S128240C Behaving Strangely

Without a demo board to verify the display, I'm simultaneously debugging software and hardware.

The display portion of the schematic is attached. The display generates Vlcd at 16.3 V, but only after the initialization function tells it to. That means the display is accepting data from the processor. However, the display won’t drive the data bus on a read command. Oddly, the data bus is ALMOST passive during the read operation. The 100-kohm pull-ups take the data bus only to about 2.9 V, with Vdd at 3.3 V. Also, the data lines have several mV of ripple on them at the LCD bias oscillator frequency. With the LCD unplugged, the data lines really are passive, as you would expect.

Contrast control partly works. At some point in the range, one step in either direction switches Vlcd between 16.2 V and 12.8 V. V0..V4 correspond to Vlcd, but not as I would expect. 

     Low  High
Vlcd 12.0 16.3
V0   11.9  3.9
V1   11.0  3.7
V2    8.7  3.6
V3    1.8  0.3
V4    1.0  0.3

Two displays I have behave this way. Both seem to accept control data, refuse display data, and avoid driving the data bus.

Thanks.

Rod

Attachment: Schematic.pdf (90.0KB)