Datasheet

DS1742
4 of 17
SETTING THE CLOCK
As shown in Table 2, bit 7 of the Control register is the W (write) bit. Setting the W bit to 1 halts
updates to the DS1742 registers. The user can subsequently load correct date and time values
into all eight registers, followed by a write cycle of 00h to the Control register to clear the W bit
and transfer those new settings into the clock, allowing timekeeping operations to resume from
the new set point.
Again referring to Table 2, bit 6 of the Control register is the R (read) bit. Setting the R bit to 1
halts updates to the DS1742 registers. The user can subsequently read the date and time
values from the eight registers without those contents possibly changing during those I/O
operations. A subsequent write cycle of 00h to the Control register to clear the R bit allows
timekeeping operations to resume from the previous set-point.
The pre-existing contents of the Control register bits 0:5 (Century value) are ignored/unmodified
by a write cycle to Control if either the W or R bits are being set to 1 in that write operation.
The pre-existing contents of the Control register bits 0:5 (Century value) will be modified by a
write cycle to Control if the W bit is being cleared to 0 in that write operation.
The pre-existing contents of the Control register bits 0:5 (Century value) will not be modified by
a write cycle to Control if the R bit is being cleared to 0 in that write operation.
STOPPING AND STARTING THE CLOCK OSCILLATOR
The clock oscillator may be stopped at any time. To increase the shelf life, the oscillator can be
turned off to minimize current drain from the battery. The
OSC
bit is the MSB (bit 7) of the
seconds registers, see Table 2. Setting it to a 1 stops the oscillator.
FREQUENCY TEST BIT
As shown in Table 2, bit 6 of the day byte is the frequency test bit. When the frequency test bit
is set to logic 1 and the oscillator is running, the LSB of the seconds register will toggle at 512
Hz. When the seconds register is being read, the DQ0 line will toggle at the 512 Hz frequency
as long as conditions for access remain valid (i.e.,
CE
low,
OE
low,
WE
high, and address for
seconds register remain valid and stable).
CLOCK ACCURACY
The DS1742 is guaranteed to keep time accuracy to within ±1 minute per month at 25°C. Dallas
Semiconductor calibrates the RTC at the factory using nonvolatile tuning elements. The
DS1742 does not require additional calibration. For this reason, methods of field clock
calibration are not available and not necessary. Clock accuracy is also affected by the electrical
environment and caution should be taken to place the RTC in the lowest level EMI section of
the PCB layout. For additional information, refer to Application Note 58.