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RECOMMENDED LAYOUT FOR CRYSTAL

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

SYMBOL

MIN

TYP

MAX

UNITS

Nominal Frequency

32.768

kHz

Series Resistance

ESR

45

kΩ

Load Capacitance

6

pF

Application 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 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.. Values in the day register that correspond to the day of the week are

user-defined, but must be sequential (e.g. if 1 equals Sunday, 2 equals Monday and so on). The day

register increments at midnight. Illogical time and date entries result in undefined operation.

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.

READING FROM THE CLOCK REGISTERS

Buffers are used to copy the time and date register at the beginning of a read. When reading in burst

mode, the user copy is static while the internal registers continue to increment.

Local ground plane (Layer 2)

crystal

X1

X2

GND