DS1305
RECOMMENDED LAYOUT FOR CRYSTAL
Local ground plane (Layer 2)
X1
crystal
X2
GND
CLOCK ACCURACY
The accuracy of the clock is dependent upon the accuracy of the crystal and the accuracy of the match
between the capacitive load of the oscillator circuit and the capacitive load for which the crystal was
trimmed. Additional error is added by crystal frequency drift caused by temperature shifts. External
circuit noise coupled into the oscillator circuit can result in the clock running fast. Refer to Application
Note 58, “Crystal Considerations with Dallas Real-Time Clocks” for detailed information.
Table 1. Crystal Specifications
PARAMETER
Nominal Frequency
Series Resistance
Load Capacitance
SYMBOL MIN
TYP
32.768
MAX
UNITS
kHz
kΩ
fO
ESR
CL
45
6
pF
Note: The crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to
Applications Note 58: Crystal Considerations for Dallas Real-Time Clocks for additional specifications.
CLOCK, CALENDAR, AND ALARM
The time and calendar information is obtained by reading the appropriate register bytes. The RTC
registers and user RAM are illustrated in Figure 2. The time, calendar, and alarm are set or initialized by
writing the appropriate register bytes. Note that some bits are set to 0. These bits always read 0 regardless
of how they are written. Also note that registers 12h to 1Fh (read) and registers 92h to 9Fh are reserved.
These registers always read 0 regardless of how they are written. The contents of the time, calendar, and
alarm registers are in the BCD format. The day register increments at midnight. Values that correspond to
the day of week are user-defined but must be sequential (e.g., if 1 equals Sunday, 2 equals Monday and so
on). Illogical time and date entries result in undefined operation.
Except where otherwise noted, the initial power on state of all registers is not defined. Therefore, it is
important to enable the oscillator (EOSC = 0) and disable write protect (WP = 0) during initial
configuration.
WRITING TO THE CLOCK REGISTERS
The internal time and date registers continue to increment during write operations. However, the
countdown chain is reset when the seconds register is written. Writing the time and date registers within
one second after writing the seconds register ensures consistent data.
Terminating a write before the last bit is sent aborts the write for that byte.
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