
Electrical Specifications
195
December 2003 Revised March 2005
SPRS231D
5.5
Clock Specifications
This section provides the timing requirements and switching characteristics for the OMAP5912 system clock
signals.
5.5.1 32-kHz Oscillator and Input Clock
The 32.768-kHz clock signal (often abbreviated to 32-kHz) may be supplied by either the on-chip 32-kHz
oscillator (requiring an external crystal) or an external CMOS signal.
The on-chip oscillator requires an external 32.768-kHz crystal connected across the OSC32K_IN and
OSC32K_OUT pins. The connection of the required circuit, consisting of the crystal and two load capacitors,
is shown in Figure 52. The load capacitors, C
1
and C
2
, should be chosen such that the equation below is
satisfied (recommended values are C
1
= C
2
= 10 pF). C
L
in the equation is the load specified for the crystal.
All discrete components used to implement the oscillator circuit should be placed as close as possible to the
associated oscillator pins (OSC32K_IN and OSC32K_OUT) and to the V
SS
pin closest to the oscillator pins
(ZZG ball Y13; ZDY ball H8).
NOTE 1:
Do not connect V
SS
(ZZG ball Y13; ZDY ball H8) to the common board ground but only to the
oscillator circuit as shown.
C
L
C
1
C
2
(
C
1
C
2
)
OSC32K_IN
OSC32K_OUT
C1
C2
Crystal
32.768 kHz
V
SS
(ZZG ball Y13)
(ZDY ball H8)
Do not connect V
SS
(ZZG ball Y13; ZDY ball H8) to the common board ground but only to the oscillator circuit as shown.
Figure 52. 32-kHz Oscillator External Crystal
NOTE 2:
Recommended maximum series resistance specification of the crystal is 100 k
or less at 32 kHz.
Series resistance at any other parasitic resonance of the crystal should be greater than 100 k
. For crystals
whose parasitic resonance has maximum series resistor less than 100 k
, a PI-network (Figure 53) is
needed between the OSC32K_OUT pin and the crystal to suppress oscillation at frequencies other than
32 kHz. The PI-network creates a pole to reduce the negative resistance at frequencies greater than 32 kHz.
The recommended PI-network for use with these crystals is CPI = 10 pF total and RPI = 390 k
±
5%