1997 Mar 28

5

Philips Semiconductors

Product specification

Clock/calendar with Power Fail Detector

PCF8573

7

FUNCTIONAL DESCRIPTION

7.1

Oscillator

The PCF8573 has an integrated crystal-controlled

oscillator which provides the timebase for the prescaler.

The frequency is determined by a single 32.76 kHz crystal

connected between OSCI and OSCO. A trimmer is

7.2

Prescaler and time counter

The prescaler provides a 128 Hz signal at the FSET output

for fine adjustment of the crystal oscillator without loading

it. The prescaler also generates a pulse once a second to

advance the seconds counter. The carry of the prescaler

and the seconds counter are available at the outputs SEC,

The mark-to-space ratio of both signals is 1 : 1. The time

counter is advanced one count by the falling edge of output

signal MIN. A transition from HIGH-to-LOW of output

signal SEC triggers MIN to change state. The time counter

counts minutes, hours, days and months, and provides a

full calendar function which needs to be corrected only

once every four years - to allow for leap-year. Cycle

lengths are shown in Table 1.

7.3

Alarm register

The alarm register is a 24-bit memory. It stores the

time-point for the next setting of the status flag COMP.

Details of writing and reading of the alarm register are

included in the description of the characteristics of the

7.4

Comparator

The comparator compares the contents of the alarm

register and the time counter, each with a length of 24 bits.

When these contents are equal the flag COMP will be set

4 ms after the falling edge of MIN. This set condition

occurs once at the beginning of each minute. This

information is latched, but can be cleared by an instruction

immediately after the flag is set and will be executed. Flag

COMP information is also available at the output COMP.

The comparison may be based upon hours and minutes

only if the internal flag NODA (no date) is set. Flag NODA

can be set and cleared by separate instructions via the

instruction has been received. Both COMP and NODA

Table 1

Cycle length of the time counter

Note

1. During February of a leap-year the ‘Time Counter Days’ may be set to 29 by directly writing into it using the ‘execute

address’ function. Leap-years must be tracked by the system software.

UNIT

NUMBER OF BITS

COUNTING CYCLE

CARRY FOR

FOLLOWING UNIT

CONTENT OF MONTH

COUNTER

minutes

7

00 to 59

59

→ 00

hours

6

00 to 23

23

→ 00

6

01 to 28

28

→ 01

2 (note 1)

or 29

→ 01

2 (note 1)

01 to 30

30

→ 01

4, 6, 9, 11

01 to 31

31

→ 01

1, 3, 5, 7, 8, 10, 12

months

5

01 to 12

12

→ 01