HCMS-39X2 Datasheet, PDF (20/22 Pages) Agilent(Hewlett-Packard) – 3.3 V High Performance CMOS 5x7 AlphaNumeric Displays

The logic ground is internally
connected to the LED ground by
a substrate diode. This diode
becomes forward biased and
conducts when the logic ground
is 0.4 V greater than the LED
ground. The LED ground and the
logic ground should be connected
to a common ground, which can
withstand the current introduced
by the switching LED drivers.
When separate ground
connections are used, the LED
ground can vary from -0.3 V to
+0.3 V with respect to the logic
ground. Voltages below -0.3 V
can cause all the dots to be ON.
Voltage above +0.3 V can cause
dimming and dot mismatch.
Using a decoupling capacitor
between the power supply and
ground will help prevent any
supply noise in the frequency
range greater than that of the
functioning display from
interfering with the display’s
internal circuitry. The value of
the capacitor depends on the
series resistance from the ground
back to the power supply and the
range of frequencies that need to
be suppressed. It is also
advantageous to use the largest
ground plane possible.
Electrostatic Discharge
The inputs to the ICs are
protected against static discharge
and input current latch up.
However, for best results,
standard CMOS handling
precautions should be used.
Before use, the HCMS-39XX
should be stored in antistatic
tubes or in 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 buildup. Input current latch
up is caused when the CMOS
inputs are subjected to either a
voltage below ground
(V_IN < ground) or to a voltage
higher than V_LOGIC
(V_IN > V_LOGIC) and when a high
current is forced into the input.
To prevent input current latch up
and ESD damage, unused inputs
should be connected to either
ground or V_LOGIC. Voltages
should not be applied to the
inputs until V_LOGIC has been
applied to the display.
Appendix C. Oscillator
The oscillator provides the
internal refresh circuitry with a
signal that is used to synchronize
the columns and rows. This
ensures that the right data is in
the dot drivers for that row. This
signal can be supplied from
either an external source or the
internal source.
A display refresh rate of 100 Hz
or faster ensures flicker-free
operation. Thus, for an external
oscillator the frequency should be
greater than or equal to 512 x
100 Hz = 51.2 kHz. Operation
above 1 MHz without the
prescaler or 8 MHz with the
prescaler may cause noticeable
pixel-to-pixel mismatch.
20