SAA1062 LCD Driver
As you can see elsewhere on my site I own a nice little Philips frequency counter PM6668.
This little counter contains a seven digit LCD display which is controlled by three SAA1062 LCD controllers.
Each controller can drive up to 20 LCD segments.
Remember that in the old days LCD segments were not multiplexed, which means that each segment required its own segment driver.
A capacitor of 22n is connected between the OSC input and the GND pin which will generate a typical segment clock of approximately 60 Hz.
In this mode the SAA1062 operates as the master device.
The outputs O1 to O20 directly drive the LCD segments, whereas the BPD pin is connected to the LCD's back plane terminal.
Old fashioned, non-multiplexed, LCD displays are rather simple to interface.
Simply apply an electric field between the segments you want to show and the back plane is all you need to do to make it work.
If you apply a static electric field like that for a longer period of time you might as well remove the electric field after 30 minutes because the segments will remain visible for ever after.
This is because the segments will "burn in" if you apply a DC voltage over a longer period of time between the segment and the back plane.
Thus the polarity is switched at a rate of approximately 50 Hz.
This means that at one time the voltage between a visible segment and the back plane is +5 V, and during the next half period the voltage is -5 V.
Don't be tempted to simply switch the segment voltage on/off, because that still leaves an average DC component between the segment and back plane of 2.5 V.
The simplest way to drive an LCD segment is by using an Exclusive OR gate.
One input of the EXOR gate receives the 50 Hz back plane square wave, while the other input receives the segment status.
If this input is 0 the output of the EXOR is in sync with the back plane, which results in the segment being invisible.
A 1 on this input results in the output being inverted compared to the back plane signal, which results in the segment being visible.
The diagram above shows the simplified control timing.
Although it is called a timing diagram I don't know many real timing details like data setup time and maximum clock speed.
In the application of the SB6668 the minimum clock high and low times are 5µs.
I have no idea if the SAA1062 can go any faster than this 100kHz.