ISL1221 Datasheet, PDF (16/24 Pages) Intersil Corporation – Low Power RTC with Battery Backed

ISL1221
BATTERY MODE ATR SELECTION (BMATR <1:0>)
Since the accuracy of the crystal oscillator is dependent on
the V_DD/V_BAT operation, the ISL1221 provides the capability
to adjust the capacitance between V_DD and V_BAT when the
device switches between power sources.
TABLE 12.
BMATR1
0
BMATR0
0
DELTA
CAPACITANCE
(C_BAT TO C_VDD)
0pF
0
1
-0.5pF (≈ +2ppm)
1
0
+0.5pF (≈ -2ppm)
1
1
+1pF (≈ -4ppm)
DIGITAL TRIMMING REGISTER (DTR <2:0>)
The digital trimming bits DTR0, DTR1, and DTR2 adjust the
average number of counts per second and average the ppm
error to achieve better accuracy.
• DTR2 is a sign bit. DTR2 = “0” means frequency
compensation is >0. DTR2 = “1” means frequency
compensation is <0.
• DTR1 and DTR0 are both scale bits. DTR1 gives 40ppm
adjustment and DTR0 gives 20ppm adjustment.
A range from -60ppm to +60ppm can be represented by
using these three bits (Table 13).
TABLE 13. DIGITAL TRIMMING REGISTERS
DTR2
DTR REGISTER
DTR1
DTR0
ESTIMATED
FREQUENCY
PPM
0
0
0
0 (default)
0
0
1
+20
0
1
0
+40
0
1
1
+60
1
0
0
0
1
0
1
-20
1
1
0
-40
1
1
1
-60
Alarm Registers
Addresses [0Ch to 11h]
The alarm register bytes are set up identical to the RTC
register bytes, except that the MSB of each byte functions as
an enable bit (enable = “1”). These enable bits specify which
alarm registers (seconds, minutes, etc.) are used to make
the comparison. Note that there is no alarm byte for year.
The alarm function works as a comparison between the
alarm registers and the RTC registers. As the RTC
advances, the alarm will be triggered once a match occurs
between the alarm registers and the RTC registers. Any one
alarm register, multiple registers, or all registers can be
enabled for a match.
There are two alarm operation modes: Single Event and
periodic Interrupt Mode:
• Single Event Mode is enabled by setting the ALME bit to
“1”, the IM bit to “0”, and disabling the frequency output.
This mode permits a one-time match between the alarm
registers and the RTC registers. Once this match occurs,
the ALM bit is set to “1” and the IRQ output will be pulled
low and will remain low until the ALM bit is reset. This can
be done manually or by using the auto-reset feature.
• Interrupt Mode is enabled by setting the ALME bit to “1”,
the IM bit to “1”, and disabling the frequency output. The
IRQ output will now be pulsed each time an alarm occurs.
This means that once the interrupt mode alarm is set, it
will continue to alarm for each occurring match of the
alarm and present time. This mode is convenient for
hourly or daily hardware interrupts in microcontroller
applications such as security cameras or utility meter
reading.
To clear an alarm, the ALM bit in the status register must be
set to “0” with a write. Note that if the ARST bit is set to 1
(address 07h, bit 7), the ALM bit will automatically be cleared
when the status register is read.
Following are examples of both Single Event and periodic
Interrupt Mode alarms.
Example 1 – Alarm set with single interrupt (IM = “0”)
A single alarm will occur on January 1 at 11:30am.
A. Set Alarm registers as follows:
BIT
ALARM
REGISTER 7 6 5 4 3 2 1 0 HEX DESCRIPTION
SCA 0 0 0 0 0 0 0 0 00h Seconds disabled
MNA
1 0 1 1 0 0 0 0 B0h Minutes set to 30,
enabled
HRA
1 0 0 1 0 0 0 1 91h Hours set to 11,
enabled
DTA 1 0 0 0 0 0 0 1 81h Date set to 1,
enabled
MOA
1 0 0 0 0 0 0 1 81h Month set to 1,
enabled
DWA
0 0 0 0 0 0 0 0 00h Day of week
disabled
B. Also the ALME bit must be set as follows:
BIT
CONTROL
REGISTER 7 6 5 4 3 2 1 0 HEX DESCRIPTION
INT
0 1 x x 0 0 0 0 x0h Enable Alarm
16
FN6316.1
July 15, 2010