9-23
Applications
3-Level Inputs
ICM7217 has three inputs with 3-level logic states; High, Low
and Disconnected. These inputs are: LOAD REGISTER/OFF,
LOAD COUNTER/
I/O OFF and DISPLAY CONT.
The circuits illustrated on Figure 11 can be used to drive
these inputs in different applications.
Fixed Decimal Point
In the common anode versions, a fixed decimal point may be
activated by connecting the DP segment lead from the appro-
priate digit (with separate digit displays) through a 39Ω series
resistor to Ground. With common cathode devices, the DP
segment lead should be connected through a 75Ω series
resistor to V
DD
.
To force the device to display leading zeroes after a fixed
decimal point, use a bipolar transistor and base resistor in a
configuration like that shown in Figure 12 with the resistor
connected to the digit output driving the DP for left hand DP
displays, and to the next least significant digit output for right
hand DP display.
Driving Larger Displays
For displays requiring more current than the ICM7217 can
provide, the circuits of Figure 13 can be used.
LCD Display Interface
The low-power operation of the ICM7217 makes an LCD
interface desirable. The Intersil ICM7211 4-digit, BCD-to-LCD
display driver easily interfaces to the ICM7217 as shown in
Figure 14. Total system power consumption is less than 5mW.
System timing margins can be improved by using capacitance
to ground to slow down the BCD lines.
The 10kΩ - 20kΩ resistors on the switch BCD lines serve to
isolate the switches during BCD output.
Unit Counter with BCD Output
The simplest application of the ICM7217 is a 4-digit unit
counter (Figure 18). All that is required is an ICM7217, a
power supply and a 4 digit display. Add a momentary switch
for reset, an SPDT center-off switch to blank the display or
view leading zeroes, and one more SPDT switch for up/
down control. Using an ICM7217A with a common-cathode
calculator-type display results in the least expensive digital
counter/display system available.
Inexpensive Frequency Counter/ Tachometer
This circuit uses the low power ICM7555 (CMOS 555) to
generate the gating,
STORE and RESET signals as shown
in Figure 19. To provide the gating signal, the timer is con-
figured as an a stable multivibrator, using R
A
, R
B
and C to
provide an output that is positive for approximately one sec-
ond and negative for approximately 300µs - 500µs. The pos-
itive waveform time is given by t
WP
= 0.693 (R
A
+ R
B
)C
while the negative waveform is given by two = 0.693 R
B
C.
The system is calibrated by using a 5MΩ potentiometer for
R
A
as a “coarse” control and a 1kΩ potentiometer for R
B
as
a “fine” control. CD40106Bs are used as a monostable
multivibrator and reset time delay.
Tape Recorder Position Indicator/controller
The circuit in Figure 20 shows an application which uses the
up/down counting feature of the ICM7217 to keep track of
tape position. This circuit is representative of the many
applications of up/down counting in monitoring dimensional
position.
In the tape recorder application, the LOAD REGISTER,
EQUAL and ZERO outputs are used to control the recorder.
To make the recorder stop at a particular point on the tape,
the register can be set with the stop point and the
EQUAL
output used to stop the recorder either on fast forward, play
or rewind.
To make the recorder stop before the tape comes free of the
reel on rewind, a leader should be used. Resetting the
counter at the starting point of the tape, a few feet from the
end of the leader, allows the
ZERO output to be used to stop
the recorder on rewind, leaving the leader on the reel.
The 1MΩ resistor and 0.0047µF capacitor on the COUNT
INPUT provide a time constant of about 5ms to debounce
the reel switch. The Schmitt trigger on the COUNT INPUT of
the ICM7217 squares up the signal before applying it to the
counter. This technique may be used to debounce
switch-closure inputs in other applications.
Precision Elapsed Time/Countdown Timer
The circuit in Figure 21 uses an ICM7213 precision one
minute/one second timebase generator using a 4.1943MHz
crystal for generating pulses counted by an ICM7217B. The
thumbwheel switches allow a starting time to be entered into
the counter for a preset-countdown type timer, and allow the
register to be set for compare functions. For instance, to
make a 24-hour clock with BCD output the register can be
preset with 2400 and the
EQUAL output used to reset the
counter. Note the 10K resistor connected between the LOAD
COUNTER terminal and Ground. This resistor pulls the
LOAD COUNTER input low when not loading, thereby
inhibiting the BCD output drivers. This resistor should be
eliminated and SW4 replaced with an SPDT center-off
switch if the BCD outputs are to be used.
This technique may be used on any 3-level input. The 100kΩ
pullup resistor on the count input is used to ensure proper
logic voltage swing from the ICM7213. For a less expensive
(and less accurate) timebase, an ICM7555 timer may be
used in a configuration like that shown in Figure 19 to
generate a 1Hz reference.
8-Digit Up/Down Counter
This circuit (Figure 22) shows how to cascade counters and
retain correct leading zero blanking. The NAND gate detects
whether a digit is active since one of the two segments
a or b
is active on any unblanked number. The flip flop is clocked
by the least significant digit of the high order counter, and if
this digit is not blanked, the Q output of the flip flop goes high
and turns on the NPN transistor, thereby inhibiting leading
zero blanking on the low order counter.
ICM7217
