Shift position after 'clear screen' instruction

dreiss

• September 17, 2015 01:32

On the NHD-0420CW, is the shift position expected to be restored to 0 when issuing a "clear screen" instruction (command 0x01)?  What about "return home" when the display is off (the "D" bit in the "Display ON / OFF Control" command)?  I'm seeing some very strange behavior.

After completely powering off the module (0 volts, all capacitors drained), I power it on and issue these commands:

// Set 4-line mode
command(0x3A);
command(0x09);
command(0x38);
// Display off
command(0x08);
// Clear screen
command(0x01);
// Display on
command(0x0C);
// Write two characters at current location.
data(0x30);
data(0x41);
// Move to second line and write two more characters.
command(0xA0);
data(0x32);
data(0x42);

I expect that all four characters will be on the left side of the screen (or right, because it is reversed), but instead I see them split: two on the left and two on the right.

This behavior is very fragile.  In *only* happens after a power-on.  After asserting the reset pin, the same command sequence gives the proper result.  Also, small changes to the code fix it.  For example, replacing the

command(0x08)

with

0x08,0x0C,0x08

(display off, on, off) fixes it.  Issuing a

command(0x02)

(return home) fixes it, but only if it is *after*

command(0x08)

. Can anyone else replicate this behavior?  I am using I2C.

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  Saurabh_B

  • September 17, 2015 16:37

  I was wondering if you could post a picture of what you are seeing.

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  dreiss

  • September 18, 2015 02:45

  This is what the code produces after a hard power off (which seems wrong):

  

  This is what the modified code produces, or what the original code produces after asserting the reset pin:

  

  I'm not concerned about the mirror-image effect.  I can fix that with the bidirection register.

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  Saurabh_B

  • September 18, 2015 12:16

  This might have something to do with the initialization of the display. This seems like it might be an issue with the Entry Mode command where the display isn't incrementing the position after a datawrite properly.
  
  If you are sending an initialization sequence can you post that as well?

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  dreiss

  • September 18, 2015 14:39

  The first code snippet is my initialization sequence.

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  Saurabh_B

  • September 18, 2015 15:50

  I would take a look at page 22 on the https://newhavendisplay.com/content/specs/NHD-0420CW-AB3.pdf  datasheet of the display for a example initialization sequence. The sequence does address the flipped bits. I would suggest adding those along with what you already have.
  
  I will wire one up to see if I can recreate what you were seeing.

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