DS1305

PIN DESCRIPTION (continued)

PIN

NAME

FUNCTION

DIP

TSSOP

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

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

PF

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

the PF pin is driven low.

16

20

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

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