DS1305EN Datasheet DS1305EN, DS1305N, DS1305 | P4-P6

DS1305

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PIN DESCRIPTION (continued)

DIP TSSOP

NAME

FUNCTION

12 15 SDI

Serial Data Input. When SPI communication is selected, the SDI pin is the

serial data input for the SPI bus. When 3-wire communication is selected, this

pin must be tied to the SDO pin (the SDI and SDO pins function as a single I/O

pin when tied together).

13 16 SDO

Serial Data Output. When SPI communication is selected, the SDO pin is the

serial data output for the SPI bus. When 3-wire communication is selected, this

pin must be tied to the SDI pin (the SDI and SDO pins function as a single I/O

pin when tied together).

14 17 V

CCIF

Interface Logic Power-Supply Input. The V

CCIF

pin allows the DS1305 to drive

SDO and PF output pins to a level that is compatible with the interface logic,

thus allowing an easy interface to 3V logic in mixed supply systems. This pin is

physically connected to the source connection of the p-channel transistors in

the output buffers of the SDO and PF pins.

15 18

Active-Low Power-Fail Output. The PF pin is used to indicate loss of the

primary power supply (V

CC1

). When V

CC1

is less than V

CC2

or is less than V

BAT

the PF pin is driven low.

16 20 V

CC1

Primary Power Supply. DC power is provided to the device on this pin.

OPERATION

The block diagram in Figure 1 shows the main elements of the serial alarm RTC. The following

paragraphs describe the function of each pin.

Figure 1. BLOCK DIAGRAM

1Hz

OSCILLATOR AND

COUNTDOWN CHAIN

DS1305

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RECOMMENDED LAYOUT FOR CRYSTAL

CLOCK ACCURACY

The accuracy of the clock 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 temperature shifts. External

circuit noise coupled into the oscillator circuit can result in the clock running fast. Refer to Application

Note 58, “Crystal Considerations with Dallas Real-Time Clocks” for detailed information.

Table 1. Crystal Specifications

PARAMETER SYMBOL MIN TYP MAX UNITS

Nominal Frequency f

32.768 kHz

Series Resistance ESR 45 k

Load Capacitance C

6 pF

Note: The crystal, traces, and crystal input pins should be isolated from RF generating signals. Refer to

Applications Note 58: Crystal Considerations for Dallas Real-Time Clocks for additional specifications.

CLOCK, CALENDAR, AND ALARM

The time and calendar information is obtained by reading the appropriate register bytes. The RTC

registers and user RAM are illustrated in Figure 2. The time, calendar, and alarm are set or initialized by

writing the appropriate register bytes. Note that some bits are set to 0. These bits always read 0 regardless

of how they are written. Also note that registers 12h to 1Fh (read) and registers 92h to 9Fh are reserved.

These registers always read 0 regardless of how they are written. The contents of the time, calendar, and

alarm registers are in the BCD format. The day register increments at midnight. Values that correspond to

the day of week are user-defined but must be sequential (e.g., if 1 equals Sunday, 2 equals Monday and so

on). Illogical time and date entries result in undefined operation.

Except where otherwise noted, the initial power on state of all registers is not defined. Therefore, it is

important to enable the oscillator (EOSC = 0) and disable write protect (WP = 0) during initial

configuration.

WRITING TO THE CLOCK REGISTERS

The internal time and date registers continue to increment during write operations. However, the

countdown chain is reset when the seconds register is written. Writing the time and date registers within

one second after writing the seconds register ensures consistent data.

Terminating a write before the last bit is sent aborts the write for that byte.

Local ground plane (Layer 2)

crystal

GND

DS1305

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READING FROM THE CLOCK REGISTERS

Buffers are used to copy the time and date register at the beginning of a read. When reading in burst

mode, the user copy is static while the internal registers continue to increment.

Figure 2. RTC REGISTERS AND ADDRESS MAP

HEX ADDRESS

READ WRITE

Bit7 Bit6 Bit5 Bit4 Bit3 Bit2 Bit1 Bit0 RANGE

00h 80h 0 10 Seconds Seconds 00–59

01h 81h 0 10 Minutes Minutes 00–59

P 01–12 + P/A

02h 82h 0

24 10

10 Hour Hours

00–23

03h 83h 0 0 0 0 Day 1–7

04h 84h 0 0 10 Date Date 1–31

05h 85h 0 0 10 Month Month 01–12

06h 86h 10 Year Year 00–99

— —

Alarm 0

—

07h 87h M 10 Seconds Alarm Seconds Alarm 00–59

08h 88h M 10 Minutes Alarm Minutes Alarm 00–59

01–12 + P/A

09h 89h M

24 10

10 Hour Hour Alarm

00–23

0Ah 8Ah M 0 0 0 Day Alarm 01–07

— —

Alarm 1

—

0Bh 8Bh M 10 Seconds Alarm Seconds Alarm 00–59

0Ch 8Ch M 10 Minutes Alarm Minutes Alarm 00–59

01–12 + P/A

0Dh 8Dh M

24 10

10 Hour Hour Alarm

00–23

0Eh 8Eh M 0 0 0 Day Alarm 01–07

0Fh 8Fh Control Register —

10h 90h Status Register —

11h 91h Trickle Charger Register —

12h–1Fh 92h–9Fh Reserved —

20h–7Fh A0h–FFh 96 Bytes User RAM 00–FF

Note: Range for alarm registers does not include mask’m’ bits.

The DS1305 can be run in either 12-hour or 24-hour mode. Bit 6 of the hours register is defined as the

12- or 24-hour mode select bit. When high, the 12-hour mode is selected. In the 12-hour mode, bit 5 is the

AM/PM bit with logic high being PM. In the 24-hour mode, bit 5 is the second 10-hour bit (20 to 23

hours).

The DS1305 contains two time-of-day alarms. Time-of-day Alarm 0 can be set by writing to registers 87h

to 8Ah. Time-of-day Alarm 1 can be set by writing to registers 8Bh to 8Eh. The alarms can be

programmed (by the INTCN bit of the control register) to operate in two different modes; each alarm can

drive its own separate interrupt output or both alarms can drive a common interrupt output. Bit 7 of each

of the time-of-day alarm registers are mask bits (Table 2). When all of the mask bits are logic 0, a time-

of-day alarm only occurs once per week when the values stored in timekeeping registers 00h to 03h

match the values stored in the time-of-day alarm registers. An alarm is generated every day when bit 7 of

the day alarm register is set to a logic 1. An alarm is generated every hour when bit 7 of the day and hour

alarm registers is set to a logic 1. Similarly, an alarm is generated every minute when bit 7 of the day,

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P22

DS1305EN

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Real Time Clock Serial Alarm RTC 3-Wire

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