13
FN6690.1
September 26, 2008
DIGITAL OUTPUT SELECTION BIT (OUT)
This bit selects the output status of the FT/OUT. 512Hz
Frequency Output Enable bit (FT) must be set to “0”
(disable) for OUT to take effect on FT/OUT pin. When the
OUT is set to “1” and FT is set to “0”, the FT/OUT pin is set
to logic level high. The FT/OUT pin voltage level is controlled
by the voltage of the pull-up resistor on FT/OUT pin. When
the OUT is set to “0” and FT is set to “0”, the FT/OUT pin is
set to logic level low. The voltage level of FT/OUT is set to
VOL level. The OUT bit is set to “1” on power-up. The
FT/OUT pin is an open drain output requires the use of a
pull-up resistor.
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 and
sub-second, and the register order for Alarm register is not a
100% matching to the RTC register so please take caution
on programming the alarm function.
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.
To clear an alarm, the ALM status bit must be set to “0” with
a write. Note that if the ARST bit is set to “1” (address 0Bh,
bit 7), the ALM bit will automatically be cleared when the
status register is read.
I
2
C Serial Interface
The ISL12008 supports a bidirectional bus oriented protocol.
The protocol defines any device that sends data onto the
bus as a transmitter and the receiving device as the receiver.
The device controlling the transfer is the master and the
device being controlled is the slave. The master always
initiates data transfers and provides the clock for both
transmit and receive operations. Therefore, the ISL12008
operates as a slave device in all applications.
All communication over the I
2
C bus is conducted by sending
the MSB of each byte of data first.
Protocol Conventions
Data states on the SDA line can change only during SCL
LOW periods. SDA state changes during SCL HIGH are
reserved for indicating START and STOP conditions (see
Figure 10). On power-up of the ISL12008, the SDA pin is in
the input mode.
All I
2
C bus operations must begin with a START condition,
which is a HIGH to LOW transition of SDA while SCL is
HIGH. The ISL12008 continuously monitors the SDA and
SCL lines for the START condition and does not respond to
any command until this condition is met (see Figure 10). A
START condition is ignored during the power-up sequence.
All I
2
C bus operations must be terminated by a STOP
condition, which is a LOW to HIGH transition of SDA while
SCL is HIGH (see Figure 10). A STOP condition at the end
of a read operation or at the end of a write operation to
memory only places the device in its standby mode.
An acknowledge (ACK) is a software convention used to
indicate a successful data transfer. The transmitting device,
either master or slave, releases the SDA bus after
transmitting 8 bits. During the ninth clock cycle, the receiver
pulls the SDA line LOW to acknowledge the reception of the
8 bits of data (see Figure 11).
The ISL12008 responds with an ACK after recognition of a
START condition followed by a valid Identification Byte, and
once again after successful receipt of an Address Byte. The
ISL12008 also responds with an ACK after receiving a Data
Byte of a write operation. The master must respond with an
ACK after receiving a Data Byte of a read operation.
FIGURE 10. VALID DATA CHANGES, START, AND STOP CONDITIONS
SDA
SCL
START
DATA DATA
STOP
STABLE CHANGE
DATA
STABLE
ISL12008
