STN LCD AC Bias frequency problem

Yes it does make sense. If you want the AC bias pin to clock at (hsync rate) you put 0 in that register. If you want it to clock at (hsync rate / 2) you put 1 in there.

LP usually means "line pulse", or rather horizontal sync; are you sure you're not misreading the datasheet for the LCD?

Last time I used an STN LCD was 12 or so years ago, but I am pretty sure I remember the same signal coming out of the Hitachi chip I used then (along with plenty of external logic). It would just toggle every V-sync, IIRC it was used by the LCD module to keep reversing its internal bias (DC would damage it). To my memory, all LCD modules were pretty much the same regarding this signal, so my guess is you can just use the one supplied by the chip, if this is what it is mean for and if it looks like what I described above, more or less. Nowadays, I use TFTs, which keep this sort of thing internal, but I would be surprised if the mono LCDs have changed a lot since those days.

Dimiter

------------------------------------------------------ Dimiter Popoff Transgalactic Instruments

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Not Really Me wrote:

Scott,

• The 401 is the one with the colour controller rather than the mono but I believe that that's OK.

• I think that the LP pin is not the AC bias. Typically it is called M. Some displays generate it internally.

• The Sharp docs are as confusing as ever. I think the "ACB = (lineclocks)-1" ought to be frame clocks. Normally the M signal will toggle every few frames - in your case every frame, so I'd expect ACB = 0. However, the text in chapter 14 also suggests line clocks using different words, so it might be possible thats what they mean but I doubt it.

• 184 Hz frame rate seems a bit high to me. I'd expect 75-100 Hz.

I have a LH75410 based system here with mono 320x240 (landscape) display but changing ACB makes no visible difference. Perhaps the display I have isn't using M, some do some don't. I'd have to take the machine apart and prod with a scope to find out what frequency it's running at. I wonder if I can see the change on the port pin from inside the chip...

regards, Peter

Thanks to the others that replied also.

Peter, you are correct, it is the version with the color controller. The Sharp spec says it does mono also. The chip was chosen over the mono only version for some other feature.

The Truly datasheet seems to be adjusted whenever we ask a question. I really think they make it up as they go. The display is a 4 bit wide I/O monochrome. The first version of the datasheet was so completely wrong that it listed the interface as a simple latched 8 bit port. I guess you put up with this type of problem to save 40-50% over competing units. The LP signal is the AC Bias. We have tried all values from 0 to 31 in the ACB field of the register and only 31 gives a stable display. I am also concerned that the frame rate is too high, as well as the clock frequency which may be exceeding the LCD spec. I getting someone to verify that now.

As a bit of probably important information I only received yesterday, the real problem is temperature related. The display works fine at room temp, but is intended for an outdoor unit. When stored over night in a freezer at -20 C, the display still works fine. As the temperature is brought up, we start to get a very noticeably flicker (particularly on the upper 1/3 to

1/2 of the LCD) at around 0 C and it doesn't go away until around 18 - 20 C. On some displays it never goes away and the seems to have damaged the display permanently. The display is rated for operation down to -20 C and storage to -30 C. In the production version we will use a heater to keep it at 0 C or above.

Scott