Newhaven Display Forum

Newhaven Products => Character LCDs => Topic started by: pabbott on September 10, 2015, 08:23:22 AM

Title: software controlled contrast is not consistent
Post by: pabbott on September 10, 2015, 08:23:22 AM
Hi,
We have the NHD-C0216CIZ-FSW-FBW-3V3 in an application.  We have built over 500 units. 
The contrast on a small percentage of the 500 units is very dark. 

The contrast is controlled via software in the initialization.  Each unit has the same code, so it sends the same contrast value.  So then why are some units much darker than most? 

Is the contrast dependent on any hardware (external to the COG/LCD module) differences/tolerances?  (I ask this because we have tried replacing the LCD module on a bad unit and the contrast problem persists- suggesting the fault is not with the LCD module.  But we've only done this once, so it's not conclusive.)

FYI, here's my init code:
Code: [Select]
const char LCD16_initString1[] = {
LCD16_Comsend,
0x38,  // function
0x39,  // function
0x14,  // Internalosc,
0x78,  // Contrast,
0x56,  // Powercontrol,
0x6D,  // Followercontrol, 
};

const char LCD16_initString2[] = {
LCD16_Comsend,
0x0C,  // Displayon,
LCD16_Clear,
0x06  // Entrymode, 
};

void LCD_init(void){
   
    LCD_RST_SetVal();
    Cpu_Delay100US( 400);
    write_i2c_LCD(LCD16_initString1,sizeof(LCD16_initString1));
    Cpu_Delay100US( 400);
    write_i2c_LCD(LCD16_initString2,sizeof(LCD16_initString2));
    Cpu_Delay100US( 400);
}

Title: Re: software controlled contrast is not consistent
Post by: Saurabh_B on September 10, 2015, 12:31:38 PM
Hi,

The hardware will play a role in the contrast. The 2 capacitors are part of a booster circuit that steps up the voltage required to create the contrast.

I was wondering how big of a difference you are seeing in the contrast? If possible to take a picture comparing 2 displays.

Also are there any differences between the boards on which the displays look a little bit darker? (Voltage, Resistance, or capacitance)
Title: Re: software controlled contrast is not consistent
Post by: pabbott on September 10, 2015, 02:17:08 PM
Thanks for the reply.
The contrast difference is quite large, and the darker one is unreadable at some angles.  Attached a photo showing a good and bad unit side-by-side. 

I measured the voltages on all 8 pins.  There is no voltage difference above ~8mV between the two samples.

1.  3.01V
2.  3.01V
3.  3.01V
4.  0 (gnd)
5.  3.01V
6.  5.86V
7.  4.43V
8.  1.51V

Any other ideas?