DS17885S-3 查看數據表（PDF） - Maxim Integrated

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DS17885S-3

Real-Time Clocks

Maxim Integrated 

DS17285/DS17287/

DS17485/DS17487/

DS17885/DS17887

Detailed Description

The DS17x85 is a successor to the DS1285 real-time

clock (RTC). The device provides 18 bytes of real-time

clock/calendar, alarm, and control/status registers and 114

bytes of nonvolatile battery-backed RAM. The device also

provides additional extended RAM in either 2k/4k/8kbytes

(DS17285/DS17485/DS17885). A time-of-day alarm, six

maskable interrupts with a common interrupt output, and

a programmable square-wave output are available. It also

operates in either 24-hour or 12-hour format with an AM/

PM indicator. A precision temperature-compensated cir-

cuit monitors the status of VCC. If a primary power-supply

failure is detected, the device automatically switches to

a backup supply. The backup supply input supports a

primary battery, such as a lithium coin cell. The device is

accessed by a multiplexed address/data bus.

Oscillator Circuit

The DS17x85 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, and Figure 2 shows

a functional schematic of the oscillator circuit. The oscil-

lator is controlled by an enable bit in the control register.

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 startup times. A circuit using a crystal with the

recommended characteristics and proper layout usually

starts within one second.

An external 32.768kHz oscillator can also drive the

DS17x85. In this configuration, the X1 pin is connected

to the external oscillator signal and the X2 pin is left

unconnected.

Clock Accuracy

The accuracy of the clock 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 capaci-

tive 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.

Figure 3 shows a typical PC board layout for isolation of

the crystal and oscillator from noise. Refer to Application

Note 58: Crystal Considerations with Dallas Real-Time

Clocks for detailed information.

Clock Accuracy (DS17287,

DS17487, and DS17887)

The encapsulated DIP (EDIP) modules are trimmed at the

factory to ±1 minute per month accuracy at 25°C.

www.maximintegrated.com

Real-Time Clocks

Table 1. Crystal Specifications* (DS17x85

Only)

PARAMETER

Nominal

Frequency

Series

Resistance

Load

Capacitance

SYMBOL MIN

fO

ESR

CL

TYP

32.768

6 or

12.5

MAX

50

UNITS

kHz

kΩ

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 addi-

tional specifications.

COUNTDOWN

CHAIN

CL1

CL2 RTC REGISTERS

DS17285/87

DS17485/87

DS17885/87

X1

X2

CRYSTAL

Figure 2. Oscillator Circuit Showing Internal Bias Network

LOCAL GROUND PLANE (TOP LAYER)

CRYSTAL

NOTE: 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 DEVICE PACKAGE.

Figure 3. Layout Example

X1

X2

GND

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