Detailed Description

The DS1372 is a 32-bit binary counter designed to con-

tinuously count time in seconds. An additional counter

is provided that can generate a periodic alarm. An

interrupt output can be driven when the alarm condition

is met. The device includes a unique, factory-lasered

64-bit ROM ID. The device is programmed serially by

Oscillator Circuit

The DS1372 is designed to operate with a standard

32.768kHz quartz crystal having a 12.5pF specified

selection and crystal layout considerations, refer to

Application Note 58:

Crystal Considerations with Dallas

Real-Time Clocks (RTCs). An external 32.768kHz oscil-

lator can be used as the DS1372’s time base. In this

configuration, the X1 pin is connected to the external

oscillator signal and the X2 is floated. The EOSC bit in

the Control Register controls oscillator operation.

Clock Accuracy

The initial clock accuracy is dependent upon the accu-

racy 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 cou-

pled into the oscillator circuit can result in the clock run-

ning fast. Figure 2 shows a typical PCB layout for

isolation of the crystal and oscillator from noise. Refer to

Application Note 58:

Crystal Considerations with Dallas

Real-Time Clocks (RTCs) for detailed information.

Operation

The block diagram in Figure 1 shows the DS1372’s main

elements. As shown, communications to and from the

DS1372 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 a

register address. Subsequent registers can be accessed

sequentially until a STOP condition is executed.

Address Map

Table 1 shows the address map for the DS1372 regis-

ters. During a multibyte access, when the address

pointer reaches the end of the register space (10h) it

address pointer incrementing to location 00h, the cur-

rent time is transferred to a second set of registers. The

time information is read from these secondary registers,

while the clock may continue to run. This eliminates the

need to reread the registers in case the main registers

update during a read.

Clock Operation

The clock counter is a 32-bit up counter. The counter

counts up once per second. The contents can be read

or written by accessing the address range 00h–03h. On

00h, the current value is latched into a register, which is

output on the serial data line while the counter contin-

ues to increment. When writing to the registers, the

divider chain is reset when register 00h is written. Once

the divider chain is reset, the remaining clock registers

should be written within one second to avoid rollover

issues. Additionally, to avoid rollover issues the clock

registers must also be written from LSB to MSB, and all

four bytes should always be written.

_______________________________________________________________________________________

5

CRYSTAL

X1

X2

LOCAL GROUND PLANE (LAYER 2)

Figure 2. Layout Example