Using Pin 13 for LCD

Will there be any unforeseen problems using pin 13 (that has the built in LED) to operate my 2x16 LCD display's pin 6 (H/L Enable signal)?

I don't foresee any unforeseen problems.


Rob

I do foresee an unforeseen problem. Is there some reason why you want to use pin 13 for Enable (as opposed to using that pin for RS or one of the data lines). If the added load of the LED and it's resistor is going to cause any problems it is most likely to cause such problems with the Enable line. The other signals are relatively stable, that is they are high or low for relatively long periods of time, whereas the Enable line is pulsed high then low for every data transmission. There are some constraints on the rise and fall time of this signal and the presence of the LCD/resistor may not mess this up, but it certainly won't help things either.

Don

It should work fine. I recently used pin13 to supply the clock signal to a SPI 2 channel DAC module and had no problems.

Lefty

With all due respects an SPI 2 channel DAC module is not an LCD module. If there is some reason why he has to put the Enable line on pin 13 then he will probably get away with it. On the other hand if one of the other signals can be put on that pin it would be a good idea to do so.

Don

floresta:
With all due respects an SPI 2 channel DAC module is not an LCD module. If there is some reason why he has to put the Enable line on pin 13 then he will probably get away with it. On the other hand if one of the other signals can be put on that pin it would be a good idea to do so.

Don

I'm willing to listen to any electrical reason why he can't/shouldn't use pin13. The load of the led/resistor is on the AVR output pin and should have no bearing on the LCD module. As the resistor is 1k ohm for the led, the AVR output pin will have a load of around 3.5ma, so whatever extra load the LCD's enable input pin adds is, I'm pretty sure the AVR will drive it fine. Or do you have a some other concern in mind? Or to put it another reason, why would it not be a good idea for him to use pin 13?

Lefty

Or do you have a some other concern in mind? Or to put it another reason why would it not be a good idea for him to use pin 13?

It's not the AVR that I am concerned with, it's the possible degradation of the Enable pulse by the presence of the LED/resistor combination. I thought I spelled this out pretty clearly in reply #2.

Don

floresta:

Or do you have a some other concern in mind? Or to put it another reason why would it not be a good idea for him to use pin 13?

It's not the AVR that I am concerned with, it's the possible degradation of the Enable pulse by the presence of the LED/resistor combination. I thought I spelled this out pretty clearly in reply #2.

Don

Well as I stated in my reply #3, I'm driving a clock output signal at around 100khz using pin 13 and I don't see any degradation of the signal on my oscilloscope. An LED does not represent any significant capacitance or inductive reactance, so as long as the AVR output pin has plenty of current grunt to spare I still just don't understand your concern.

Lefty

What I am trying to get across is that the Enable signal seems to be the most critical one on the LCD module. I am not saying the the LED/resistor is affecting the Enable pulse, but I am not saying that it is not affecting that pulse either. I am saying that there is no good reason to use a connection that will even possibly degrade that signal as long as other options are available.

I do not buy your argument that since the input signal to a DAC module is not affected by the LED/resistor combination then neither will the Enable pulse be affected. Your oscilloscope observations on a DAC module are really irrelevant unless you are sure that the input circuit of the LCD module is essentially the same as that of the DAC module.

Don

I guess we can leave it as just agreeing to disagree on the use of pin 13. I've kind of tired of the subject. :wink:

Lefty

With a very tight home pcb in the making, it's quite beneficial to use pin as the enable instead of an other. If you haven't had any problem with DAC, I doubt I ever will with the LCD...