Hi,
I'm designing a PIC devlopment board (again) and want to have a 7 segment display with a standard character LCD. Nothing special there. The LCD will have its R/W pin tied to ground, forcing the data lines as permenant inputs, and the seven segment display (4 digits) is multiplexed.
THey aren't meant to be used at the same time,.. I have hidden the LED display under the LCD, which is pluggable, this saves precious board area.
The thing is, as I'm making this single sided, and cramming everything on a 160x100mm board, routing it without a mass of top jumper wires is proving difficult. What I've found to make things easier is if I connect the LCD data lines directly to the LED cathodes on the display. This means they are connected AFTER the LED series resistor. I have provided a picture.
Now, assuming we don't turn on any of the 7seg displays transistor drivers, no current can flow through the LED's, the display is off, and the LCD display will see normal logic levels. But when running the LED display.... n the off chance the 'user' plugs in an LCD, even though the LCD display won't be strobed (the E line is jumpered) its data lines will see 5v - LED vf.
That means, a logic 1 would appear as 5v, a logic 0 would be 5-2 = 3v. Obviously this isn't going to communicate with the LCD, but it isn't meant to beused at the same time anyway.
My question is (finally got there), would this possibly damage the LCD? or perhaps cause the inputs to draw any significant current? Or am I just being overly cautious?
Please note, this schem isn't finished. I am trying to make the whole board DIY friendly... so I am having to change the schematic 'on the fly' to make it easier to route.
I'm designing a PIC devlopment board (again) and want to have a 7 segment display with a standard character LCD. Nothing special there. The LCD will have its R/W pin tied to ground, forcing the data lines as permenant inputs, and the seven segment display (4 digits) is multiplexed.
THey aren't meant to be used at the same time,.. I have hidden the LED display under the LCD, which is pluggable, this saves precious board area.
The thing is, as I'm making this single sided, and cramming everything on a 160x100mm board, routing it without a mass of top jumper wires is proving difficult. What I've found to make things easier is if I connect the LCD data lines directly to the LED cathodes on the display. This means they are connected AFTER the LED series resistor. I have provided a picture.
Now, assuming we don't turn on any of the 7seg displays transistor drivers, no current can flow through the LED's, the display is off, and the LCD display will see normal logic levels. But when running the LED display.... n the off chance the 'user' plugs in an LCD, even though the LCD display won't be strobed (the E line is jumpered) its data lines will see 5v - LED vf.
That means, a logic 1 would appear as 5v, a logic 0 would be 5-2 = 3v. Obviously this isn't going to communicate with the LCD, but it isn't meant to beused at the same time anyway.
My question is (finally got there), would this possibly damage the LCD? or perhaps cause the inputs to draw any significant current? Or am I just being overly cautious?
Please note, this schem isn't finished. I am trying to make the whole board DIY friendly... so I am having to change the schematic 'on the fly' to make it easier to route.