Resistors before LCD logic lines

[mrkenneth]

I know this may be a stupid question, but here goes:

Will a typical 8-bit parallel LCD display work if all eight of the logic lines are connected to a microcontroller (Microchip PIC) in series with 330-ohm resistors? The 330-ohm resistors are supposed to be for an LED 7-segment display, but I would like the ability to disconnect that and use the eight I/O pins for controlling an LCD. The datasheet of the LCDs that I have looked at say that the logic current is 2-3 mA, and needs a minimum of 4.7V for it to be considered a '1' or a 'high'. Would it still work with the resistors, or will the voltage drop be too high? (Is there a formula for calculating the voltage drop? I know, I am bad at math. )

Thank you in advance!

 

[maged541946]

Hi,
The voltage drop is I*R=V so
3*330=660 mv or 0.66 Volts
now you did not say the logic 1 is How many volts
TTL would give 3.5 to 4.9, cmos would be able to give up to 14.5 volts.

Maged A. Mohamed

http://magedm.freeyellow.com

http://www.magedsoft.com

 

[ijl]

The resistors are hardly a problem, but there is another issue that must be considered.

LCD's require a special driver, because driving a liquid crystal display at direct current will cause permanent damage to the display unit.

See, for example

http://ww1.microchip.com/downloads/en/AppNotes/00563c.pdf

 

[mrkenneth]

Thanks for the formula. I didn't know this ubiquitous equation could be used for voltage drop calculations.

Forgot to mention that logic 1 is 5V (TTL from PIC) and that I will be using an "intelligient" LCD display with an integrated controller.

Another question: is it possible to multiplex an LCD on the same 8-bit lines as a 7-segment LED display? I was thinking about turning on the write line with LCD, and then turning it off and displaying 7-segment LED data. Will there be too much flicker that way?

Thank you in advance!

 

[maged541946]

I can't tell about flicker, you are the one to deside..
Flicker starts to be obvious if updateing is less than 16 times per second, so according to your code (jobs the pic has to do) then if there is time to write all displays 20 times per sec (once each 50 milli sec) note you can sandwitch the dislay sub in code segments if code should take longer.
A good pic programming software will show you how much time your program segments take (in clock cycles or machine code) so you can antissipate where you should jump to update displays then return.

always you can get another pic with more I/O pins, consider the money you spend on hardware mutiplexing, (and time to "debug" it or fix its timing problems and adjust the software to correctly write to this or that) is spent to get the more I/O pins, simplify your design and save on coffie and asprin
Best regards and any help

Maged A. Mohamed

http://magedm.freeyellow.com

http://www.magedsoft.com

 

[felixc]

If you speak to an intelligent LCD controller (like those Optrex modules), then I doubt that there is a 2-3mA drive required. This is usually a bus driven device, with CMOS interface, so the series resistors should pose no problem.
Keeping the LED on the same bus will pose a problem though, as the LEDs are diodes at some 1.4 volts shunted to ground or VCC. The LEDs will fool the signals at the LCD controller end. Unless you can tap the LCD lines right at the PIC lines.

 

[IRstuff]

Your peripheral vision is sensitive to flicker up to 60Hz. You can test this by putting your computer monitor at the extreme edge of your peripheral vision. Most monitors are set up for 50 Hz to 70 Hz non-interlaced frame rates.

TTFN