FUNCTIONAL DEVICE OPERATION
CLOCK GENERATION AND REAL TIME CLOCK
data that has been provided, so occasional refreshing is recommended to ensure that any drift influencing environmental factors
have not skewed the clock beyond desired tolerances.
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 the UVDET threshold with LICELL greater than BP. A small
capacitor should be placed from LICELL to ground under all circumstances.
Upon initial insertion of the coincell, it is not immediately connected to the on chip circuitry. The cell gets connected when the
IC powers on, or after enabling the coincell charger when the IC was already on. During operation, coincells can get damaged
and their lifetime reduced when deeply discharged. In order to avoid such, the internal circuitry supplied from LICELL is
automatically disconnected for voltages below the coincell disconnect threshold. The cell gets reconnected again under the same
conditions as for initial insertion.
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 that
if/when the main cell gets depleted, that 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 20. Coin cell Charger Voltage Specifications
VCOIN[2:0]
Output Voltage
000
001
010
011
100
101
110
111
2.50
2.70
2.80
2.90
3.00
3.10
3.20
3.30
Table 21. Coin cell Charger Specifications
Parameter
Typ
Units
Voltage Accuracy
Coin Cell Charge Current in On and Watchdog modes ICOINHI
100
60
mV
μA
μA
%
Coin Cell Charge Current in Off and Low Power Off modes (User Off / Memory Hold) ICOINLO
Current Accuracy
10
30
LICELL Bypass Capacitor
100
4.7
100
4.7
nF
μF
nF
μF
LICELL Bypass Capacitor as coin cell replacement
LICELL Bypass Capacitor
LICELL Bypass Capacitor as coin cell replacement
13892
Analog Integrated Circuit Device Data
Freescale Semiconductor
54
FREESCALE [ Freescale ]
