MC34708 datasheet(29/159 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

MC34708

Functional Block Description

7.3.2.2

Real Time Clock

A Real Time Clock (RTC) is provided with time and day counters as well as an alarm function. The RTC utilizes the 32.768 kHz

crystal oscillator for the time base and is powered by the coin cell backup supply when BP has dropped below operational range.

In configurations where the SRTC is used, the RTC can be disabled to conserve current drain by setting the RTCDIS bit to a 1

(defaults on at power up).

Time and Day Counters

The 32.768 kHz clock is divided down to a 1.0 Hz time tick which drives a 17 bit Time Of Day (TOD) counter. The TOD counter

counts the seconds during a 24 hour period from 0 to 86,399 and will then roll over to 0. When the roll over occurs, it increments

the 15 bit DAY counter. The DAY counter can count up to 32767 days. The 1.0 Hz time tick can be used to generate a 1HZI

interrupt if unmasked.

Time Of Day Alarm

A Time Of Day Alarm (TODA) function can be used to turn on the application and alert the processor. If the application is already

on, the processor will be interrupted. The TODA and DAYA registers are used to set the alarm time. When the TOD counter is

equal to the value in TODA and the DAY counter is equal to the value in DAYA, the TODAI interrupt will be generated.

Timer Reset

As long as the supply at BP is valid, the real time clock will be supplied from VCOREDIG. If BP is not valid, the real time clock

can be backed up from a coin cell via the LICELL pin. When the VSRTC voltage drops to the range of 0.9 - 0.8 V, the RTCPORB

reset signal is generated and the contents of the RTC will be reset. Additional registers backed up by coin cell will also reset with

RTCPORB. To inform the processor that the contents of the RTC are no longer valid due to the reset, a timer reset interrupt

function is implemented with the RTCRSTI bit.

RTC Timer Calibration

A clock calibration system is provided to adjust the 32,768 cycle counter that generates the 1.0 Hz timer for RTC timing registers.

The general implementation relies on the system processor to measure the 32.768 kHz crystal oscillator against a higher

frequency and more accurate system clock such as a TCXO. If the RTC timer needs a correction, a 5-bit 2’s complement

calibration word can be sent via the SPI to compensate the RTC for inaccuracy in its reference oscillator.

VSRTC

Output Voltage VOUT

•VINMIN < VIN < VINMAX

•ILMIN < IL < ILMAX

• PUMS[4:0]

 (0110, 0111, 1000, 1001)

• On mode (On, Standby, Standby LPM)

1.15

1.2

1.25

VSRTC

Output Voltage VOUT

•VINMIN < VIN < VINMAX

•ILMIN < IL < ILMAX

• PUMS[4:0] = (0110, 0111, 1000, 1001)

• On mode (On, Standby, Standby LPM)

1.25

1.3

1.35

ISRTCQ

Active Mode Quiescent Current

VINMIN < VIN < VINMAX, IL = 0

• VSRTC = 1.2 V

• VSRTC = 1.3 V

1.7

2.7

A

Table 16. VSRTC Electrical Specifications

Characteristics noted under conditions BP = 3.6 V, VBUS = 5.0 V, -40 C  TA  85 C, unless otherwise noted. Typical values

at BP = 3.6 V and TA = 25 °C under nominal conditions, unless otherwise noted.

Symbol

Characteristic

Min

Typ

Max

Unit

Notes

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