8
FN6749.1
December 15, 2011
takes a finite amount of time, there is the possibility that the
clock could change during the course of a read operation. In
this device, the time is latched by the read command (falling
edge of the clock on the ACK bit prior to RTC data output) into
a separate latch to avoid time changes during the read
operation. The clock continues to run. Alarms occurring during
a read are unaffected by the read operation.
Writing to the Real-Time Clock
The time and date may be set by writing to the RTC
registers. RTC Register should be written ONLY with Page
Write. To avoid changing the current time by an incomplete
write operation, write to the all 8 bytes in one write operation.
When writing the RTC registers, the new time value is
loaded into a separate buffer at the falling edge of the clock
during the Acknowledge. This new RTC value is loaded into
the RTC Register by a stop bit at the end of a valid write
sequence. An invalid write operation aborts the time update
procedure and the contents of the buffer are discarded. After
a valid write operation, the RTC will reflect the newly loaded
data beginning with the next “one second” clock cycle after
the stop bit is written. The RTC continues to update the time
while an RTC register write is in progress and the RTC
continues to run during any non-volatile write sequences.
Accuracy of the Real-Time Clock
The accuracy of the real-time clock depends on the
accuracy of the quartz crystal that is used as the time base
for the RTC. Since the resonant frequency of a crystal is
temperature dependent, the RTC performance will also be
dependent upon temperature. The frequency deviation of
the crystal is a function of the turnover-temperature of the
crystal from the crystal’s nominal frequency. For example, a
>20ppm frequency deviation translates into an accuracy of
>1 minute per month. These parameters are available from
the crystal manufacturer. Intersil’s RTC family provides
on-chip crystal compensation networks to adjust
load-capacitance to tune oscillator frequency from -34ppm to
+80ppm when using a 12.5pF load crystal. For more detailed
information, see the “Application Section” on page 20.
Clock/Control Registers (CCR)
The Control/Clock Registers are located in an area separate
from the EEPROM array, and are only accessible following a
slave byte of “1101111x”, and reads or writes to addresses
[0000h:003Fh]. The clock/control memory map has memory
addresses from 0000h to 003Fh. The defined addresses are
described in Table 1. Writing to and reading from the
undefined addresses are not recommended.
CCR Access
The contents of the CCR can be modified by performing a
byte or a page write operation directly to any address in the
CCR. Prior to writing to the CCR (except the status register),
however, the WEL and RWEL bits must be set using a three
step process (see section See “Writing to the Clock/Control
Registers” on page 12.)
The CCR is divided into 6 sections. These are:
1. Alarm 0 (8 bytes; non-volatile)
2. Alarm 1 (8 bytes; non-volatile)
3. Control (5 bytes; non-volatile)
4. Unique ID (8 bytes; non-volatile)
5. Real-Time clock (8 bytes; volatile)
6. Status (1 byte; volatile)
Each register is read and written through buffers. The
non-volatile portion (or the counter portion of the RTC) is
updated only if RWEL is set, and only after a valid write
operation and stop bit. A sequential read or page write
operation provides access to the contents of only one
section of the CCR per operation. Access to another section
requires a new operation. A read or write can begin at any
address in the CCR.
It is not necessary to set the RWEL bit prior to writing the
status register. Section 5 (status register) supports a single
byte read or write only. Continued reads or writes from this
section terminates the operation.
The state of the CCR can be read by performing a random
read at any address in the CCR at any time. This returns the
contents of that register location. Additional registers are
read by performing a sequential read. The read instruction
latches all Clock registers into a buffer, so an update of the
clock does not change the time being read. A sequential
read of the CCR will not result in the output of data from the
memory array. At the end of a read, the master supplies a
stop condition to end the operation and free the bus. After a
read of the CCR, the address remains at the previous
address +1 so the user can execute a current address read
of the CCR and continue reading the next Register.
Real-Time Clock Registers
SC, MN, HR, DT, MO, YR: Clock/Calendar Registers
These registers depict BCD representations of the time. As
such, SC (Seconds) and MN (Minutes) range from 00 to 59,
HR (Hour) is 1 to 12 with an AM or PM indicator (H21 bit) or
0 to 23 (with MIL = 1), DT (Date) is 1 to 31, MO (Month) is 1
to 12, YR (Year) is 0 to 99.
DW: Day of the Week Register
This register provides a day of the week status, and uses
three bits DY2 to DY0 to represent the seven days of the
week. The counter advances in the cycle
0-1-2-3-4-5-6-0-1-2-… The assignment of a numerical value
to a specific day of the week is arbitrary, and may be decided
by the system software designer. The default value is
defined as ‘0’.
ISL12024IRTC
