Analog Integrated Circuit Device Data
Freescale Semiconductor
101
PC34708
Functional Block Requirements and Behaviors
32.768 kHz Crystal Oscillator RTC Block Description and Application Information
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 switcher 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.
7.9.1.2
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.
Table 105. Oscillator and Clock Main 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
OSCILLATOR AND CLOCK OUTPUT
VINRTC
Operating Voltage
Oscillator and RTC Block from BP
Oscillator and RTC Block from LICELL
1.8
-
4.5
3.6
V
IINRTC
Operating Current Crystal Oscillator and RTC Module
All blocks disabled, no main battery attached, coin cell is attached
to LICELL
-
2.0
5.0
μA
tSTART-RTC RTC oscillator startup time
Upon application of power
-
1.0
sec
VRTCLO
Output Low
CLK32K Output sink 100
μA
CLK32KMCU Output source 50
μA
0
-
0.2
V
VRTCHI
Output High
CLK32K Output source 100
μA
CLK32KMCU Output sink 50
μA
CLK32KVC
C -0.2
VSRTC-0.2
-
CLK32KVC
C
VSRTC
V
tCLK32KET CLK32K Rise and Fall Time, CL = 50 pF
CLK32KDRV [1:0] = 00
CLK32KDRV [1:0] = 01 (default)
CLK32KDRV [1:0] = 10
CLK32KDRV [1:0] = 11
-
6.0
2.5
3.0
2.0
-
ns
tCKL32KMCUE
T
CLK32KMCU Rise and Fall Time
CL = 12 pF
-
22
-
ns