HCMS-39X6 Datasheet, PDF (20/22 Pages) Agilent(Hewlett-Packard) – 3.3 V High Performance CMOS 5x7 AlphaNumeric Displays

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.
Appendix D. Refresh Circuitry
This display driver consists of 20
one-of-eight column decoders and
20 constant current sources, 1
one-of-eight row decoder and
eight row sinks, a pulse width
modulation control block, a peak
current control block, and the
circuit to refresh the LEDs. The
refresh counters and oscillator
are used to synchronize the
columns and rows.
The 160 bits are organized as 20
columns by 8 rows. The IC
illuminates the display by
sequentially turning ON each of
the 8 row-drivers. To refresh the
display once takes 512 oscillator
cycles. Because there are eight
row drivers, each row driver is
selected for 64 (512/8) oscillator
cycles. Four cycles are used to
briefly blank the display before
the following row is switched on.
Thus, each row is ON for 60
oscillator cycles out of a possible
64. This corresponds to the
maximum LED on time.
The temperature of the display
will also affect the LED bright-
ness as shown in Figure 10.
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