Rainbow-electronics DS1374 Manual de usuario descargar pdf (Pagina 9)

Detailed Description

The DS1374 is a real-time clock with an I

C serial inter-

face. It provides elapsed seconds from a user-defined

starting point in a 32-bit counter (Figure 4). A 24-bit

counter can be configured as either a watchdog

counter or an alarm counter. An on-chip oscillator cir-

cuit uses a customer-supplied 32.768kHz crystal to

keep time. A power-control circuit switches operation

from V

to V

BACKUP

and back when power on V

cycled. If a rechargeable backup supply is used, a

trickle charger can be enabled to charge the backup

supply while V

is on.

Oscillator Circuit

The DS1374 uses an external 32.768kHz crystal. The

oscillator circuit does not require any external resistors

or capacitors to operate. Table 1 specifies several crys-

tal parameters for the external crystal. Figure 5 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.

Clock Accuracy

Clock accuracy 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 tempera-

ture shifts. External circuit noise coupled into the oscilla-

tor circuit can result in the clock running fast. Figure 6

shows a typical PC board layout for isolating the crystal

and oscillator from noise. Refer to Application Note 58:

Crystal Considerations with Dallas Real-Time Clocks for

detailed information.

DS1374C Only

The DS1374C integrates a standard 32,768Hz crystal

into the package. Typical accuracy at nominal V

and

25°C is approximately 10ppm. See Application Note 58

for information about crystal accuracy vs. temperature.

Address Map

Table 2 shows the address map for the DS1374 regis-

ters. During a multibyte access, the address pointer

wraps around to location 00h when it reaches the end of

the register space (08h). On an I

C START, STOP, or

address pointer incrementing to location 00h, the current

time is transferred to a second set of registers. These

secondary registers read the time information, while the

clock continues to run. This eliminates the need to reread

the registers in case of an update of the main registers

during a read.

Time-of-Day Counter

The time-of-day counter is a 32-bit up counter. The

contents can be read or written by accessing the

DS1374

C, 32-Bit Binary Counter Watchdog RTC with

Trickle Charger and Reset Input/Output

_____________________________________________________________________ 9

COUNTDOWN

CHAIN

CRYSTAL

RTC

REGISTERS

DS1374

Figure 5. Oscillator Circuit Showing Internal Bias Network

LOCAL GROUND PLANE (LAYER 2)

NOTE: AVOID ROUTING SIGNALS IN THE CROSSHATCHED AREA (UPPER LEFT-HAND

QUADRANT) OF THE PACKAGE UNLESS THERE IS A GROUND PLANE BETWEEN THE

SIGNAL LINE AND THE PACKAGE.

CRYSTAL

GND

Figure 6. Layout Example

PARAMETER

SYMBOL MIN

TYP

MAX

UNITS

Nominal

Frequency

32.768

kHz

Series

Resistance

ESR 45 kΩ

Load

Capacitance

6pF

Table 1. 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 addi-

tional specifications.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

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