7
Display Internal Block Diagram
Figure 2 shows the HDLX-1414 display internal block
diagram. The CMOS IC consists of a 4 x 7 Character RAM,
a 128-character ASCII decoder and the refresh circuitry
necessary to synchronize the decoding and driving of four
5 x 7 dot matrix characters.
Four 7-bit ASCII words are stored in the Character RAM.
The IC reads the ASCII data and decodes it vita the
128-character ASCII decoder. This decoder includes the
64-character set of the HPDL-1414, 32 lower case ASCII
symbols, and 32 foreign language symbols.
Data Entry
Figure 1 shows a truth table for the HDLX-1414 display.
Address inputs A
0
and A
1
are used to select the digit
location in the display. When A
0
and A
1
are both logic low,
data is loaded into the right most character.
Data inputs D
0
–D
6
are used to load information into the
display. Data is latched into the given character address on
the rising edge of the \WR signal. Data and Address inputs
must be held stable during the write cycle to ensure that
correct data is stored in the display.
Mechanical and Electrical Considerations
The HDLX-1414 is a 12-pin DIP package that can be
stacked to create a character array of any size. The display
is designed to operate reliably in –40˚C to +85˚C ambient
temperatures.
Electrical Description
Pin Function Description
\Write (\WR, Pin 3) WR must be logic 0 to store data in the display.
Address Inputs A
0
and A
1
select a speci c location in the display memory. Address 00 accesses the far right
(A
1
and A
0
, Pins 4 and 5) character, and address 11 accesses the far left position.
V
DD
(Pin 6) V
DD
is the positive power supply input.
Ground (GND, Pin 7) Ground is the display ground.
Data Inputs D0–D6 are used to specify the input data for the display.
(D0–D6, Pins 1, 2, 8-12)
The HDLX-1414 is assembled by die attaching and wire
bonding 140 LEDs and a CMOS IC to a high tempera-
ture printed circuit board. A polycarbonate lens is placed
over the PC board, creating a protective air gap over the
LED wire bonds. Back ll epoxy environmentally seals the
display package. This package construction makes the
display highly tolerant to temperature cycling and allows
wave soldering.
ESD and Latchup Protection
The inputs to the CMOS IC are protected against static
discharge and input current latchup. However, for best
results, standard CMOS handling precautions should be
used. Prior to use, the HDLX-1414 should be stored in
antistatic tubes or conductive material. During assembly
a grounded conductive work area should be used, and
assembly personnel should wear conductive wrist straps.
Lab coats made of synthetic material should be avoided
since they are prone to static charge build-up. Input
current latchup can be caused when the CMOS inputs are
subjected either to a voltage below ground (V
in
< ground)
or higher than V
DD
(V
in
> V
DD
) and when a high current is
forced into the input. To prevent input current latchup and
ESD damage, unused inputs should be connected either
to ground or to V
DD
. Voltages should not be applied to the
inputs until V
DD
has been turned on, and high transient
input voltages should be eliminated.
\WR A1 A0 D6 D5 D4 D3 D2 D1 D0 Function
1 X X X X X X X X X No Change
0 0 0 Digit 0 ASCII Data (Right Most Character)
0 0 1 Digit 1 ASCII Data
0 1 0 Digit 2 ASCII Data
0 1 1 Digit 3 ASCII Data (Left Most Character)
0 = Logic 0; 1 = Logic 1; X = Do Not Care.
Figure 1. Display truth table.
Write to
Character
RAM
