Is this safe? Another shift register question...

Other Hardware General Electronics
1

Hi all,
I have a 128x64 LCD that I am controlling using a couple of shift registers ( 74hc595 ).
I wish to extend its interface and connect a 74hc165 to add read capabilities to the LCD.

I'm not sure weather this can be done, but attached is a simple diagram of what I intend.
The LCD is actually driven off two 74hc595's, I am only showing the data connection ( DB0 - DB7 ).

I thought of using diodes ( 1n4001 ) on the 74hc595 outputs to protect them during read operations. Would this be required?

Also my main trail of thought is based on:
When sending the read operation to the LCD, the 74hc595 data lines can be set to 0 so as not to interfere with the 74hc165 read operation.

Is this even possible?
I have a hunch hope that it is. :slight_smile:

2

the 74hc595 data lines can be set to 0 so as not to interfere with the 74hc165 read operation.

No a logic zero will interfere just as much as a logic one. A zero is pulling down, that is sinking current so it will short out the LCD's output.

However, if you the output enable pin on the 595 and set it high it puts the outputs of that chip into a tri-state, that is high impedance. Then you can read the output of the LCD.

3

Thanks for the reply, that seems perfect, I can use one of the free outputs on the control 74hc595 to set the output enable on the data 74hc595 when it sets a read operation.

Cheers for the help