Story about LCD hacking


It started long time ago, just after velvet revolution. Borders were opened and we were allowed visit other country. My father bought on flea market at vienna radioclub some intresting electronics. After years we found at home one piece of LCD bought there. Daddy said: “it could be nice in QRP TRX to show current QRG, do you know control it?” And it was challenge to me..

Front is LCD, Buttons “Clear”, “Total”. On back side is switch with “Line”, “Time” position and buttons “+”,“-”. Inside device is PCB with PC8574 I/O expander, PCF8… 1kb RAM with battery and several transistors. Othere PCB is LCD with driver PCF8566. Original function of this device is realy mystery to me.

I downloaded datasheet, which was key to success.

I found with buzzer:

  • Ucc and Gnd
  • Buttons signals
  • Address of SA0 – Bit for cluster addressing
  • A0,A1,A2 – Bit for device addressing
  • Number of Backplanes – BP1 and BP3 are connected because they are in same phase → 1:3 Multiplex

Finally, after some time studying datasheet and PCB, I wrote short code and send into chip 2 command bytes and several data bytes. Nothing happened, rechecked it and found that Enable LCD bit was not set properly. On second try, LCD REVIVE!

Another task is map LCD segments and data bits in PCF8566 chip. Result is shown in table on photo. Some numbers were printed on wrong position when I wanted see “01234567”.. So, I swaped those numbers in code..

Consumption of this LCD is 33uA(micro), it is ideal to any portable device supply from battery.

Mission successful. Next challange is implement it to my counter.