DS1305

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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.

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.

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