Hitachi HD44780 LCD - I unplug Vdd but LCD stays on?

[lee321987]

I'm working on a project controlling an HD44780 LCD with a PIC microcontroller.
The data lines on the LCD get fed from a 74HC164 shift register that if controlled by the PIC.

I had it running, but wanted to keep the LCD off until the shift register has a known value, so I unplugged Vdd on the LCD (to connect it to a MOSFET), but the LCD STAYED ON. The 60ma LED back-light stayed on too!!

From where could the LCD be sucking this power??

I have the RS and EN pins on the LCD connected to digital I/O pins on the pic.

Am I supposed to have resistors between any of the HD44780 lines and the PIC or 74164??

 

[betwixt]

The pins of most IC's probably including the HD44780 have protection diodes conneced to ground and their supply lines. They are there to protect the IC in case excessive voltage is put on a pin, the idea is they safely conduct the excess into either ground or the supply. If you remove the VDD connection, the supply line voltage drops below that of the signals on the pins so the diodes start to conduct and because they are connected to the internal VDD line, they supply power to it.

What you are doing is wrong though and can damage the 74HC164, PIC and the LCD. Those diodes are not intended to carry constant current, especially to the backlight which has quite a high demand and the shift register probably has it's outputs overloaded as well. The biggest danger is that when you reconnect the VDD you will get what's known as "latch-up" occurring. This is a state where the input stages of the LCD can go into a state where they draw very high current and potentially burn out. If you want to prevent the LCD showing anything, either turn off all the power to devices connected to it as well, or tie the read/write line into read state (high) so no data is written to it.

Brian.

 

[lee321987]

Thank you very very much!!
Great, informative reply!

 

[ark5230]

The back light supply is on Pins 15 and 16 of LCD display where as the VDD supply for LCD module is at Pin 2. The back light will not go if you remove VDD, removing the supply to back light LED Pin 15 and 16 will stop back light.