9-24
It is possible to use separate thumbwheel switches for
presetting, but since the devices load data with the oscillator
free-running, the multiplexing of the two devices is difficult to
synchronize.
Precision Frequency Counter/Tachometer
The circuit shown in Figure 23 is a simple implementation of
a four digit frequency counter, using an ICM7207A to provide
the one second gating window and the
STORE and RESET
signals. In this configuration, the display reads hertz directly.
With Pin 11 of the ICM7027A connected to V
DD
, the gating
time will be 0.1s; this will display tens of hertz at the least
significant digit. For shorter gating times, an ICM7207 may
be used (with a 6.5536MHz crystal), giving a 0.01s gating
with Pin 11 connected to V
DD
, and a 0.1s gating with Pin 11
open.
To implement a four digit tachometer, the ICM7207A with
one second gating should be used. To get the display to read
directly in RPM, the rotational frequency of the object to be
measured must be multiplied by 60. This can be done
electronically using a phase-locked loop, or mechanically by
using a disc rotating with the object with the appropriate
number of holes drilled around its edge to interrupt the light
from an LED to a photo-dector. For faster updating, use 0.1s
gating, and multiply the rotational frequency by 600.
Auto-Tare System
This circuit uses the count-up and count-down functions of
the ICM7217, controlled via the
EQUAL and ZERO outputs,
to count in SYNC with an ICL7109A and ICL7109D Con-
verter as shown in Figure 24. By
RESETing the ICM7217 on
a “tare” value conversion, and
STORE-ing the result of a true
value conversion, an automatic fare subtraction occurs in the
result.
The ICM7217 stays in step with the ICL7109 by counting up
and down between 0 and 4095, for 8192 total counts, the
same number as the ICL7109 cycle. See applications note
No. A047 for more details.
TABLE 2. CONTROL INPUT DEFINITIONS ICM7217
INPUT TERMINAL VOLTAGE FUNCTION
STORE 9 V
DD
(or floating)
V
SS
Output Latches Not Updated
Output Latches Updated
UP/DOWN 10 V
DD
(or floating)
V
SS
Counter Counts Up
Counter Counts Down
RESET 14 V
DD
(or floating)
V
SS
Normal Operation
Counter Reset
LOAD COUNTER/
I/O OFF
12 Unconnected
V
DD
V
SS
Normal Operation
Counter Loaded with BCD data
BCD Port Forced to Hi-Z Condition
LOAD REGlSTER/
OFF
11 Unconnected
V
DD
V
SS
Normal Operation
Register Loaded with BCD Data
Display Drivers Disabled; BCD Port
Forced to Hi-Z Condition, mpx Counter
Reset to D4; mpx Oscillator Inhibited
DISPLAY CONTrol 23 Common Anode
20 Common Cathode
Unconnected
V
DD
V
SS
Normal Operation
Segment Drivers Disabled
Leading Zero Blanking Inhibited
ICM7217
