Rainbow Electronics DS1338 - Manual (Page 8)

DS1338 I 2 C RTC with 56-Byte NV RAM
8 of 15
OPERATION
The DS1338 operates as a slave device on the serial bus. Access is obtained by implementing a START condition
and providing a device identification code, followed by data. Subsequent registers can be accessed sequentially
until a STOP condition is executed. The device is fully accessible and data can be written and read when V
CC
is
greater than V
PF
. However, when V
CC
falls below V
PF
, the internal clock registers are blocked from any access. If
V
PF
is less than V
BAT
, the device power is switched from V
CC
to V
BAT
when V
CC
drops below V
PF
. If V
PF
is greater
than V
BAT
, the device power is switched from V
CC
to V
BAT
when V
CC
drops below V
BAT
. The registers are maintained
from the V
BAT
source until V
CC
is returned to nominal levels. The block diagram (Figure 3) shows the main elements
of the DS1338.
An enable bit in the seconds register controls the oscillator. Oscillator startup times are highly dependent upon
crystal characteristics, PC board leakage, and layout. High ESR and excessive capacitive loads are the major
contributors to long start-up times. A circuit using a crystal with the recommended characteristics and proper layout
usually starts within 1 second.
OSCILLATOR CIRCUIT
The DS1338 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. Figure 4 shows a
functional schematic of the oscillator circuit. The startup time is usually less than 1 second when using a crystal
with the specified characteristics.
Table 1. Crystal Specifications
*
PARAMETER SYMBOL MIN TYP MAX UNITS
Nominal Frequency f
O
32.768 kHz
Series Resistance ESR 45 k W
Load Capacitance C
L
12.5 pF
*The 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 4. Oscillator Circuit Showing Internal Bias Network
COUNTDOWN
CHAIN
RTC
X1 X2
CRYSTAL
C
L
1 C
L
2 RTC
REGISTERS