9 Replies Latest reply on May 23, 2015 12:33 AM by ravi.m

    Refresh Rate of LCD

    ravi.m

       What is meant by Refresh Rate mentioned in Segment LCD component?

        • 1. Re: Refresh Rate of LCD
          user_1377889

          Welcome in the forum.

             

          Since the LCD component is not made for a static LCD -glass device, but one that needs fewer inputs for maintaining all the segments, some kind of multiplexing has to be made. This switching frequency is named "Frame-rate" and is in the range of 30Hz to 150Hz, default is 60Hz allowing for flicker-free display.

             

           

             

          Bob

          • 2. Re: Refresh Rate of LCD
            user_14586677

            There is an associated problem with LCDs, that is writing at high rep rates display data. Typical

               

            character LCD, like a 16 x 2, have their own timing domain, are async to processor. As such if you

               

            are in a tight loop writing repeatedly the LCD you can get display "artifacts", annoying and just plain

               

            sloppy appearance.

               

             

               

            The way to combat this is to create a buffer in RAM, in this case dispbuff[ 2, 17 ], additional char to

               

            hold NULL char, string termination. Then call a routine that first checks if the data contains a change

               

            in the string to be written from what is already written to LCD, in the buffer. If no change then no LCD

               

            write. If change write only the char(s) that have changed, and update buffer to reflect what yopu have

               

            written to LCD.

               

             

               

            This will eliminate need for delays in LCD update folks normally use, and virtually eliminate all display

               

            artifacts.

               

             

               

            This works as long as data changes slowly relative to human eye response. If real data changing rapidly

               

            LCD display not exactly useful output device( unless you average it), unless of course doing bargraph

               

            or something akin. And then what good is a bargraph bouncing around at high rates, one can't conclude

               

            much from that either.

               

             

               

            Regards, Dana.

            • 3. Re: Refresh Rate of LCD
              ravi.m

               Thanks all for the responses. I am trying to understand LCD terminology. 

                 

              so, its only for  flicker free display, the frame rate should be chosen. Is there any min limitation for the duration over which a pixel is addressed in a frame (sub-frame). because, if we assume 60Hz Frame rate and as the # of commons increases, the sub-frame duration reduces. 

                 

              Also, can you explain how contrast is implemented in LCD?

              • 4. Re: Refresh Rate of LCD
                user_1377889

                I would suggest you to read the LCD datasheet. On page 32 the principle of the "Dead-Time Period" contrast setting is explained with a diagram.

                   

                 

                   

                Bob

                • 5. Re: Refresh Rate of LCD
                  ravi.m

                   Thanks Bob. This is what I am looking for. To reduce the contrast, Sub-frame period is reduced and dead time period is increased at  the same frame rate. 

                     

                  Rvi

                  • 6. Re: Refresh Rate of LCD
                    user_1377889

                    You are always welcome, Rvi.

                       

                     

                       

                    Bob

                    • 7. Re: Refresh Rate of LCD
                      user_14586677

                      Contrast -

                         

                       

                         

                      www.newhavendisplay.com/contrastvoltage_circuit.html

                         

                       

                         

                      More basic, LCD displays rely on V fields to control twist in the shutter.

                         

                      The molecule that is twisting determines the light level thru the pixel.

                         

                      Fully shut off there is still a small amount of light that comes thru, hence

                         

                      dark level not totally dark. You can google a lot of info on technologies,

                         

                      reflective, transmissive, transflective.

                         

                       

                         

                      Regards, Dana.

                      • 8. Re: Refresh Rate of LCD
                        user_14586677

                        Is there any min limitation for the duration over which a pixel is addressed in a frame (sub-frame).

                           

                         

                           

                        There are specs on raw glass performance on graphic LCDs. The basic pixel and

                           

                        its row column drivers are all pretty capacitive, and their intrinsic chemistry turn on turn

                           

                        off  time. Agravated by cold I might add. There are newer fluids that are doing better

                           

                        at the cold problem. Older displays, if you ever get to see them in cold, you can write

                           

                        a book waiting for the molecules in the fluid to twist due to viscous forces. Its where HV

                           

                        would be great to produce field strength but physical parameters and geometries limit

                           

                        E field strengh before arcover, damage occur. And we don't like our processors running

                           

                        at Vdd = KV levels ( which would be great for outer space applications, re problem with

                           

                        ionizing radiation upset event in memory).

                           

                         

                           

                            

                           

                                 

                           

                        http://www.pacificdisplay.com/lcd_oper_basics.htm

                           

                        http://www.orientdisplay.com/LCD-basics.html

                           

                         

                           

                         

                           

                        Regards, Dana.

                        • 9. Re: Refresh Rate of LCD
                          ravi.m

                          Fully shut off there is still a small amount of light that comes thru, hence dark level not totally dark."

                             

                          The same is also explained in component datasheet by the term Discrimination ratio. It depends on the duty ratio and bias level chosen. They have also given a table with the recommended bias type for every duty ratio.

                             

                          Thanks Dana for sharing the info. 

                             

                          Rvi