Rainbow Electronics DS1747P - Manual (Page 4)

DS1747/DS1747P
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DS1747 TRUTH TABLE Table 1
V
CC
CE OE WE MODE DQ POWER
V
IH
X X DESELECT HIGH-Z STANDBY
V
IL
X V
IL
WRITE DATA IN ACTIVE
V
IL
V
IL
V
IH
READ DATA OUT ACTIVE
V
CC
>V
PF
V
IL
V
IH
V
IH
READ HIGH-Z ACTIVE
V
SO
<V
CC
<V
PF
X X X DESELECT HIGH-Z CMOS STANDBY
V
CC
<V
SO
<V
PF
X X X DESELECT HIGH-Z 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 BCD 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 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 (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 Dallas Semiconductor using nonvolatile tuning elements, and does not
require additional calibration. For this reason, methods of field clock calibration are not available and
not necessary. Clock accuracy is also effected 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
please see application note 58.
CLOCK ACCURACY (POWERCAP MODULE)
The DS1747 and DS9034PCX are each individually tested for accuracy. Once mounted together, the
module will typically keep time accuracy to within
±1.53 minutes per month (35 ppm) at 25°C. Clock
accuracy is also effected 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 please see application note
58.