ICM7217_01 Datasheet, PDF (8/20 Pages) Intersil Corporation – 4-Digit LED Display, Programmable Up/Down Counter

ICM7217
sponding to average segment currents of 3.1mA. Figure 1
shows the multiplex timing. The DISPLAY pin controls the
display output using three level logic. The pin is self-biased
to a voltage approximately ¹/₂ (V_DD); this corresponds to
normal operation. When this pin is connected to V_DD, the
segments are disabled and when connected to V_SS, the
leading zero blanking feature is inhibited. For normal opera-
tion (display on with leading zero blanking) the pin should be
left open. The display may be controlled with a 3 position
SPDT switch; see Test Circuit.
Multiplex SCAN Oscillator
The on-board multiplex scan oscillator has a nominal free-
running frequency of 2.5kHz. This may be reduced by the
addition of a single capacitor between the SCAN pin and the
positive supply. Capacitor values and corresponding nomi-
nal oscillator frequencies, digit repetition rates, and loading
times are shown in Table 1.
R1
10kΩ
1MΩ
R2
20kΩ
SCAN INPUT
ICM7217
C
0.01µF
FIGURE 10A.
1MΩ
500Ω
500Ω
SCAN INPUT
ICM7217
0.01µF
FIGURE 10B.
V_DD = 5V
10kΩ
78
4
3kΩ
200Ω
0.05µF
ICM7555
3
2
61
8s
0.05µF
SCAN INPUT
ICM7217
0V
FIGURE 10C.
FIGURE 10. BRIGHTNESS CONTROL CIRCUITS
TABLE 1. ICM7217 MULTIPLEXED RATE CONTROL
SCAN
CAPACITOR
None
20pF
90pF
NOMINAL
OSCILLATOR
FREQUENCY
2.5kHz
1.25kHz
600Hz
DIGIT
REPETITION
RATE
625Hz
300Hz
150Hz
SCAN
CYCLE
TIME
(4 DIGITS)
1.6ms
3.2ms
8ms
The internal oscillator output has a duty cycle of
approximately 25:1, providing a short pulse occurring at the
oscillator frequency. This pulse clocks the four-state counter
which provides the four multiplex phases. The short pulse
width is used to delay the digit driver outputs, thereby provid-
ing inter-digit blanking which prevents ghosting. The digits
are scanned from MSD (D4) to LSD (D1). See Figure 1 for
the display digit multiplex timing.
During load counter and load register operations, the
multiplex oscillator is disconnected from the SCAN input and
is allowed to free-run. In all other conditions, the oscillator
may be directly overdriven to about 20kHz, however the
external oscillator signal should have the same duty cycle as
the internal signal, since the digits are blanked during the
time the external signal is at a positive level (see Figure 1).
To insure proper leading zero blanking, the interdigit blank-
ing time should not be less than about 2µs. Overdriving the
oscillator at less than 200Hz may cause display flickering.
The display brightness may be altered by varying the duty
cycle. Figure 10 shows several variable-duty-cycle oscilla-
tors suitable for brightness control at the ICM7217 SCAN
input. The inverters should be CMOS CD4000 series and
the diodes may be any inexpensive device such as lN914.
Counting Control, STORE, RESET
As shown in Figure 2, the counter is incremented by the
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