MAX44009环境光传感器LCD背光亮度的控制应用

最后一步就是在传感器和执行器之间建立连接，通过微控制器实现。有人可能首先要问："环境光强如何映射到背光亮度?"事实上，有些文献专门介绍了相关方案。其中一种映射方式是，Microsoft®针对运行Windows® 7¹操作系统的计算机提出的。图7所示曲线是由Microsoft提供的，它可以将环境光强度映射到显示屏亮度(以全部亮度的百分比表示)。

　　图7. 将环境光强映射为最佳显示屏亮度的曲线示例

　　这种特殊曲线可以用以下函数表示：

　　如果设备采用的是已集成亮度控制功能的LCD控制芯片，就可通过向芯片发送指令，轻松设置背光亮度。如果设备采用的是PWM直接控制亮度，则要考虑如何将比例信号映射至显示屏亮度。

　　在MAX1698示例中，根据其产品说明书的介绍，可以将驱动电流映射为电压。通过这个示例，我们可以假设LED电流强度几乎与其电流呈线性关系。这样，我们就可以在上述等式中乘上一个系数，计算出PWM所映射的有效电压，该电压再被映射至LED电流，最后转化成显示屏亮度。

　　方案实施

　　最好不要从一个亮度级直接跳转到另一个亮度级，而是平滑上调和下调背光亮度，确保不同亮度等级之间无缝过渡。为了达到这一目的，最好采用带有固定或不同亮度步长、可逐步调节亮度的定时中断，也可采用带有可控制LED输入电流的PWM值的定时中断，或者是能够发送到显示屏控制器的串行指令的定时中断。图8提供了这种算法的一个示例。

　　图8. 步进式亮度调节的算法示例

　　另一个问题是，系统响应环境光强变化的速度。我们应尽量避免过快地改变亮度等级。这是因为光强的瞬间变化(譬如一扇窗户打开或瞬间有一束光扫过)可能导致背光亮度发生不必要的变化，这往往会造成用户感觉不适。并且，较长的响应时间还有助于减少微控制器对光传感器的检测次数，从而可以释放一定的微控制器资源。

　　最初级的方法就是每隔一两秒钟检查一次光传感器，然后相应地调整背光亮度。更好的方法是，只有光线强度偏离特定范围一定时间后，才对背光亮度进行调节。譬如，如果正常光强是200lux，我们可能只会在光强降到180lux以下或升至220lux以上，而且持续时间超过数秒的情况下才调节亮度。幸运的是，MAX9635和MAX44009都集成了中断引脚和阈值寄存器，可轻松实现这个目的。请参考应用笔记4786"MAX9635环境光传感器的接口程序"，获取更多详细信息。

    		
源代码
#define MAX44009_ADDR	0x96
// begin definiTIon of slave addresses for MAX44009
#define INT_STATUS	0x00
#define INT_ENABLE	0x01
#define CONFIG_REG	0x02
#define HIGH_BYTE	0x03
#define LOW_BYTE	0x04
#define THRESH_HIGH	0x05
#define THRESH_LOW	0x06
#define THRESH_TIMER	0x07
// end definiTIon of slave addresses for MAX44009
 
extern float SCALE_FACTOR;	// captures scaling factors to map from % brightness to PWM
float currentBright_pct;	// the current screen brightness, in % of maximum
float desiredBright_pct;	// the desired screen brightness, in % of maximum
float stepSize;			// the step size to use to go from the current 
				// brightness to the desired brightness
uint8 lightReadingCounter;	
 
/**
 *	FuncTIon:	SetPWMDutyCycle
 *
 *	Arguments:	uint16 dc - desired duty cycle
 *
 *	Returns:	none
 *
 *	Description:	Sets the duty cycle of a 16-bit PWM, assuming that in this 
 *			architecture, 0x0000 = 0% duty cycle
 * 			0x7FFF = 50% and 0xFFFF = 100%
**/
extern void SetPWMDutyCycle(uint16 dc);
 
/**
 *	Function:	I2C_WriteByte
 *
 *	Arguments:	uint8 slaveAddr - address of the slave device
 *			uint8 command - destination register in slave device
 *			uint8 data - data to write to the register
 *
 *	Returns:	ACK bit
 *
 *	Description:	Performs necessary functions to send one byte of data to a 
 *			specified register in a specific device on the I2C bus
**/
uint8 2C_WriteByte(uint8 slaveAddr, uint8 command, uint8 data);
 
/**
 *	Function:	I2C_ReadByte
 *
 * 	Arguments:	uint8 slaveAddr - address of the slave device
 *			uint8 command - destination register in slave device
 *			uint8 *data - pointer data to read from the register
 * 
 *	Returns:	ACK bit
 *
 *	Description:	Performs necessary functions to get one byte of data from a 
 *			specified register in a specific device	on the I2C bus
**/
uint8 I2C_ReadByte(uint8 slaveAddr, uint8 command, uint8* data);
 
/**
 *	Function:	getPctBrightFromLuxReading
 *	
 *	Arguments:	float lux - the pre-computed ambient light level
 *
 *	Returns:	The % of maximum brightness to which the backlight should be set 
 *			given the ambient light (0 to 1.0)
 *
 *	Description:	Uses a function to map the ambient light level to a backlight 
 *			brightness by using a predetermined function
**/
float getPctBrightFromLuxReading(float lux);
 
/**
 *	Function:	mapPctBrighttoPWM
 *
 *	Arguments:	float pct
 *
 *	Returns:	PWM counts needed to achieve the specified % brightness (as 
 *			determined by some scaling factors)
**/	
uint16 mapPctBrighttoPWM(float pct);
 
/**
 *	Function:	getLightLevel
 *
 *	Arguments:	n/a
 *
 *	Returns:	the ambient light level, in lux
 *
 *	Description:	Reads both the light registers on the device and returns the 
 *			computed light level
**/
float getLightLevel(void);
 
/**
 *	Function:	stepBrightness
 *
 *	Arguments:	n/a
 *
 *	Returns:	n/a
 *
 *	Description:	This function would be called by an interrupt. It looks at the 
 *			current brightness setting, then the desired brightness setting. 
 *			If there is a difference between the two, the current brightness 
 *			setting is stepped closer to its goal.
**/
void stepBrightness(void);
 
/**
 *	Function:	timerISR
 *
 *	Arguments:	n/a
 *
 *	Returns:	n/a
 *
 *	Description:	An interrupt service routine which fires every 100ms or so. This 
 *			handles all the ambient light sensor and backlight
 *			control code.
**/
void timerISR(void);
 
void main() {
	
	SetupMicro();				// some subroutine which initializes this CPU
	
	I2C_WriteByte(MAX44009_ADDR, CONFIG_REG, 0x80);	// set to run continuously
	
	lightReadingCounter = 0;
	stepSize = .01;
	currentBright_pct = 0.5;
	desiredBright_pct = 0.5;
	SetPWMDutyCycle(mapPctBrighttoPWM(currentBright_pct));
	InitializeTimerInterrupt();		// set this to fire every 100ms
	
	while(1) {
		// do whatever else you need here, the LCD control is done in interrupts
		Idle();
	}
} // main routine
 
// the point at which the function clips to 100%
#define MAXIMUM_LUX_BREAKPOINT	1254.0
float getPctBrightFromLuxReading(float lux) {
	if (lux > MAXIMUM_LUX_BREAKPOINT)
		return 1.0;
	else
		return (9.9323*log(x) + 27.059)/100.0;
} // getPctBrightFromLuxReading
 
uint16 mapPctBrighttoPWM(float pct) {
	return (uint16)(0xFFFF * pct * SCALE_FACTOR);
} // mapPctBrighttoPWM
 
float getLightLevel(void) {
	uint8* lowByte;
	uint8* highByte;
	uint8 exponent;
	uint8 mantissa;
	float result;
	
	I2C_ReadByte(MAX44009_ADDR, HIGH_BYTE, highByte);
	I2C_ReadByte(MAX44009_ADDR, LOW_BYTE, lowByte);
	
	exponent = (highByte & 0xF0) >> 4;// upper four bits of high byte register
	mantissa = (highByte & 0x0F) << 4;// lower four bits of high byte register = 
 					  // upper four bits of mantissa
	mantissa += lowByte & 0x0F; 	  // lower four bits of low byte register = 
					  // lower four bits of mantissa
	
	result = mantissa * (1 << exponent) * 0.045;
	
	return result;
} //getLightLevel
 
void stepBrightness(void) { 
	// if current is at desired, don't do anything
	if (currentBright_pct == desiredBright_pct)
		return;
	// is the current brightness above the desired brightness?
	else if (currentBright_pct > desiredBright_pct) {
		// is the difference between the two less than one step?
		if ( (currentBright_pct-stepSize) < desiredBright_pct)
			currentBright_pct = desiredBright_pct;
		else	
			currentBright_pct -= stepSize;
	} // else if
	else if (currentBright_pct < desiredBright_pct) {
		// is the difference between the two less than one step?
		if ( (currentBright_pct+stepSize) > desiredBright_pct)
			currentBright_pct = desiredBright_pct;
		else	
			currentBright_pct += stepSize;
	} // else if
	
	SetPWMDutyCycle(mapPctBrighttoPWM(currentBright_pct));
	return;
} // stepBrightness
 
void timerISR(void) {
	float lux;
	float pctDiff;
	
	stepBrightness();
	if (lightReadingCounter)
		lightReadingCounter--;
	else {
		lightReadingCounter = 20;	// 2 second delay
		lux = getLightLevel();
		desiredBright_pct = getPctBrightFromLuxReading(lux);
		pctDiff = abs(desiredBright_pct - currentBright_pct);
		stepSize = (pctDiff <= 0.01) ? 0.01:pctDiff/10;
	} // else
	
	ClearInterruptFlag();
} // timerISR