ISL12025 Intersil Corporation, ISL12025 Datasheet - Page 21

• Manufacturer Part Number: ISL12025
• Description: Real-Time Clock/Calendar
• Manufacturer: Intersil Corporation

Application Section
Crystal Oscillator and Temperature Compensation
Intersil has now integrated the oscillator compensation
circuity on-chip, to eliminate the need for external
components and adjust for crystal drift over temperature and
enable very high accuracy time keeping (<5ppm drift).
The Intersil RTC family uses an oscillator circuit with on-chip
crystal compensation network, including adjustable
load-capacitance. The only external component required is
the crystal. The compensation network is optimized for
operation with certain crystal parameters which are common
in many of the surface mount or tuning-fork crystals available
today. Table 7 summarizes these parameters.
Table 8 contains some crystal manufacturers and part
numbers that meet the requirements for the Intersil RTC
products.
The turnover temperature in Table 7 describes the
temperature where the apex of the of the drift vs.
temperature curve occurs. This curve is parabolic with the
drift increasing as (T-T0)
example, the turnover temperature is typically +25°C, and a
peak drift of >110ppm occurs at the temperature extremes of
Frequency
Frequency Tolerance
Turnover Temperature
Operating Temperature Range
Parallel Load Capacitance
Equivalent Series Resistance
SIGNALS FROM
THE MASTER
SDA BUS
SIGNALS FROM
THE SLAVE
PARAMETER
2
. For an Epson MC-405 device, for
21
TABLE 7. CRYSTAL PARAMETERS REQUIRED FOR INTERSIL RTC’S
ADDRESS
SLAVE
1
A
C
K
FIGURE 26. SEQUENTIAL READ SEQUENCE
MIN
-40
20
DATA
(1)
32.768
TYP
12.5
25
ISL12025
A
C
K
DATA
MAX
±100
(2)
30
85
50
-40 and +85°C. It is possible to address this variable drift by
adjusting the load capacitance of the crystal, which will result
in predictable change to the crystal frequency. The Intersil
RTC family allows this adjustment over temperature since
the devices include on-chip load capacitor trimming. This
control is handled by the Analog Trimming Register, or ATR,
which has 6 bits of control. The load capacitance range
covered by the ATR circuit is approximately 3.25pF to
18.75pF, in 0.25pF increments. Note that actual capacitance
would also include about 2pF of package related
capacitance. In-circuit tests with commercially available
crystals demonstrate that this range of capacitance allows
frequency control from +116ppm to -37ppm, using a 12.5pF
load crystal.
In addition to the analog compensation afforded by the
adjustable load capacitance, a digital compensation feature
is available for the Intersil RTC family. There are three bits
known as the Digital Trimming Register or DTR, and they
operate by adding or skipping pulses in the clock signal. The
range provided is ±30ppm in increments of 10ppm. The
default setting is 0ppm. The DTR control can be used for
coarse adjustments of frequency drift over temperature or for
crystal initial accuracy correction.
A
C
K
UNITS
ppm
kHz
k
°C
°C
pF
(n is any integer greater than 1)
Ω
DATA
(n-1)
Down to 20ppm if desired
Typically the value used for most crystals
For best oscillator performance
A
C
K
DATA
(n)
NOTES
O
S
T
P
October 18, 2006
FN6371.1