DS1747_12 Datasheet, PDF (5/16 Pages) Maxim Integrated Products – Y2K-Compliant, Nonvolatile Timekeeping RAMs

DS1747/DS1747P Y2K-Compliant, Nonvolatile Timekeeping RAMs
Table 1. Truth Table
V_CC
CE OE WE
V_IH
X
X
V_CC>V_PF
V_IL
X
V_IL
V_IL
V_IL
V_IH
V_IL
V_IH
V_IH
V_SO<V_CC<V_PF
X
X
X
V_CC<V_SO<V_PF
X
X
X
MODE
Deselect
Write
Read
Read
Deselect
Deselect
DQ
High-Z
Data In
Data Out
High-Z
High-Z
High-Z
POWER
Standby
Active
Active
Active
CMOS Standby
Data-Retention
Mode
SETTING THE CLOCK
As shown in Table 2, bit 7 of the century register is the write bit. Setting the write bit to a one, like the
read bit, halts updates to the DS1747 registers. The user can then load them with the correct day, date
and time data in 24-hour format. Resetting the write bit to a zero then transfers those values to the actual
clock counters and allows normal operation to resume.
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 one 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 512Hz. When the
seconds register is being read, the DQ0 line will toggle at the 512Hz 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 (DIP MODULE)
The DS1747 is guaranteed to keep time accuracy to within ±1 minute per month at +25°C. The RTC is
calibrated at the factory by Maxim using nonvolatile tuning elements, and does not require additional
calibration. For this reason, methods of field clock calibration are not available and not necessary. The
electrical environment also affects the clock accuracy, and caution should be taken to place the RTC in
the lowest-level EMI section of the PC board layout. For additional information, refer to Application
Note 58.
CLOCK ACCURACY (PowerCap MODULE)
The DS1747 and DS9034PCX are each individually tested for accuracy. Once mounted together, the
module typically keeps time accuracy to within ±1.53 minutes per month (35 ppm) at +25°C. 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 PC board layout. For additional information, refer to Application
Note 58.
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