DS1307 64 x 8, Serial, I
C Real-Time Clock
The DS1307 uses an external 32.768kHz crystal. The oscillator circuit does not require any external
resistors or capacitors to operate. Table 1 specifies several crystal parameters for the external crystal.
characteristics, the startup time is usually less than one second.
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 will be added by crystal frequency drift caused by temperature shifts. External
circuit noise coupled into the oscillator circuit may result in the clock running fast. Refer to
Note 58: Crystal Considerations with Dallas Real-Time Clocks
for detailed information.
Table 1. Crystal Specifications*
crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to
Application Note 58: Crystal Considerations for Dallas Real-Time Clocks
for additional specifications.
Figure 2. Recommended Layout for Crystal
LOCAL GROUND PLANE (LAYER 2)
AVOID ROUTING SIGNAL LINES IN THE CROSSHATCHED
AREA (UPPER LEFT QUADRANT) OF THE PACKAGE UNLESS
THERE IS A GROUND PLANE BETWEEN THE SIGNAL LINE AND THE
RTC AND RAM ADDRESS MAP
Table 2 shows the address map for the DS1307 RTC and RAM registers. The RTC registers are located in
address locations 00h to 07h. The RAM registers are located in address locations 08h to 3Fh. During a
multibyte access, when the address pointer reaches 3Fh, the end of RAM space, it wraps around to
location 00h, the beginning of the clock space.
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