Can I disable LCD module by disconnecting Power to it

Question

I am designing a development board for Atmega328. I want to keep LCD module optional (Hitachi HD44780, 2x16) . That means I will solder the LCD module on my board, but I may not use it if I want to use port pins for other purpose. So I was thinking of providing only one jumper link for VCC line of LCD (instead of providing several jumpers for data and other signals) so that I can remove the jumper link and use the port pins for other purpose if LCD is not needed. Can I do this? Will my port pins get loaded if I remove power to LCD module or will LCD module gets isolated? I looked at the datasheet, but could not see internal connectivity of pins. May be my question applies to many other devices as well. By disabling power to a device, can we isolate that device?

Answer 1

Most likely your LCD module will have some Chip Select or Slave Select pin (CS). Only if that pin is asserted (usually by pulling it low) the module will actually listen to or send out data over its data/address lines and be sensitive to clock.

Other modules you connect to the controller can have a CS as well.

By connecting the CS pin to some GPIO you can select which component you actually want to talk to, allowing you to connect multiple components to the very same parallel data/address bus and multiplex communications. This is how it's done usually (and used to happen for a long time to talk to plug in cards, ISA and old parallel PCI in PCs before there was PCI-Express).

Note that displays may come with two kinds of CS, one for write and one for read access. If both lines are left unasserted (usually high), then the peripheral is neutral. Pulling both lines asserted it invalid. Pulling WE (write enable) would make it sensitive to data writes xor pulling OE (output enable) would make it output data on its pins.

Update:

That means I will solder the LCD module on my board, but I may not use it if I want to use port pins for other purpose. So I was thinking of providing only one jumper link for VCC line of LCD (instead of providing several jumpers for data and other signals) so that I can remove the jumper link and use the port pins for other purpose if LCD is not needed. Can I do this?

Most likely not! The reason for this is, that most digital circuitry contains protective clamping diodes that short circuit any voltages outside the operative range to either GND or VCC. There are two kind of VCC range protection: Rectifier to VCC or Zener to GND. In case of a Rectifier to VCC any voltage on the signal lines that greater than about 0.2 above the peripheral's supply gets clamped. Which for an unpowered device either means, that the device parasitically draws current from the signal, powering its circuitry (bad) or that the signals are simply clamped to GND+0.2V (not good either).

If your LCD doesn't provide CS pins (unlikely) you can insert a bidirectional logic buffer (e.g. a 74FCT162245T) with direction pin to act as a CS circuit.