DC to DC Converter
A Simple -5VDC OR -5 / -10 VDC, DC TO DC Power Converter
If you need minus five volts (-5VDC) for the LCD display contrast control voltage and
the power supply does not have -5VDC output, a simple circuit using a ICL-7660 chip
is the solution to the problem. (See figure1) Another simple circuit using an ICL-
7661 IC can be used to provide -5VDC or -10VDC to the LCD contrast control
voltage circuit. The circuit in "figure 2" can be preset to -5VDC output, to -10VDC
output, or can be switchable. The ICL-7660 is the "C-MOS" version of the popular 555 chip and
ICL-7661 is the "C-MOS" version of the dual ICL-7660.
Solutions to Interface Problems
During the design and development of new product, hardware or software problems
between the host processor and the display module sometimes occur. These problems
are usually the result of design error or an improper understanding of the host
microprocessor or display module application rules. When these problems occur,
especially when the display remains blank, the immediate conclusion is often, "I must
have received a bad display." Experience has shown that the display will not be the
problem. As an aid to the designer, a step-by-step trouble-shooting procedure
follows which will help pin-point the cause of most troubles.
Step 1: Power Supply
Refer to the "Power Supply Schematics" in the next section.
Remove the questionable module from the system. On the test bench, hook up the
three power supply leads EXACTLY as shown in the appropriate schematic. The
proper fluid MUST be matched to the proper schematic. Use a 20K pot for a VR.
Rotating the pot iin one direction will produce a blank display. Rotating the pot in the
opposite direction will produce a display with all pixels "ON" in the top character line
or in the case of four-line displays, all pixels in the first and third character lines will
be "ON". This shows the display to be functioning properly. Now rotate the pot so the
pixel pattern is barely visible. DISCUSSION: Display modules require two voltages to
function properly. Both schematics require +5VDC across VDD and VSS. This voltage
powers the ICs on the module. The fluid bias voltage is across VDD and VO and is
derived from the pot VR. The fluid bias voltage can be adjusted from 0 to +10V in the
schematic to the right and from 0 to +5V in the schematic to the left.
Step 2: Initialization
Refer to the "Initialization or Reset" information in the previous Section.
Hookup the remaining I/O lines to the display module. Make sure the
pixel pattern is barely visible. Send the appropriate HEX codes to
initialize the display. For a two-line display, send HEX
38,38,06,0E,01. Remember, as each HEX code is put on the data bus,
the enable line must be strobe. The duration of the enable strobe pulse
must be 450ns minimum. Information on the data bus will be
transferred to the display instruction register on the falling edge of the
strobe pulse. Once all HEX codes of the initialization procedure are
strobed into the display instruction register a steady state cursor will
appear at the first character position of the top line. If the cursor
appears, the display has been initialized. If no cursor appears, transfer
of data is not taking place and there is most likely a problem with the
enable strobe pulse timing. Refer to "Timing Characteristics" below.
DISCUSSION: Display modules are designed with a built-in
initialization on "Power-Up." This built-in procedure initializes all
displays as on line and with the display function off (blank). During the
developmental phase of a new product, display modules should be
reinitialized via software using the HEX codes provided. This will allow
the designer to visually confirm when the display has been
initialized. Failure to initialize at this point is almost always due to
timing errors (see following sections). The above procedures are valid for
most Shelly displays. Some occasional troubles and solutions are listed below. Should
your display not function as described above, please consult the factory.
Symptom Possible Cause
Transmitting too
Erratic or unreliable operation a.Afast to display
A b.ABad solder joints at
header
ESD damage
Disconnection of
Missing or weak pixels a.A
EL lamp terminals
(usually permanent damage) b.A
with inverter still
A A
"ON"
A c.A
Disassembly of
display module
This display has
aA only one IC, and
Only the first 8 characters of a16 character display will function
A must be initialized
A
A as though it is a
two-line display.
Timing Characteristics
Write Operation
Read Operation
Note: Rs line must be on 140 µs or more before the E line goes high.
Power Supply Requirements
+ 5 VDC + 5 VDC & - 5 VDC
Fig. 3 Fig. 4
VR 10K - 20K
Module Fluid
Type Type
Power Supply Voltage Input
TN S
Fig. 3 + 5VDC
TN H
Fig. 4 + 5VDC & - 5VDC
TN W
Fig. 4 + 5VDC & - 5VDC
STN H
Fig. 3 + 5VDC
STN M
Fig. 4 + 5VDC & - 5VDC
STN T
Fig. 4 + 5VDC & - 5VDC
The End