Driving a little LCD

It's worse than that, isn't it? You have to drive "on" segments with an AC square wave. DC is 'off' for LCD.

Been using the Maxim MAXQ2000 for a LCD project, it's good for 4x 132 segments but it's biggest package is a 100pin LQFP. It's got a free

The GCC compiler for MSP430s. For some reason I can't fathom, it's not part of the gcc tree (bush?).

Back on peripherals, the only bit that has ever caused tears for me was the rather frightening frequency locked loop (RFFFL) on the clock generator, which I ended up ignoring.

Paul Burke

or serial to color LCD at a cheaper price:

or serial to brilliant color OLED:

Don...

Don't forget to add the postage. I tried to buy a "cheaper" unit from Canada. Then I saw the delivery charge and the handling fee. It doubled the price.

Be careful.

Al

--- The way it's usually done for a static display is that the 'on' segments are driven out of phase with the backplane and the 'off' segments are driven in phase with the backplane.

The driving waveform should be square so as not to leave a net DC between the segments and the backplane, like this: _ _ _ _ BP _| |_| |_| |_| |_ _ _ _ _ OFF _| |_| |_| |_| |_ _ _ _ _ _ ON |_| |_| |_| |_| This is easily done by continuously driving the backplane with a square wave at about 30 Hz and the segment's on/off signal to the segment through an exclusive-or, like this: (View in Courier)

SEGa>---------A EXOR Y---[seg]---+ +----B | | | SEGb>----|----A | | EXOR Y---[seg]---+ +----B | | | SEGc>----|----A | | EXOR Y---[seg]---+ +----B | | | SEGd>----|----A | | EXOR Y---[seg]---+ +----B | | | SEGe>----|----A | | EXOR Y---[seg]---+ +----B | | | SEGf>----|----A | | EXOR Y---[seg]---+ +----B | | | SEGg>----|----A | | EXOR Y---[seg]---+ +----B | | | BP>------+-------------------+

So, in order "light up" a segment, the appropriate SEG signal would be held high and to kill it would be pulled low.

Since the liquid crystal fluid is non-conductive it becomes the dielectric between the capacitances formed by the segments and the backplane, so when the SEG lines go low the voltage across the corresponding segments and the backplane will be the same, resulting in a voltage of zero between them, which will turn the segment off.

However, when the SEG signal is pulled high, the voltage between the segment and the backplane will be twice the peak-to-peak output voltage of the EXOR, (if the EXOR's output can swing rail-to-rail and is the same amplitude as the square wave on the backplane) causing the segment to turn on.

-- JF

Not much worse- static drive requires a backplane plus one line per segment, so 33 pins for 4 digits plus decimal points, or 32 if you can lose one of the decimal points.

Best regards, Spehro Pefhany

Eeyore wrote:

Dave wrote:

They are multiplexed, but since LCDs require AC drive, thy don't really have anodes and cathodes. A small multiplexed segment LCD typically has 2, 3, or 4 "backplane" lines, and 16 to 64 "segment" lines, depending on the total segment count.