TUTORIALS 4786

Interface Code Implementation for the MAX9635 Ambient Light Sensor

By: Prashanth Holenarsipur

Abstract: This application note describes the proper use of interrupt output functionality in the MAX9635 ambient light sensor for LCD backlight management in portable devices like smartphones and tablets. The interrupt functionality allows adjustment of light threshold values for multiple illumination zones without the need to poll the light sensor repeatedly. When used properly, this function allows the system to remain in a low-power sleep mode or to dedicate other resources for other user-defined tasks. The interrupt functionality significantly improves energy efficiency, system performance, and the user's experience in different lighting conditions. This article also gives an example of I²C C pseudocode for programming the interrupt.

Introduction

The MAX9635 ambient light sensor uses advanced techniques to detect the brightness of ambient light. This device is useful in multiple applications including, but not limited to, display LCD backlight adjustment in portable and home electronics and room lighting. The MAX9635's extremely low operating power (just 0.65µA) and its 1.8V operating voltage (designed to facilitate easy interaction with a microcontroller I/O ports) make it attractive for use in a number of sensor and security applications. The backlight adjustment and low operating power extend battery life and increase the energy efficiency of the lighting application.

One of the MAX9635's most valuable features is a highly versatile interrupt output pin. This pin allows the system to stay in a low-power sleep state or dedicate resources for other user-valued tasks.

This application note explains how to code this interrupt output functionality to ensure optimal system performance. Some example C-style pseudocode is given.

Register Presets

The table below shows the register map of the MAX9635, together with power-on reset (POR) states.

Register Bit Register address Power-on RESET state R/W
7 6 5 4 3 2 1 0
Status
Interrupt status INTS 0x00 0x00 R
Interrupt enable INTE 0x01 0x00 R/W
Configuration
Configuration CONT MANUAL CDR TIM[2:0] 0x02 0x03 R/W
LUX reading
LUX high byte E3 E2 E1 E0 M7 M6 M5 M4 0x03 0x00 R
LUX low byte M3 M2 M1 M0 0x04 0x00 R
Threshold set
Upper threshold high byte UE3 UE2 UE1 UE0 UM7 UM6 UM5 UM4 0x05 0xFF R/W
Lower threshold high byte LE3 LE2 LE1 LE0 LM7 LM6 LM5 LM4 0x06 0x00 R/W
Threshold timer T7 T6 T5 T4 T3 T2 T1 T0 0x07 0xFF R/W

The preset registers are Configuration, Interrupt Enable, and Threshold Timer.

Power-up settings for the Configuration register (Address 0x02), CONT=0 and MANUAL=0, are sufficient for most user applications. These settings tell the MAX9635 to automatically scale its sensitivity up and down based on the appropriate ambient light level.

To enable interrupt functionality, the master, i.e., the microcontroller, first writes a 1 to the Interrupt Enable register (Address 0x01).

The master then writes a suitable delay to the Threshold Timer register (Address 0x07). Normally, this setting does not change. There are two principal reasons for writing this threshold delay. First, writing a nonzero value to this register prevents nuisance trips due to fleeting or momentary changes in light conditions. A fleeting light change could occur when a shadow passes over the light sensor after a user gesture or movement of the device. Second, an intentional delay in the response of display brightness allows time for a defined user-interface algorithm. An example of this would be a mobile application like an iPad™ device. In this example one would not want the display brightness to fluctuate rapidly when passing through a dark corridor like a subway punctuated by periodic lights.

Threshold Register Settings

During normal operation the user repeatedly programs the Upper Threshold register (Address 0x05) and Lower Threshold register (Address 0x06). An interrupt is triggered (INTS bit in register 0x00 is set to 1 and the active-low INT hardware pin is pulled low) when the ambient light level exceeds the window levels defined by these registers. This interrupt will last longer than the delay defined by the Threshold Timer register (Address 0x07).

To program the Threshold register settings, the master first reads the lux counts from the data registers, LUX High Byte (address 0x03) and LUX Low Byte (address 0x04), to find the current operating zone. The master then sets appropriate Upper Threshold register and Lower Threshold register counts.

Example Algorithm for Backlight Control

The human eye perceives brightness logarithmically, in much the same way as the human ear perceives loudness. As a result, backlight strength is typically programmed so that it also responds in a logarithmic fashion to ambient light levels. Therefore, there are finer steps in low-light levels, while the backlight strength does not change as much in bright ambient conditions. The host processor would, ideally, also implement additional advanced image-processing algorithms such as contrast and color adjustment based on this ambient light-level information.

A typical algorithm for brightness control could have five threshold levels for control. Quite often the type of interface glass and size of physical opening can reduce the light that the light sensor "sees" down to as little as 5% to 10% of external ambient light. This scaling should be taken into account when setting the threshold levels.

The following table is one example of backlight strength and upper and lower thresholds. To convert the threshold lux to threshold counts, simply divide the target lux setting by 0.045.

Illumination Zone External Lux (typ) Backlight Strength (%) External Lux, Lower Threshold (typ) External Lux, Upper Threshold (typ) Lower Threshold (10% Glass) Upper Threshold (10% Glass)
Dark 4 25 < 0 > 10 < 0 > 1
Dim 20 45 < 10 > 50 < 1 > 5
Home 100 65 < 50 > 200 < 5 > 20
Office 400 85 < 200 > 1000 < 20 > 100
Sunlight > 2000 100 < 1000 > Maximum < 100 > Maximum

Changing backlight strength with external lighting conditions.
Changing backlight strength with external lighting conditions.

Implementing an Interrupt

The following figure shows a typical example of a flowchart implemented by the master microcontroller.

Example of a flowchart implemented by the master microcontroller.

Algorithm Threshold Levels and Ambient Measurement: Counts vs. Lux

It is more straightforward to implement the algorithm in terms of counts rather than lux values. Doing so bypasses the need to use any floating-point math and allows simple fixed-point microcontroller code.

Ambient light count 2^(Exponent) × Mantissa
Exponent = 8xE3 + 4xE2 + 2xE1 + E0
Mantissa = 128xM7 + 64xM6 + 32xM5 + 16xM4 + 8xM3 + 4xM2 + 2xM1 + M0
Upper threshold count 2^(Exponent) × Mantissa
Exponent = 8xE3 + 4xE2 + 2xE1 + E0
Mantissa = 128xM7 + 64xM6 + 32xM5 + 16xM4 + 15
Lower threshold count 2^(Exponent) × Mantissa
Exponent = 8xE3 + 4xE2 + 2xE1 + E0
Mantissa = 128xM7 + 64xM6 + 32xM5 + 16xM4

Using the desired thresholds from the table above, one can calculate threshold register bytes for use as limits in the pseudocode for each illumination zone. These thresholds are simply compared to ambient light count calculated from the equation above.

Zone Lower Threshold, 10% Glass (Lux) Upper Threshold, 10% Glass (Lux) Desired Lower Threshold Counts Desired Upper Threshold Counts Lower Threshold Register Byte Upper Threshold Register Byte Actual Lower Threshold Counts Actual Upper Threshold Counts Actual Lower Threshold (Lux) Actual Upper Threshold (Lux)
Dark < 0 > 1 0 22 0000
0000
0000
0001
0 31 < 0 > 1.395
Dim < 1 > 5 22 111 0000
0001
0000
0110
16 111 < 0.72 > 4.995
Home < 5 > 20 111 556 0000
0110
0010
1001
96 636 < 4.32 > 28.62
Office < 20 > 100 556 2222 0010
1001
0100
1000
576 2288 < 25.92 > 102.96
Sunlight < 100 > Maximum 2222 4177920 0100
1000
1110
1111
2048 4177920 < 92.16 > 188006

It should be noted that if the operating light level is quite close to the border of a defined illumination zone, backlight levels can fluctuate more frequently and cause discomfort to a user. For this reason, a small overlap zone has been defined between the upper threshold of one illumination zone and the lower threshold of the next higher illumination zone. This provides a natural hysteresis to act as a shield against small light fluctuations. These overlaps can be expanded further if desired.

The algorithm described here is only a general guideline for one possible implementation of backlight brightness control. Those skilled in the art of backlight control have developed many different algorithms to deliver a sophisticated and transparent feel to the end user.

Sample C Pseudocode Implementation

// begin definition of slave device address
#define MAX9635_WR_ADDR		0x96
#define MAX9635_RD_ADDR		0x97

// begin definition of slave register addresses for MAX9635
#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 MAX9635

// define some lookup tables for the upper and lower thresholds as well as the
// brightness. All tables values are taken from text of application notes
#define NUM_REGIONS		5
uint8 upperThresholds[NUM_REGIONS] = {0x01, 0x06, 0x29, 0x48, 0xEF};
uint8 lowerThresholds[NUM_REGIONS] = {0x00, 0x01, 0x06, 0x29, 0x48};
uint8 backlightBrightness[NUM_REGIONS] = {0x40, 0x73, 0xA6, 0xD9, 0xFF};

/**
	Function:	SetPWMDutyCycle

	Arguments:	uint8 dc - desired duty cycle

	Returns:	none

	Description:	sets the duty cycle of a 8-bit PWM, assuming that in this
			architecture, 0x00 = 0% duty cycle 0x7F = 50% and 0xFF = 100%
**/
extern void SetPWMDutyCycle(uint8 dc);
extern void SetupMicro(void);
extern void Idle(void);

/**
	Function:	I2C_WriteByte

	Arguments:	uint8 slaveAddr - address of the slave device
			uint8 regAddr - 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
**/
extern uint8 I2C_WriteByte(uint8 slaveAddr, uint8 regAddr, uint8 data);

/**
	Function:	I2C_ReadByte

	Arguments:	uint8 slaveAddr - address of the slave device
			uint8 regAddr - 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
**/
extern uint8 I2C_ReadByte(uint8 slaveAddr, uint8 regAddr, uint8* data);

/**
	Function:	findNewThresholdsAndBrightness

	Arguments:	uint8 luxCounts - light counts High Byte
			uint8 *highThresh - pointer to memory storing upper threshold byte
			uint8 *lowThresh - pointer to memory storing lower threshold byte

	Returns:	none

	Description:	Based on what the lux reading was (in counts), this routine
			determines the current operating illumination zone. The zones
			are defined by upper and lower bounds in a lookup table. After
			knowing the operating zone, this function may set new interrupt
			thresholds and a backlight brightness. Since the interrupt only
			fires when the lux reading is outside the defined region, these
			threshold and brightness settings are not overwritten with the
			same data repeatedly.
**/
void findNewThresholdsAndBrightness(uint8 luxCounts, uint8 *highThresh,
  				    uint8 *lowThresh);

void main() {

	uint8 *highThresholdByte;	// upper and lower threshold bytes
	uint8 *lowThresholdByte;
	uint8 *timerByte;
	uint8 max9635Interrupt = 0;	// status of MAX9635 interrupt register
	uint8 luxCounts;		// computed as shown below

	SetupMicro();			// some subroutine which initializes this CPU
	*highByte = 0;
	*lowByte = 0;
	*highThresholdByte = 0xEF;	// upper threshold counts
					// initially = POR setting (maximum possible = 0xEF)
	*lowThresholdByte = 0x00;	// lower threshold counts
					// initially POR setting (minimum possible = 0x00)
	*timerByte = 0x14;		// initial timer delay for thresholds:
					// 0x14 * 100ms = 2 seconds

					// initialize MAX9635 threshold and timer registers
	I2C_WriteByte(MAX9635_WR_ADDR, THRESH_HIGH, *highThresholdByte);
	I2C_WriteByte(MAX9635_WR_ADDR, THRESH_LOW, *lowThresholdByte);
	I2C_WriteByte(MAX9635_WR_ADDR, THRESH_TIMER, *timerByte);
	I2C_WriteByte(MAX9635_WR_ADDR, INT_ENABLE, 0x01);// enable sensor interrupts

	while(1) {

		// do other tasks until an interrupt fires
		// assume that this function waits for the status of a GPIO-type pin to
		// change states
		while (! GPIO_StatusChanged() ) {
			// some idling subroutine, shown with polling a port for
			// simplicity - but alternate interrupt-based routines are more
			// efficient
			Idle();
		} // loop until an interrupt occurs

		// ok... an interrupt fired! was it from the MAX9635?
		I2C_ReadByte(MAX9635_RD_ADDR, INT_STATUS, max9635Interrupt);

		/**
			Place code to check other devices here, if desired
		**/

		if (max9635Interrupt) {
			// get the current lux reading from the MAX9635
			I2C_ReadByte(MAX9635_RD_ADDR, HIGH_BYTE, luxCounts);
			findNewThresholdsAndBrightness(luxCounts, highThresholdByte,
     							lowThresholdByte);

			// write to the threshold and timer registers with new data
			I2C_WriteByte(MAX9635_WR_ADDR, THRESH_HIGH, *highThresholdByte);
			I2C_WriteByte(MAX9635_WR_ADDR, THRESH_LOW, *lowThresholdByte);

			max9635Interrupt = 0;	// interrupt serviced, clear the bits
		} // only executes if the MAX9635's interrupt fired

		// perform other tasks which are only done after change of a GPIO pin
	} // loop forever

} // main routine

void findNewThresholdsAndBrightness(uint8 luxCounts, uint8 *highThresh, uint8 *lowThresh) {

	uint8 i;

	for (i=0; i < NUM_REGIONS; ++i) {
		if ((luxCounts >= lowerThresholds[i]) && (luxCounts <= upperThresholds[i])){
			*highThresh = upperThresholds[i];
			*lowThresh = lowerThresholds[i];
			// PWM duty cycle sets the brightness of the backlight
			SetPWMDutyCycle(backlightBrightness[i]);
			return;  // found the region -- no point in continuing the loop
		} // found the right region
	} // check where the lux reading lies in terms of threshold regions

} // findNewThresholdsAndBrightness