LCD screen and boot pull-ups

[cospan]

I’m working on getting my LCD screen up and running and I ran into an issue.

The LCD data lines are used to specify the boot configuration and so I used 100K pull-ups to SYS_VDD1_3P3V on the appropriate signals. Without the LCD screen the board boots up fine but when I attached the LCD screen the board didn’t boot. I couldn’t get into U-Boot. I suspected something was wrong with the pull-ups/pull-downs on the LCD pins and when I measured them with a multimeter I noticed that without the LCD screen attached the SYS_VDD1_3P3V measured 3.3V but on the other side of a pull-up the measured value was 2.9V. The SYS_VDD1_3P3V was always at 3.3V regardless of if I used the LCD or not.

This isn’t that bad, but on my board I have no other pull-down components on that line so the only place I can imagine a pull-down is within the SOC.

I measured the voltage at the same location after I put on the LCD screen and the value was 1.0V. From some back the envelope calculations it would appear that the LCD screen had a ~43kOhm pull-down resistor.

There are BeagleBone capes that have LCD screens on them, even the same LCD screen that I used:

http://www.newhavendisplay.com/specs/NHD-5.0-800480TF-ATXL-CTP.pdf

They all seem to wire up to the LCD in the same way that I have but there is no mention of a boot-up problem.

I looked into the datasheet for the on LCD controller and it doesn’t specifically mention that there is are pull-up/pull-down on any of the LCD data lines.

I managed to work around this issue by using 10K pull-up resistors instead of 100K.

I am curious if this is just a problem with my board. If someone has a Redboard would they measure the voltage on the pull-up lines to determine if they measure 2.9V.

Thanks,
Dave

[Erik Welsh]

Dave,

Glad you were able to figure out and work around the issue.

Yes, approximately 2.9V is reasonable for the voltage on the input line with a 100K pull up. If you look at the DC Electrical Characteristics of the AM3358 for “All other LVCMOS pins (VDDSHVx = 3.3 V; x = 1 to 6)” (See page 91 of the AM3358 datasheet, http://www.ti.com/lit/ds/symlink/am3358.pdf), you can see that Input leakage current and Ioz are both a max of 18uA.  Based on the voltage drop and the pullup resistance, you can see that there is about 4uA of current being consumed which is well within these leakage current limits.

You will see this same behavior on the OSD3358-SM-RED.

Thanks,

Erik

[7Usan]

This is solved then. You can see that there is about 4uA of current being consumed which is well within these leakage current limits.

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