HP Classic replacement PCB - multi emulator

[Harald]

(11-12-2017 07:26 PM)Alejandro Paz(Germany) Wrote:  I'm curious about the voltage level translators, are thy needed or say 3.3/5 V are everything that is needed by the LCD drivers ?

There is one voltage doubler charge pump on the PCB the provides the supply voltage for the display drivers.
The micro controllers are running directly of the battery voltage and the data lines of the display drivers can be driven directly from the port pins. The RCD line needs level shifting, but I have done that very simple by an LED that serves as a zener diode and a pullup resistor. The clock from the display drivers to the µC is connected via a 10k resistor to protect the µC from the higher voltage. All fairly simple hardware wise. The real trick is to properly drive the display driver data lines.

Here is the schematic for those interested:

  cce33.pdf (Size: 34.89 KB / Downloads: 48)

Cheers,
Harald