ICM7226A_01 Datasheet, PDF (11/19 Pages) Intersil Corporation – 8-Digit, Multi-Function, Frequency Counter/Timer

ICM7226A, ICM7226B
Overflow Indication
When overflow happens in any measurement it will be indicated
on the decimal point of the digit 8. A separate LED indicator can
be used. Figure 14 shows how to connect this indicator.
When timing repetitive signals, it is not necessary to “prime”
the lCM7226A and lCM7226B as the first alternating signal
states automatically prime the device. See Figure 1.
During any time interval measurement cycle, the ICM7226A
and lCM7226B requires 200ms following B going low to
update all internal logic. A new measurement cycle will not
take place until completion of this internal update time.
a
fb
g
ec
d
DP
LED overflow indicator connections: Overflow will be
indicated on the decimal point output of digit 8.
DEVICE
ICM7226A
ICM7226B
CATHODE
Decimal Point
D8
ANODE
D8
Decimal Point
FIGURE 14. SEGMENT IDENTIFICATION AND DISPLAY FONT
Time Interval Measurement
When in the time interval mode and measuring a single
event, the lCM7226A and lCM7226B must first be “primed”
prior to measuring the event of interest. This is done by first
generating a negative going edge on Channel A followed by a
negative going edge on Channel B to start the “measurement
interval”. The inputs are then primed ready for the measure-
ment. Positive going edges on A and B, before or after the
priming, will be needed to restore the original condition.
Priming can be easily accomplished using the circuit in
Figure 15.
SIGNAL A
2
INPUT A
V_DD
SIGNAL B
V_DD
2
INPUT B
N.O.
PRIME
1
150K
1
100K
1N914
0.1µF
1
10K
1
10nF
Oscillator Considerations
The oscillator is a high gain complementary FET inverter. An
external resistor of 10MΩ or 22MΩ should be connected
between the oscillator input and output to provide biasing.
The oscillator is designed to work with a parallel resonant
10MHz quartz crystal with a static capacitance of 22pF and
a series resistance of less than 35Ω. Among suitable
crystals is the 10MHz CTS KNIGHTS ISI-002.
For a specific crystal and load capacitance, the required g_M
can be calculated as follows:
g_M
=
ω²
C_IN
C_OUT
R_S



1
+
-^CC----^O-_L--_{}
2
where C_L
=



-_C--^C--_I--_N-^I--^N--+--^C---_C--^O---_O-^U---_U--^T--_T--_{}
C_O = Crystal Static Capacitance
R_S = Crystal Series Resistance
C_IN = Input Capacitance
C_OUT = Output Capacitance
ω = 2πf
The required g_M should not exceed 50% of the g_M specified
for the lCM7226 to insure reliable startup. The OSCillator
INPUT and OUTPUT pins each contribute about 4pF to C_IN
and C_OUT. For maximum stability of frequency, C_IN and
C_OUT should be approximately twice the specified crystal
static capacitance.
In cases where non decade prescalers are used, it may be
desirable to use a crystal which is neither 10MHz or 1MHz.
In that case both the multiplex rate and time between
measurements will be different. The multiplex rate is:
f_MUX
1MHz
=_m-_2o--^f--^O_d-_×---_e-^S₁---_.0-^C---_T4- _{hfeorti1m0eMbHeztwmeoednemaenadsu}f_{MreUmX e}=_nt-_2s--^f--^O-_×i--_s--^S₁---₀-^C-_{2-----3f---O-×----S}f_1--o_--C0-r_--6-thine
the 10MHz mode and ²---_f--_O-^×----_S¹----⁰_C---⁵- in the 1MHz mode.
V_SS
V_SS
V_SS
DEVICE
1
2
TYPE
CD4049B Inverting Buffer
CD4070B Exclusive - OR
FIGURE 15. PRIMING CIRCUIT, SIGNALS A AND B BOTH HIGH
OR LOW
Following the priming procedure (when in single event or 1
cycle range) the device is ready to measure one (only) event.
The buffered oscillator output should be used as an oscillator
test point or to drive additional logic; this output will drive one
low power Schottky TTL load. When the buffered oscillator
output is used to drive CMOS or the external oscillator input,
a 10kΩ resistor should be added from the buffered oscillator
output to V_DD .
The crystal and oscillator components should be located as
close to the chip as practical to minimize pickup from other
signals. Coupling from the EXTERNAL OSClLLATOR INPUT
to the OSClLLATOR OUTPUT or INPUT can cause
undesirable shifts in oscillator frequency.
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