUNT);
	Xil_AssertVoid(InstancePtr->IsReady == XIL_COMPONENT_IS_READY);

	/*
	 * Loop through the Options table, turning the enable on or off
	 * depending on whether the bit is set in the incoming Options flag.
	 */

	for (Index = 0; Index < XTC_NUM_OPTIONS; Index++) {
		if (Options & OptionsTable[Index].Option) {

			/*
			 * Turn the option on
			 */
			CounterControlReg |= OptionsTable[Index].Mask;
		}
		else {
			/*
			 * Turn the option off
			 */
			CounterControlReg &= ~OptionsTable[Index].Mask;
		}
	}

	/*
	 * Write out the updated value to the actual register
	 */
	XTmrCtr_WriteReg(InstancePtr->BaseAddress, TmrCtrNumber,
			  XTC_TCSR_OFFSET, CounterControlReg);
}

/*****************************************************************************/
/**
*
* Get the options for the specified timer counter.
*
* @param	InstancePtr is a pointer to the XTmrCtr instance.
* @param	TmrCtrNumber is the timer counter of the device to operate on
*		Each device may contain multiple timer counters. The timer
*		number is a zero based number with a range of
*		0 - (XTC_DEVICE_TIMER_COUNT - 1).
*
* @return
*
* The currently set options. An option which is set to a '1' is enabled and
* set to a '0' is disabled. The options are bit masks such that multiple
* options may be set or cleared. The options are described in xtmrctr.h.
*
* @note		None.
*
******************************************************************************/
u32 XTmrCtr_GetOptions(XTmrCtr * InstancePtr, u8 TmrCtrNumber)
{

	u32 Options = 0;
	u32 CounterControlReg;
	u32 Index;

	Xil_AssertNonvoid(InstancePtr != NULL);
	Xil_AssertNonvoid(TmrCtrNumber < XTC_DEVICE_TIMER_COUNT);
	Xil_AssertNonvoid(InstancePtr->IsReady == XIL_COMPONENT_IS_READY);

	/*
	 * Read the current contents of the control status register to allow
	 * the current options to be determined
	 */
	CounterControlReg = XTmrCtr_ReadReg(InstancePtr->BaseAddress,
					       TmrCtrNumber, XTC_TCSR_OFFSET);
	/*
	 * Loop through the Options table, turning the enable on or off
	 * depending on whether the bit is set in the current register settings.
	 */
	for (Index = 0; Index < XTC_NUM_OPTIONS; Index++) {
		if (CounterControlReg & OptionsTable[Index].Mask) {
			Options |= OptionsTable[Index].Option;	/* turn it on */
		}
		else {
			Options &= ~OptionsTable[Index].Option;	/* turn it off */
		}
	}

	return Options;
}
/** @} */