CDP1879 Intersil Corporation, CDP1879 Datasheet - Page 12

• Manufacturer Part Number: CDP1879
• Description: CMOS Real-Time Clock
• Manufacturer: Intersil Corporation

NOTE: CDP1879C-1 only.
Design Considerations for Stable Crystal Oscillation
1. Stray capacitances should be minimized for best oscilla-
2. A signal line or power source line must not cross or go
3. It is advisable to put a 0.1 F capacitor between V
FIGURE 5. CONNECTIONS FOR AN EXTERNAL FREQUENCY
tor performance. Circuit board traces should be kept to a
maximum of 1 inch, and there should be no parallel
traces.
near the oscillator circuit line.
V
Crystal Impedance
SS
PARAMETER
TABLE 3. TYPICAL OSCILLATOR CIRCUIT PARAMETERS FOR SUGGESTED OSCILLATOR CIRCUIT, SEE FIGURE 6
of the CDP1879.
R
R
C
C
C
F
0
1
S
L
SOURCE APPLIED TO REAL-TIME CLOCK
CDP1879
XTAL
XTAL
5
P
F
PIN 23
PIN 22
4.197MHz
39
P
22
39
73
FIGURE 7. TYPICAL EXTERNAL CLOCK-SOURCE CIRCUIT
F
5
-
-
EXTERNAL
FREQUENCY
SOURCE
R
L
200K
22 MEG.
CDP1879, CDP1879C-1
R
F
2.097MHz
DD
200
22
39
5
-
-
1/3 CD54/74HC04
and
1
2
3
4-115
Standby (Timekeeping) Voltage Operation
When any one of the four specified crystals is used with the
on-board oscillator, the Real-Time Clock can operate at a
minimum of 4V V
(timekeeping only, no device READ/WRITE accesses) down
to 3V at -40
To achieve this low voltage operation, an external 32kHz clock
source must be supplied at the XTAL input (see Figure 7). The
standby requirements for CHIP SELECT/DESELECT are
listed in Table 4, and Figure 8 indicates the timing waveforms.
Figure 9 illustrates the typical timekeeping curve over the full
temperature range.
FIGURE 6. SUGGESTED OSCILLATOR CIRCUIT APPLIED TO
14
4
CDP1879
1.049MHz
XTAL
XTAL
o
200
REAL-TIME CLOCK (SEE TABLE 3)
+3V
22
39
C to +85
5
-
-
PIN 23
PIN 22
DD
. However, at 32kHz the clock will run
o
24
23
22
C and 2.5V at 0
CDP1879
32768Hz (NOTE)
R
C
F
L
50K (max.)
R
S
200
22
39
91
5
o
C
RESONANT
PARALLEL
O
to +70
CRYSTAL
o
C.
UNITS
M
k
pF
pF
pF
C
I