DS1338 Maxim, DS1338 Datasheet - Page 8

• Manufacturer Part Number: DS1338
• Description: The DS1338 serial real-time clock (RTC) is a low-power, full binary-coded decimal (BCD) clock/calendar plus 56 bytes of NV SRAM
• Manufacturer: Maxim

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
greater than V
V
than V
timekeeping functions are maintained from the V
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.
POWER CONTROL
The power-control function is provided by a precise, temperature-compensated voltage reference and a
comparator circuit that monitors the V
V
access. If V
greater than V
maintained from the V
and write access is allowed after t
registers are reset to 01/01/00 01 00:00:00 (DD/MM/YY DOW HH:MM:SS). The CH bit in the seconds register will be set
to a 0.
Table 1. Power Control
OSCILLATOR CIRCUIT
The DS1338 uses an external 32.768kHz crystal. The oscillator circuit does not require any external resistors or
capacitors to operate. Table 2 specifies several crystal parameters for the external crystal. Figure 3 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 2. Crystal Specifications
*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.
Nominal Frequency
Series Resistance
Load Capacitance
PF
CC
V
V
V
V
CC
CC
CC
CC
is less than V
is greater than V
CONDITION
< V
< V
> V
> V
PARAMETER
BAT
SUPPLY
PF
PF
PF
PF
, the device power is switched from V
, V
, V
, V
, V
PF
CC
CC
CC
CC
is less than V
PF
BAT
. However, when V
< V
> V
< V
> V
, the device power is switched from V
BAT
BAT
BAT
BAT
BAT
, the device power is switched from V
BAT
PF
. However, when V
source until V
READ/WRITE
BAT
ACCESS
SYMBOL
, the device power is switched from V
Yes
Yes
No
No
ESR
C
f
O
L
REC
CC
CC
falls below V
*
CC
(Figure 1). On the first application of power to the device the time and date
level. The device is fully accessible and data can be written and read when
is returned to nominal levels (Table 1). After V
POWERED
MIN
CC
V
V
V
V
BY
falls below V
BAT
CC
CC
CC
BAT
CC
PF
32.768
TYP
12.5
, the internal clock registers are blocked from any access. If
8 of 16
to V
source until V
CC
CC
to V
BAT
PF
to V
, the internal clock registers are blocked from any
MAX
when V
BAT
50
BAT
when V
CC
CC
when V
to V
UNITS
CC
is returned to nominal levels. The block
kHz
kΩ
pF
CC
DS1338 I
drops below V
BAT
CC
drops below V
when V
drops below V
2
C RTC with 56-Byte NV RAM
CC
CC
drops below V
BAT
returns above V
BAT
. The oscillator and
PF
. The registers are
. If V
PF
PF
is greater
. If V
PF
, read
CC
PF
is
is