Datasheet

Analog Integrated Circuit Device Data

31 Freescale Semiconductor

MC34708

Functional Block Description

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.

The available correction range should be sufficient to ensure drift accuracy in compliance with standards for DRM time keeping.

Note that the 32.768 kHz oscillator is not affected by RTCCAL settings; calibration is only applied to the RTC time base counter.

Therefore, the frequency at the clock output CLK32K is not affected.

The RTC system calibration is enabled by programming the RTCCALMODE[1:0] for desired behavior by operational mode.

The RTC Calibration circuitry can be automatically disabled when main battery contact is lost or if it is so deeply discharged that

the RTC power draw is switched to the coin cell (configured with RTCCALMODE=01).

Because of the low RTC consumption, RTC accuracy can be maintained through long periods of the application being shut down,

even after the main battery has discharged. However, the calibration can only be as good as the RTCCAL data provided, so

occasional refreshing is recommended to ensure any drift influencing environmental factors have not skewed the clock beyond

desired tolerances.

7.3.3 Coin Cell Battery Backup

The LICELL pin provides a connection for a coin cell backup battery or supercap. If the main battery is deeply discharged,

removed, or contact-bounced (i.e., during a power cut), the RTC system and coin cell maintained logic will switch over to the

LICELL for backup power. This switch over occurs for a BP below 1.8 V threshold with LICELL greater than BP. A small capacitor

should be placed from LICELL to ground under all circumstances.

Upon initial insertion of the coin cell, it is not immediately connected to the on chip circuitry. The cell gets connected when the IC

powers on, or after enabling the coin cell charger when the IC was already on.

The coin cell charger circuit will function as a current-limited voltage source, resulting in the CC/CV taper characteristic typically

used for rechargeable Lithium-Ion batteries. The coin cell charger is enabled via the COINCHEN bit. The coin cell voltage is

programmable through the VCOIN[2:0] bits. The coin cell charger voltage is programmable in the ON state where the charge

current is fixed at ICOINHI.

If COINCHEN=1 when the system goes into Off or User Off state, the coin cell charger will continue to charge to the predefined

voltage setting but at a lower maximum current ICOINLO. This compensates for self discharge of the coin cell and ensures if and/

or when the main cell gets depleted, the coin cell will be topped off for maximum RTC retention. The coin cell charging will be

stopped for the BP below UVDET. The bit COINCHEN itself is only cleared when an RTCPORB occurs.

Table 17. RTC Calibration Settings

Code in RTCCAL[4:0] Correction in Counts per 32768 Relative correction in ppm

01111 +15 +458

00011 +3 +92

00001 +1 +31

00000 0 0

11111 -1 -31

11101 -3 -92

10001 -15 -458

10000 -16 -488

Table 18. RTC Calibration Enabling

RTCCALMODE Function

RTC Calibration disabled (default)

RTC Calibration enabled in all modes except coin cell only

Reserved for future use. Do not use.

RTC Calibration enabled in all modes