MC34708 datasheet(26/159 Pages) FREESCALE

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Analog Integrated Circuit Device Data



Freescale Semiconductor

MC34708

Functional Block Description

7.3

Clocking and Oscillators

7.3.1

Clock Generation

A system clock is generated for internal digital circuitry as well as for external applications utilizing the clock output pins. A crystal

oscillator is used for the 32.768 kHz time base and generation of related derivative clocks. If the crystal oscillator is not running

(for example, if the crystal is not present), an internal 32 kHz oscillator will be used instead.

Support is also provided for an external Secure Real Time Clock (SRTC) which may be integrated on a companion system

processor IC. For media protection in compliance with Digital Rights Management (DRM) system requirements, the

CLK32KMCU can be provided as a reference to the SRTC module where tamper protection is implemented.

7.3.1.1

Clocking Scheme

The internal 32 kHz oscillator is an integrated backup for the crystal oscillator, and provides a 32.768 kHz nominal frequency at

60% accuracy, if running. The internal oscillator only runs if a valid supply is available at BP, and would not be used as long as

the crystal oscillator is active. In absence of a valid supply at the BP supply node (for instance due to a dead battery), the crystal

oscillator continues running supplied from the coin cell battery. All control functions will run off the crystal derived frequency,

occasionally referred to as “32 kHz” for brevity’s sake.

During the switch-over between the two clock sources (such as when the crystal oscillator is starting up), the output clock is

maintained at a stable active low or high phase of the internal 32 kHz clock to avoid any clocking glitches. If the XLTAL clock

source suddenly disappears during operation, the IC will revert back to the internal clock source. Given the unpredictable nature

of the event and the startup times involved, the clock may be absent long enough for the application to shut down during this

transition due to for instance a sag in the regulator output voltage or absence of a signal on the clock output pins.

A status bit, CLKS, is available to indicate to the processor which clock is currently selected: CLKS = 0 when the internal RC is

used and CLKS = 1 if the crystal source is used. The CLKI interrupt bit will be set whenever a change in the clock source occurs,

and an interrupt will be generated if the corresponding CLKM mask bit is cleared.

7.3.1.2

Oscillator Specifications

The crystal oscillator has been optimized for use in conjunction with the Micro Crystal CC7V-T1A32.768 kHz-9.0 pF-30 ppm or

equivalent (such as Micro Crystal CC5V-T1A or Epson FC135) and is capable of handling its parametric variations.

The electrical characteristics of the 32 kHz Crystal oscillator are given in the following table, taking into account the crystal

characteristics noted above. The oscillator accuracy depends largely on the temperature characteristics of the used crystal.

Application circuits can be optimized for required accuracy by adapting the external crystal oscillator network (via component

accuracy and/or tuning). Additionally, a clock calibration system is provided to adjust the 32,768 cycle counter that generates the

1.0 Hz timer and RTC registers; see SRTC Support for more detail.

VREFCORE (BANDGAP / REGULATOR REFERENCE)

VREFCORE Output voltage

1.2

(33)

Absolute Accuracy

0.5

Temperature Drift

0.25

CREFCORE VREFCORE bypass capacitor

100

Notes

33.

3.0 V < BP < 4.5 V, no external loading on VCOREDIG, VDDLP, VCORE, or REFCORE. Extended operation down to UVDET, but no

system malfunction.

34.

35.

Maximum capacitance on VDDLP should not exceed 1000 pF, including the board capacitance.

Table 14. Core Voltages 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

MC34708 Datasheet(PDF) 26 Page - Freescale Semiconductor, Inc

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