PCD3745 datasheet(7/32 Pages) PHILIPS

7 / 32 page

1999 Feb 02

Philips Semiconductors

Product speciﬁcation

8-bit microcontroller with 4.5 kbytes OTP

memory and 32 kHz real-time clock

PCD3745A

REAL-TIME CLOCK (RTC)

The RTC consists of a 32 kHz crystal oscillator, a 32 kHz

to 1 second, 1.5 second and 1 minute divider chain, an

8-bit Frequency Adjustment Register (FAR) and the Clock

Control Register (CLCR). The complete real-time clock

section is independent of the microcontroller status, even

in Idle or Stop mode.

6.1

Oscillator

The internal 32 kHz oscillator requires an external

32.768 kHz quartz crystal (a positive deviation up to

+259 ppm is allowed by using frequency adjustment) and

an external feedback resistor (4.7 M

Ω) connected

between the RTC1 and RTC2 pins. The oscillator is

controlled by the RUN bit in the Clock Control Register.

6.2

Divider chain

The divider chain operates with the 32 kHz oscillator

output and divides this signal down to produce three

different clocks with periods of 1 second, 1.5 second and

1 minute. Depending on the state of the ITS and SITS bits

in the Clock Control Register, the falling edge of the

1 second, 1.5 second or 1 minute clock is used to set the

Clock Interrupt Flag (CIF) in the Clock Control Register.

Since the clock interrupt is used to let the microcontroller

leave the Stop mode, it is wire ORed with the external

interrupt (CE/T0) and has the same functionality, e.g. it

must be enabled in the Clock Control Register (ECI = 1)

and by execution of EN I. The clock interrupt will then be

treated as an external interrupt

Additionally, the divider chain generates a 16 kHz clock

(RCO) that can be routed through port line P1.7/RCO,

controlled by the ERCO bit in the Clock Control Register.

6.3

Frequency adjustment

Frequency adjustment is used to extend the interrupt time

by defining the number of 16 kHz clocks in the Frequency

Adjustment Register that will be counted twice within the

first 1 second or 1.5 second period after a minute interrupt.

The DIV512 is reset if its contents is equal to FAR, this will

extend the time of the next interrupt. This is done within the

first 1 second or 1.5 seconds of every minute. If the second

interrupt is used (ITS = 1 and SITS = 0), every 60th

interval may be up to 15.3 ms longer than the others as a

result of the frequency adjustment. If the 1.5 second

interrupt is used (ITS = 1 and SITS = 1), the prolongation

will affect every 40th interval. The adjusted Minute

Interrupt Time (MIT) shows now a maximum deviation of

0.5 ppm.

The frequency adjustment value of the real-time clock

section is defined by the decimal value of the contents of

the 8-bit Frequency Adjustment Register. It can be read or

written. The significance of the individual bits is illustrated

by the following equation:

Table 7 shows the recommended correction factor FAR for

all allowed real-time clock frequencies (FRCO).

The value of CLCR and FAR at reset is 00H.

Minute Interrupt Time (MIT)

FRCO

-----------------

FAR

214

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