Design of LED backlight power saving system based on S3C2440

　　The design uses a constant current LED  driver with a stable output current, ensuring the constant brightness of the backlight LED  . It is convenient to determine the output current by changing the relevant peripheral resistors, and has a highly sensitive switch control function, which can control the brightness of the LED through PWM.

　　AMC7140 is a high-power LED constant current driver chip with a wide voltage input DC range of 5~50V and a maximum output current of 700mA. It is suitable for driving 1W, 3W, and 5W LED lamps. It is packaged in TO-252-5L and has a PWM CONTROL terminal (OE pin). As shown in Figure 2, it is the pin diagram of AMC7140, where pin 1 is the power input; pin 2 is the control terminal of the output current, and the current is controlled by a high-precision resistor Rset grounded, the current Iset=1.2V/Rset, the output current Iout=500×Iset; pin 3 is grounded; pin 4 is the PWM control terminal, which is valid at high level; pin 5 is the output terminal. The application circuit of AMC7140 is shown in Figure 3.

　　2.2  Implementation of PWM control based on S3C2440

　　S3C2440  has 5 16-bit timers. Timers 0, 1, 2, and 3 have pulse width modulation (PWM) function; Timer 4 is an internal timer with no output pins; Timer 0 has a dead zone generator, which is often used in high-current devices; Timers 0 and 1 share an 8-bit pre-pulse divider, and timers 2, 3, and 4 share another one. Each timer has a clock divider that can generate 5 divided signals (1/2, 1/4, 1/8, 1/16, and TCLK). Each timer module gets its clock signal from its own clock divider, which gets its clock from the corresponding 8-bit pre-pulse divider. This 8-bit pre-pulse divider is programmable and divides PCLK according to the values ​​in the TCFG0 and TCFG1 registers. After the timer is enabled, the initial value in the timer count buffer register (TCNTBn) is loaded into the down counter, and the initial value in the timer compare buffer register (TCMPBn) is loaded into the compare register to be compared with the value of the down counter. This double buffering feature of TCNTBn and TCMPBn makes the output signal of the timer more stable when the frequency and duty cycle change. Each timer has its own clock-driven 16-bit down counter. When the counter is reduced to 0, a timer interrupt request is generated to notify the CPU that the timer operation is completed. At the same time, the value of the timer count buffer register is automatically loaded into the down counter again to continue the next operation. However, if the enable bit of the timer TCONn is cleared in normal mode, the value of TCNTBn will no longer be loaded into the counter. The value of TCNTBn is often used for PWM. When the value of the down counter is equal to the value of the compare register, the timer control logic changes the output level. Therefore, the compare register determines the opening and closing of the PWM output.

　　To set a timer, first initialize TCNTBn and TCMPBn. When initializing the timer, mainly set the following registers (taking timer 0 as an example):

　　Timer output clock frequency = PCLK/(prescaler value+1)/(divider value)

　　TCFG0 register setting: TCFG0=99; //prescaler value="99"

　　TCFG1 register setting: TCFG1=0x03; //divider value="1/16"

　　Thus, when PCLK=400M, the timer output frequency is 6.25M.

　　The timer initial value settings include:

　　TCNTB0 register setting: TCNTB0=62500; // load initial value and interrupt once every 1s

　　TCMPB0 register setting: TCMPB0=rTCNTB0》1; //50%

　　Then you can start the timer. The first time you have to load it manually: TCON=1<1;

　　After loading, change to automatic loading and start the timer: TCON=0x09.

　　2.3  Image Algorithm Design Based on S3C2440

　　The S3C2440 chip integrates an LCD controller to transmit image data to the LCD and provide necessary control signals, such as VFRAME, VLINE, VCLK, VM, etc. It can support STNLCD and TFTLCD. The mini2440 uses a 3.5in (resolution of 240×320 pixels) TFT LCD screen, configured in the commonly used 16BPP (5:6:5) mode. To display an image, just write pixel data (R(5):G(6):B(5)) to LCD_BUFFER, and the LCD controller will automatically read the data through DMA and send it to the TFTLCD for display.

　　The image algorithm is based on the image histogram for data transformation, so first you should write a subroutine and calculate the grayscale histogram of the displayed image. The algorithm is as follows (where bmp is the original grayscale image and bmp_2 is the grayscale value array):

　　for( y = 0;y < 320;y++ )

　　{for(x = 0; x < 240; x++)

　　{bmp_2 [bmp[p]] ++;

　　p = p + 1;

　　}

　　}

　　Assuming a 5% distortion, the number of pixels that need to be transformed is 240×320×5%=3,840 points. Then, based on the above algorithm principle, a point-by-point calculation method is used to calculate the corresponding (fgh-fgl) starting from grayscale 0, and finally min(fgh - fgl) is obtained by comparison.

　　The next step is to linearly shift the histogram so that the grayscale moves fgl to the dark area as a whole. In this way, the grayscale area of ​​the image is compressed from the original distribution in the interval [0,255] to the interval [0,fgh-fgl]. Next, the image should be grayscale stretched to compensate for the brightness loss caused by the backlight. If the linear stretching method is adopted, the maximum stretching multiple is obviously 255/(fgh-fgl). The algorithm is as follows (where bmp is the original grayscale image, bmp_new is the updated image, and min= min(fgh - fgl)):

　　for(y = 0;y < 320;y++)

　　{for(x = 0;x < 240;x++ )

　　{if (bmp[p]>= fgl )

　　bmp_new[p] = (bmp[p] - fgl )*255/min;

　　else

　　bmp_new[p] =0;

　　p++;

　　}

　　}

　　At this time, the pixel grayscale will not be saturated, so the backlight brightness can be reduced from 1 to (fgh-fgl)/255, and the LED drive circuit can achieve corresponding brightness control through PWM.

　　3 Experimental Results

　　As shown in FIG4 , the test image is shown. FIG4( a ) is the original image, and FIG4( b ), ( c ) and ( d ) are the test result diagrams using the histogram cropping and stretching algorithm.

　　The distortion of test image 4 (b) is 5%, and the energy saving ratio is 35%; the distortion of test image 4 (c) is 10%, and the energy saving ratio is 55%; the distortion of test image 4 (d) is 20%, and the energy saving ratio is 67%. It can be seen from the experimental results that under a certain distortion, the smaller the grayscale range of histogram clipping, the greater the reduction of backlight brightness. Compared with the original test image after histogram clipping and stretching, under the constraint of 5% distortion, the overall brightness of the image changes due to the histogram shift, but in general, there is no obvious loss in image quality.

　　4 Conclusion

　　This paper proposes a power-saving dimming method for LCD backlight based on visual characteristics, establishes a processing framework for histogram cropping and stretching, and verifies it using the ARM platform. It proves that the method in this paper can achieve about 35% backlight power saving effect with a distortion of 5% and there is no obvious loss in image quality.